initial commit

java/RunMe.java

@@ -0,0 +1,550 @@
+import java.math.BigInteger;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.*;
+import java.util.concurrent.ConcurrentSkipListSet;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+/**
+ * Run this code with provided arguments.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings("all")
+public final class RunMe {
+    private RunMe() {
+        // Utility class
+    }
+
+    public static void main(final String[] args) {
+        final byte[] password = parseArgs(args);
+
+        flag0(password);
+        System.out.println("The first flag was low-hanging fruit, can you find others?");
+        System.out.println("Try to read, understand and modify code in flagX(...) functions");
+
+        flag1(password);
+        flag2(password);
+        flag3(password);
+        flag4(password);
+        flag5(password);
+        flag6(password);
+        flag7(password);
+        flag8(password);
+        flag9(password);
+        flag10(password);
+        flag12(password);
+        flag13(password);
+        flag14(password);
+        flag15(password);
+        flag16(password);
+        flag17(password);
+        flag18(password);
+        flag19(password);
+        flag20(password);
+    }
+
+    private static void flag0(final byte[] password) {
+        // The result of print(...) function depends only on explicit arguments
+        print(0, 0, password);
+    }
+
+
+    private static void flag1(final byte[] password) {
+        while ("true".length() == 4) {
+        }
+
+        print(1, -5204358702485720348L, password);
+    }
+
+
+    private static void flag2(final byte[] password) {
+        int result = 0;
+        for (int i = 0; i < 300_000; i++) {
+            for (int j = 0; j < 300_000; j++) {
+                for (int k = 0; k < 300_000; k++) {
+                    result ^= (i * 7) | (j + k);
+                    result ^= result << 1;
+                }
+            }
+        }
+
+        print(2, -3458723408232943523L, password);
+    }
+
+
+    private static void flag3(final byte[] password) {
+        int result = 0;
+        for (int i = 0; i < 2025; i++) {
+            for (int j = 0; j < 2025; j++) {
+                for (int k = 0; k < 2025; k++) {
+                    for (int p = 0; p < 12; p++) {
+                        result ^= (i * 17) | (j + k * 7) & ~p;
+                        result ^= result << 1;
+                    }
+                }
+            }
+        }
+
+        print(3, result, password);
+    }
+
+
+    private static void flag4(final byte[] password) {
+        final long target = 8504327508437503432L + getInt(password);
+        for (long i = 0; i < Long.MAX_VALUE; i++) {
+            if ((i ^ (i >>> 32)) == target) {
+                print(4, i, password);
+            }
+        }
+    }
+
+    /* package-private */ static final long PRIME = 2025_2025_07;
+
+    private static void flag5(final byte[] password) {
+        final long n = 1_000_000_000_000_000L + getInt(password);
+
+        long result = 0;
+        for (long i = 0; i < n; i++) {
+            result = (result + i / 3 + i / 5 + i / 7 + i / 2025) % PRIME;
+        }
+
+        print(5, result, password);
+    }
+
+
+    private static void flag6(final byte[] password) {
+        /***
+            \u002a\u002f\u0077\u0068\u0069\u006c\u0065\u0020\u0028\u0022\u0031\u0022
+            \u002e\u006c\u0065\u006e\u0067\u0074\u0068\u0028\u0029\u0020\u003d\u003d
+            \u0020\u0031\u0029\u003b\u0020\u0020\u006c\u006f\u006e\u0067\u0020\u0009
+            \u0020\u0020\u0072\u0065\u0073\u0075\u006c\u0074\u0020\u003d\u0020\u000a
+            \u0035\u0037\u0034\u0038\u0035\u0037\u0030\u0032\u0034\u0038\u0033\u004c
+            \u002b\u0070\u0061\u0073\u0073\u0077\u006f\u0072\u0064\u005b\u0035\u005d
+            \u002b\u0070\u0061\u0073\u0073\u0077\u006f\u0072\u0064\u005b\u0032\u005d
+            \u003b\u002f\u002a
+        ***/
+        print(6, result, password);
+    }
+
+
+    private static void flag7(final byte[] password) {
+        // Count the number of occurrences of the most frequent noun at the following page:
+        // https://docs.oracle.com/javase/specs/jls/se21/html/jls-14.html
+
+        // The singular form of the most frequent noun
+        final String singular = "";
+        // The plural form of the most frequent noun
+        final String plural = "";
+        // The total number of occurrences
+        final int total = 0;
+        if (total != 0) {
+            print(7, (singular + ":" + plural + ":" + total).hashCode(), password);
+        }
+    }
+
+
+    private static void flag8(final byte[] password) {
+        // Count the number of bluish (#5984A1) pixels of this image:
+        // https://dev.java/assets/images/java-affinity-logo-icode-lg.png
+
+        final int number = 0;
+        if (number != 0) {
+            print(8, number, password);
+        }
+    }
+
+
+    private static final String PATTERN = "Reading the documentation can be surprisingly helpful!";
+    private static final int SMALL_REPEAT_COUNT = 10_000_000;
+
+    private static void flag9(final byte[] password) {
+        String repeated = "";
+        for (int i = 0; i < SMALL_REPEAT_COUNT; i++) {
+            repeated += PATTERN;
+        }
+
+        print(9, repeated.hashCode(), password);
+    }
+
+
+    private static final long LARGE_REPEAT_SHIFT = 29;
+    private static final long LARGE_REPEAT_COUNT = 1L << LARGE_REPEAT_SHIFT;
+
+    private static void flag10(final byte[] password) {
+        String repeated = "";
+        for (long i = 0; i < LARGE_REPEAT_COUNT; i++) {
+            repeated += PATTERN;
+        }
+
+        print(10, repeated.hashCode(), password);
+    }
+
+
+    private static void flag11(final byte[] password) {
+        print(11, 5823470598324780581L, password);
+    }
+
+
+    private static void flag12(final byte[] password) {
+        final BigInteger year = BigInteger.valueOf(-2025);
+        final BigInteger term = BigInteger.valueOf(PRIME + Math.abs(getInt(password)) % PRIME);
+
+        final long result = Stream.iterate(BigInteger.ZERO, BigInteger.ONE::add)
+                .filter(i -> year.multiply(i).add(term).multiply(i).compareTo(BigInteger.TEN) > 0)
+                .mapToLong(i -> i.longValue() * password[i.intValue() % password.length])
+                .sum();
+
+        print(12, result, password);
+    }
+
+
+    private static final long MAX_DEPTH = 100_000_000L;
+
+    private static void flag13(final byte[] password) {
+        try {
+            flag13(password, 0, 0);
+        } catch (final StackOverflowError e) {
+            System.err.println("Stack overflow :((");
+        }
+    }
+
+    private static void flag13(final byte[] password, final long depth, final long result) {
+        if (depth < MAX_DEPTH) {
+            flag13(password, depth + 1, (result ^ PRIME) | (result << 2) + depth * 17);
+        } else {
+            print(13, result, password);
+        }
+    }
+
+
+    private static void flag14(final byte[] password) {
+        final Instant today = Instant.parse("2025-09-09T12:00:00Z");
+        final BigInteger hours = BigInteger.valueOf(Duration.between(Instant.EPOCH, today).toHours() + password[1] + password[3]);
+
+        final long result = Stream.iterate(BigInteger.ZERO, hours::add)
+                .reduce(BigInteger.ZERO, BigInteger::add)
+                .longValue();
+
+        print(14, result, password);
+    }
+
+    private static void flag15(final byte[] password) {
+        // REDACTED
+    }
+
+    private static void flag16(final byte[] password) {
+        byte[] a = {
+                (byte) (password[0] + password[3]),
+                (byte) (password[1] + password[4]),
+                (byte) (password[2] + password[5])
+        };
+
+        for (long i = 1_000_000_000_000_000_000L + getInt(password); i >= 0; i--) {
+            flag16Update(a);
+        }
+
+        print(16, flag16Result(a), password);
+    }
+
+    /* package-private */ static void flag16Update(byte[] a) {
+        a[0] ^= a[1];
+        a[1] -= a[2] | a[0];
+        a[2] *= a[0];
+    }
+
+    /* package-private */ static int flag16Result(byte[] a) {
+        return (a[0] + " " + a[1] + " " + a[2]).hashCode();
+    }
+
+    /**
+     * Original idea by Alexei Shishkin.
+     */
+    private static void flag17(final byte[] password) {
+        final int n = Math.abs(getInt(password) % 2025) + 2025;
+        print(17, calc17(n), password);
+    }
+
+    /**
+     * Write me
+     * <pre>
+     *    0: iconst_0
+     *    1: istore_1
+     *    2: iload_1
+     *    3: iload_1
+     *    4: imul
+     *    5: sipush        2025
+     *    8: idiv
+     *    9: iload_0
+     *   10: isub
+     *   11: ifge          20
+     *   14: iinc          1, 1
+     *   17: goto          2
+     *   20: iload_1
+     *   21: ireturn
+     * </pre>
+     */
+    private static int calc17(final int n) {
+        return n;
+    }
+
+
+    private static void flag18(final byte[] password) {
+        final int n = 2025 + getInt(password) % 2025;
+        // Find the number of factors of n! modulo PRIME
+        final int factors = 0;
+        if (factors != 0) {
+            print(18, factors, password);
+        }
+    }
+
+
+    private static void flag19(final byte[] password) {
+        // Let n = 2025e25 + abs(getInt(password)).
+        // Consider the sequence of numbers (n + i) ** 2.
+        // Instead of each number, we write the number that is obtained from it by discarding the last 25 digits.
+        // How many of the first numbers of the resulting sequence will form an arithmetic progression?
+        final long result = 0;
+        if (result != 0) {
+            print(19, result, password);
+        }
+    }
+
+    /**
+     * Original idea by Dmitrii Liapin.
+     */
+    private static void flag20(final byte[] password) {
+        final Collection<Long> longs = new Random(getInt(password)).longs(2025_000)
+                .map(n -> n % 1000)
+                .boxed()
+                .collect(Collectors.toCollection(LinkedList::new));
+
+        // Calculate the number of objects (recursively) accessible by "longs" reference.
+        final int result = 0;
+
+        if (result != 0) {
+            print(20, result, password);
+        }
+    }
+
+    /**
+     * Original idea and implementation Igor Panasyuk.
+     */
+    private static void flag21(final byte[] password) {
+        record Pair(int x, int y) {
+        }
+
+        final List<Object> items = new ArrayList<>(Arrays.asList(
+                Byte.toUnsignedInt(password[0]),
+                (long) getInt(password),
+                new Pair(password[1], password[2]),
+                "Java SE 21 " + Arrays.toString(password)
+        ));
+
+        for (int round = 0; round < 10; round++) {
+            for (final Object item : List.copyOf(items)) {
+                // TODO: complete the switch expression using Java 21 features:
+                // items.add(
+                //     case Integer i -> square of i as long
+                //     case Long l and l is even -> l ^ 0x21L
+                //     case Long l and l is odd  -> -l
+                //     case Pair(int x, int y) -> x << 8 ^ y
+                //     case String s -> s.hashCode()
+                //     default -> 0
+                // );
+            }
+        }
+
+        long result = 0;
+        for (final Object item : items) {
+            result = result * 17 + item.toString().hashCode();
+        }
+
+        print(21, result, password);
+    }
+
+
+    // ---------------------------------------------------------------------------------------------------------------
+    // You may ignore all code below this line.
+    // It is not required to get any of the flags.
+    // ---------------------------------------------------------------------------------------------------------------
+
+    private static void print(final int no, long result, final byte[] password) {
+        System.out.format("flag %d: https://www.kgeorgiy.info/courses/prog-intro/hw1/%s%n", no, flag(result, password));
+    }
+
+    /* package-private */ static String flag(long result, byte[] password) {
+        final byte[] flag = password.clone();
+        for (int i = 0; i < 6; i++) {
+            flag[i] ^= result;
+            result >>>= 8;
+        }
+
+        return flag(flag);
+    }
+
+    /* package-private */ static String flag(final byte[] data) {
+        final MessageDigest messageDigest = RunMe.DIGEST.get();
+        messageDigest.update(SALT);
+        messageDigest.update(data);
+        messageDigest.update(SALT);
+        final byte[] digest = messageDigest.digest();
+
+        return IntStream.range(0, 6)
+                .map(i -> (((digest[i * 2] & 255) << 8) + (digest[i * 2 + 1] & 255)) % KEYWORDS.size())
+                .mapToObj(KEYWORDS::get)
+                .collect(Collectors.joining("-"));
+    }
+
+    /* package-private */ static byte[] parseArgs(final String[] args) {
+        if (args.length != 6) {
+            throw error("Expected 6 command line arguments, found: %d", args.length);
+        }
+
+        final byte[] bytes = new byte[args.length];
+        for (int i = 0; i < args.length; i++) {
+            final Byte value = VALUES.get(args[i].toLowerCase(Locale.US));
+            if (value == null) {
+                throw error("Expected keyword, found: %s", args[i]);
+            }
+            bytes[i] = value;
+        }
+        return bytes;
+    }
+
+    private static AssertionError error(final String format, final Object... args) {
+        System.err.format(format, args);
+        System.err.println();
+        System.exit(1);
+        throw new AssertionError();
+    }
+
+    /* package-private */ static int getInt(byte[] password) {
+        return IntStream.range(0, password.length)
+                .map(i -> password[i])
+                .reduce((a, b) -> a * KEYWORDS.size() + b)
+                .getAsInt();
+    }
+
+    private static final ThreadLocal<MessageDigest> DIGEST = ThreadLocal.withInitial(() -> {
+        try {
+            return MessageDigest.getInstance("SHA-256");
+        } catch (final NoSuchAlgorithmException e) {
+            throw new AssertionError("Cannot create SHA-256 digest", e);
+        }
+    });
+
+    public static final byte[] SALT = "fathdufimmonJiajFik3JeccafdaihoFrusthys9".getBytes(StandardCharsets.US_ASCII);
+
+    private static final List<String> KEYWORDS = List.of(
+            "abstract",
+            "assert",
+            "boolean",
+            "break",
+            "byte",
+            "case",
+            "catch",
+            "char",
+            "class",
+            "const",
+            "new",
+            "package",
+            "private",
+            "protected",
+            "public",
+            "return",
+            "short",
+            "static",
+            "strictfp",
+            "super",
+            "for",
+            "goto",
+            "if",
+            "implements",
+            "import",
+            "instanceof",
+            "int",
+            "interface",
+            "long",
+            "native",
+            "continue",
+            "default",
+            "do",
+            "double",
+            "else",
+            "enum",
+            "extends",
+            "final",
+            "finally",
+            "float",
+            "switch",
+            "synchronized",
+            "this",
+            "throw",
+            "throws",
+            "transient",
+            "try",
+            "void",
+            "volatile",
+            "while",
+            "record",
+            "Error",
+            "AssertionError",
+            "OutOfMemoryError",
+            "StackOverflowError",
+            "ArrayIndexOutOfBoundsException",
+            "ArrayStoreException",
+            "AutoCloseable",
+            "Character",
+            "CharSequence",
+            "ClassCastException",
+            "Comparable",
+            "Exception",
+            "IllegalArgumentException",
+            "IllegalStateException",
+            "IndexOutOfBoundsException",
+            "Integer",
+            "Iterable",
+            "Math",
+            "Module",
+            "NegativeArraySizeException",
+            "NullPointerException",
+            "Number",
+            "NumberFormatException",
+            "Object",
+            "Override",
+            "RuntimeException",
+            "StrictMath",
+            "String",
+            "StringBuilder",
+            "StringIndexOutOfBoundsException",
+            "SuppressWarnings",
+            "System",
+            "Thread",
+            "Throwable",
+            "ArithmeticException",
+            "ClassLoader",
+            "ClassNotFoundException",
+            "Cloneable",
+            "Deprecated",
+            "FunctionalInterface",
+            "InterruptedException",
+            "Process",
+            "ProcessBuilder",
+            "Runnable",
+            "SafeVarargs",
+            "StackTraceElement",
+            "Runtime",
+            "ThreadLocal",
+            "UnsupportedOperationException"
+    );
+
+    private static final Map<String, Byte> VALUES = IntStream.range(0, KEYWORDS.size())
+            .boxed()
+            .collect(Collectors.toMap(index -> KEYWORDS.get(index).toLowerCase(Locale.US), Integer::byteValue));
+}

java/base/Asserts.java

@@ -0,0 +1,84 @@
+package base;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Locale;
+import java.util.Objects;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings("StaticMethodOnlyUsedInOneClass")
+public final class Asserts {
+    static {
+        Locale.setDefault(Locale.US);
+    }
+
+    private Asserts() {
+    }
+
+    public static void assertEquals(final String message, final Object expected, final Object actual) {
+        final String reason = String.format("%s:%n     expected `%s`,%n       actual `%s`",
+                message, toString(expected), toString(actual));
+        assertTrue(reason, Objects.deepEquals(expected, actual));
+    }
+
+    public static String toString(final Object value) {
+        if (value != null && value.getClass().isArray()) {
+            final String result = Arrays.deepToString(new Object[]{value});
+            return result.substring(1, result.length() - 1);
+        } else {
+            return Objects.toString(value);
+        }
+    }
+
+    public static <T> void assertEquals(final String message, final List<T> expected, final List<T> actual) {
+        for (int i = 0; i < Math.min(expected.size(), actual.size()); i++) {
+            assertEquals(message + ":" + (i + 1), expected.get(i), actual.get(i));
+        }
+        assertEquals(message + ": Number of items", expected.size(), actual.size());
+    }
+
+    public static void assertTrue(final String message, final boolean value) {
+        if (!value) {
+            throw error("%s", message);
+        }
+    }
+
+    public static void assertEquals(final String message, final double expected, final double actual, final double precision) {
+        assertTrue(
+                String.format("%s: Expected %.12f, found %.12f", message, expected, actual),
+                isEqual(expected, actual, precision)
+        );
+    }
+
+    public static boolean isEqual(final double expected, final double actual, final double precision) {
+        final double error = Math.abs(actual - expected);
+        return error <= precision
+               || error <= precision * Math.abs(expected)
+               || !Double.isFinite(expected)
+               || Math.abs(expected) > 1e100
+               || Math.abs(expected) < precision && !Double.isFinite(actual);
+    }
+
+    public static void assertSame(final String message, final Object expected, final Object actual) {
+        assertTrue(String.format("%s: expected same objects: %s and %s", message, expected, actual), expected == actual);
+    }
+
+    public static void checkAssert(final Class<?> c) {
+        if (!c.desiredAssertionStatus()) {
+            throw error("You should enable assertions by running 'java -ea %s'", c.getName());
+        }
+    }
+
+    public static AssertionError error(final String format, final Object... args) {
+        final String message = String.format(format, args);
+        return args.length > 0 && args[args.length - 1] instanceof Throwable
+               ? new AssertionError(message, (Throwable) args[args.length - 1])
+               : new AssertionError(message);
+    }
+
+    public static void printStackTrace(final String message) {
+        new Exception(message).printStackTrace(System.out);
+    }
+}

java/base/BaseChecker.java

@@ -0,0 +1,20 @@
+package base;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public abstract class BaseChecker {
+    protected final TestCounter counter;
+
+    protected BaseChecker(final TestCounter counter) {
+        this.counter = counter;
+    }
+
+    public ExtendedRandom random() {
+        return counter.random();
+    }
+
+    public int mode() {
+        return counter.mode();
+    }
+}

java/base/Either.java

@@ -0,0 +1,95 @@
+package base;
+
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public interface Either<L, R> {
+    <NR> Either<L, NR> mapRight(final Function<? super R, NR> f);
+    <NR> Either<L, NR> flatMapRight(final Function<? super R, ? extends Either<L, NR>> f);
+    <T> T either(Function<? super L, ? extends T> lf, Function<? super R, ? extends T> rf);
+
+    boolean isRight();
+
+    L getLeft();
+    R getRight();
+
+    static <L, R> Either<L, R> right(final R value) {
+        return new Either<>() {
+            @Override
+            public <NR> Either<L, NR> mapRight(final Function<? super R, NR> f) {
+                return right(f.apply(value));
+            }
+
+            @Override
+            public <NR> Either<L, NR> flatMapRight(final Function<? super R, ? extends Either<L, NR>> f) {
+                return f.apply(value);
+            }
+
+            @Override
+            public <T> T either(final Function<? super L, ? extends T> lf, final Function<? super R, ? extends T> rf) {
+                return rf.apply(value);
+            }
+
+            @Override
+            public boolean isRight() {
+                return true;
+            }
+
+            @Override
+            public L getLeft() {
+                return null;
+            }
+
+            @Override
+            public R getRight() {
+                return value;
+            }
+
+            @Override
+            public String toString() {
+                return String.format("Right(%s)", value);
+            }
+        };
+    }
+
+    static <L, R> Either<L, R> left(final L value) {
+        return new Either<>() {
+            @Override
+            public <NR> Either<L, NR> mapRight(final Function<? super R, NR> f) {
+                return left(value);
+            }
+
+            @Override
+            public <NR> Either<L, NR> flatMapRight(final Function<? super R, ? extends Either<L, NR>> f) {
+                return left(value);
+            }
+
+            @Override
+            public <T> T either(final Function<? super L, ? extends T> lf, final Function<? super R, ? extends T> rf) {
+                return lf.apply(value);
+            }
+
+            @Override
+            public boolean isRight() {
+                return false;
+            }
+
+            @Override
+            public L getLeft() {
+                return value;
+            }
+
+            @Override
+            public R getRight() {
+                return null;
+            }
+
+            @Override
+            public String toString() {
+                return String.format("Left(%s)", value);
+            }
+        };
+    }
+}

java/base/ExtendedRandom.java

@@ -0,0 +1,89 @@
+package base;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class ExtendedRandom {
+    public static final String ENGLISH = "abcdefghijklmnopqrstuvwxyz";
+    public static final String RUSSIAN = "абвгдеежзийклмнопрстуфхцчшщъыьэюя";
+    public static final String GREEK = "αβγŋδεζηθικλμνξοπρτυφχψω";
+    @SuppressWarnings("StaticMethodOnlyUsedInOneClass")
+    public static final String SPACES = " \t\n\u000B\u2029\f";
+
+    private final Random random;
+
+    public ExtendedRandom(final Random random) {
+        this.random = random;
+    }
+
+    public ExtendedRandom(final Class<?> owner) {
+        this(new Random(7912736473497634913L + owner.getName().hashCode()));
+    }
+
+    public String randomString(final String chars) {
+        return randomChar(chars) + (random.nextBoolean() ? "" : randomString(chars));
+    }
+
+    public char randomChar(final String chars) {
+        return chars.charAt(nextInt(chars.length()));
+    }
+
+    public String randomString(final String chars, final int length) {
+        final StringBuilder string = new StringBuilder();
+        for (int i = 0; i < length; i++) {
+            string.append(randomChar(chars));
+        }
+        return string.toString();
+    }
+
+    public String randomString(final String chars, final int minLength, final int maxLength) {
+        return randomString(chars, nextInt(minLength, maxLength));
+    }
+
+    public boolean nextBoolean() {
+        return random.nextBoolean();
+    }
+
+    public int nextInt() {
+        return random.nextInt();
+    }
+
+    public int nextInt(final int min, final int max) {
+        return nextInt(max - min + 1) + min;
+    }
+
+    public int nextInt(final int n) {
+        return random.nextInt(n);
+    }
+
+    @SafeVarargs
+    public final <T> T randomItem(final T... items) {
+        return items[nextInt(items.length)];
+    }
+
+    public <T> T randomItem(final List<T> items) {
+        return items.get(nextInt(items.size()));
+    }
+
+    public Random getRandom() {
+        return random;
+    }
+
+    public  <T> List<T> random(final int list, final Function<ExtendedRandom, T> generator) {
+        return Stream.generate(() -> generator.apply(this)).limit(list).toList();
+    }
+
+    public double nextDouble() {
+        return random.nextDouble();
+    }
+
+    public <E> void shuffle(final List<E> all) {
+        Collections.shuffle(all, random);
+    }
+}

java/base/Functional.java

@@ -0,0 +1,92 @@
+package base;
+
+import java.util.*;
+import java.util.function.BiConsumer;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class Functional {
+    private Functional() {}
+
+    public static <T, R> List<R> map(final Collection<T> items, final Function<? super T, ? extends R> f) {
+        return items.stream().map(f).collect(Collectors.toUnmodifiableList());
+    }
+
+    public static <T, R> List<R> map(final List<T> items, final BiFunction<? super Integer, ? super T, ? extends R> f) {
+        return IntStream.range(0, items.size())
+                .mapToObj(i -> f.apply(i, items.get(i)))
+                .collect(Collectors.toUnmodifiableList());
+    }
+
+    public static <K, T, R> Map<K, R> mapValues(final Map<K, T> map, final Function<T, R> f) {
+        return map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, e -> f.apply(e.getValue())));
+    }
+
+    @SafeVarargs
+    public static <K, T> Map<K, T> mergeMaps(final Map<K, T>... maps) {
+        return Stream.of(maps).flatMap(m -> m.entrySet().stream())
+                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> a));
+    }
+
+    @SafeVarargs
+    public static <T> List<T> concat(final Collection<? extends T>... items) {
+        final List<T> result = new ArrayList<>();
+        for (final Collection<? extends T> item : items) {
+            result.addAll(item);
+        }
+        return result;
+    }
+
+    public static <T> List<T> append(final Collection<T> collection, final T item) {
+        final List<T> list = new ArrayList<>(collection);
+        list.add(item);
+        return list;
+    }
+
+    public static <T> List<List<T>> allValues(final List<T> vals, final int length) {
+        return Stream.generate(() -> vals)
+                .limit(length)
+                .reduce(
+                        List.of(List.of()),
+                        (prev, next) -> next.stream()
+                                .flatMap(value -> prev.stream().map(list -> append(list, value)))
+                                .toList(),
+                        (prev, next) -> next.stream()
+                                .flatMap(suffix -> prev.stream().map(prefix -> concat(prefix, suffix)))
+                                .toList()
+                );
+    }
+
+    public static <K, V> V get(final Map<K, V> map, final K key) {
+        final V result = map.get(key);
+        if (result == null) {
+            throw new NullPointerException(key.toString() + " in " + map(map.keySet(), Objects::toString));
+        }
+        return result;
+    }
+
+    public static void addRange(final List<Integer> values, final int d, final int c) {
+        for (int i = -d; i <= d; i++) {
+            values.add(c + i);
+        }
+    }
+
+    public static <T> void forEachPair(final T[] items, final BiConsumer<? super T, ? super T> consumer) {
+        assert items.length % 2 == 0;
+        IntStream.range(0, items.length / 2).forEach(i -> consumer.accept(items[i * 2], items[i * 2 + 1]));
+    }
+
+
+    public static <T> List<Pair<T, T>> toPairs(final T[] items) {
+        assert items.length % 2 == 0;
+        return IntStream.range(0, items.length / 2)
+                .mapToObj(i -> Pair.of(items[i * 2], items[i * 2 + 1]))
+                .toList();
+    }
+}

java/base/Log.java

@@ -0,0 +1,56 @@
+package base;
+
+import java.util.function.Supplier;
+import java.util.regex.Pattern;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class Log {
+    private final Pattern LINES = Pattern.compile("\n");
+    private int indent;
+
+    public static Supplier<Void> action(final Runnable action) {
+        return () -> {
+            action.run();
+            return null;
+        };
+    }
+
+    public void scope(final String name, final Runnable action) {
+        scope(name, action(action));
+    }
+
+    public <T> T scope(final String name, final Supplier<T> action) {
+        println(name);
+        indent++;
+        try {
+            return silentScope(name, action);
+        } finally {
+            indent--;
+        }
+    }
+
+    public <T> T silentScope(final String ignoredName, final Supplier<T> action) {
+        return action.get();
+    }
+
+    @SuppressWarnings("UseOfSystemOutOrSystemErr")
+    public void println(final Object value) {
+        for (final String line : LINES.split(String.valueOf(value))) {
+            System.out.println(indent() + line);
+        }
+    }
+
+    public void format(final String format, final Object... args) {
+        println(String.format(format,args));
+    }
+
+    private String indent() {
+        return "    ".repeat(indent);
+    }
+
+    protected int getIndent() {
+        return indent;
+    }
+}

java/base/MainChecker.java

@@ -0,0 +1,28 @@
+package base;
+
+import java.util.List;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings("StaticMethodOnlyUsedInOneClass")
+public final class MainChecker {
+    private final Runner runner;
+
+    public MainChecker(final Runner runner) {
+        this.runner = runner;
+    }
+
+    public List<String> run(final TestCounter counter, final String... input) {
+        return runner.run(counter, input);
+    }
+
+    public List<String> run(final TestCounter counter, final List<String> input) {
+        return runner.run(counter, input);
+    }
+
+    public void testEquals(final TestCounter counter, final List<String> input, final List<String> expected) {
+        runner.testEquals(counter, input, expected);
+    }
+
+}

java/base/Named.java

@@ -0,0 +1,15 @@
+package base;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public record Named<T>(String name, T value) {
+    public static <T> Named<T> of(final String name, final T f) {
+        return new Named<>(name, f);
+    }
+
+    @Override
+    public String toString() {
+        return name;
+    }
+}

java/base/Pair.java

@@ -0,0 +1,44 @@
+package base;
+
+import java.util.Map;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.UnaryOperator;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings({"StaticMethodOnlyUsedInOneClass", "unused"})
+public record Pair<F, S>(F first, S second) {
+    public static <F, S> Pair<F, S> of(final F first, final S second) {
+        return new Pair<>(first, second);
+    }
+
+    public static <F, S> Pair<F, S> of(final Map.Entry<F, S> e) {
+        return of(e.getKey(), e.getValue());
+    }
+
+    public static <F, S> UnaryOperator<Pair<F, S>> lift(final UnaryOperator<F> f, final UnaryOperator<S> s) {
+        return p -> of(f.apply(p.first), s.apply(p.second));
+    }
+
+    public static <F, S> BinaryOperator<Pair<F, S>> lift(final BinaryOperator<F> f, final BinaryOperator<S> s) {
+        return (p1, p2) -> of(f.apply(p1.first, p2.first), s.apply(p1.second, p2.second));
+    }
+
+    public static <T, F, S> Function<T, Pair<F, S>> tee(
+            final Function<? super T, ? extends F> f,
+            final Function<? super T, ? extends S> s
+    ) {
+        return t -> of(f.apply(t), s.apply(t));
+    }
+
+    @Override
+    public String toString() {
+        return "(" + first + ", " + second + ")";
+    }
+
+    public <R> Pair<F, R> second(final R second) {
+        return new Pair<>(first, second);
+    }
+}

java/base/Runner.java

@@ -0,0 +1,185 @@
+package base;
+
+import java.io.*;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings("unused")
+@FunctionalInterface
+public interface Runner {
+    List<String> run(final TestCounter counter, final List<String> input);
+
+    default List<String> run(final TestCounter counter, final String... input) {
+        return run(counter, List.of(input));
+    }
+
+    default void testEquals(final TestCounter counter, final List<String> input, final List<String> expected) {
+        counter.test(() -> {
+            final List<String> actual = run(counter, input);
+            for (int i = 0; i < Math.min(expected.size(), actual.size()); i++) {
+                final String exp = expected.get(i);
+                final String act = actual.get(i);
+                if (!exp.equalsIgnoreCase(act)) {
+                    Asserts.assertEquals("Line " + (i + 1), exp, act);
+                    return;
+                }
+            }
+            Asserts.assertEquals("Number of lines", expected.size(), actual.size());
+        });
+    }
+
+    static Packages packages(final String... packages) {
+        return new Packages(List.of(packages));
+    }
+
+    @FunctionalInterface
+    interface CommentRunner {
+        List<String> run(String comment, TestCounter counter, List<String> input);
+    }
+
+    final class Packages {
+        private final List<String> packages;
+
+        private Packages(final List<String> packages) {
+            this.packages = packages;
+        }
+
+        public Runner std(final String className) {
+            final CommentRunner main = main(className);
+            return (counter, input) -> counter.call("io", () -> {
+                final ByteArrayOutputStream baos = new ByteArrayOutputStream();
+                try (final PrintWriter writer = new PrintWriter(baos, false, StandardCharsets.UTF_8)) {
+                    input.forEach(writer::println);
+                }
+
+                final InputStream oldIn = System.in;
+                try {
+                    System.setIn(new ByteArrayInputStream(baos.toByteArray()));
+                    return main.run(String.format("[%d input lines]", input.size()), counter, List.of());
+                } finally {
+                    System.setIn(oldIn);
+                }
+            });
+        }
+
+        @SuppressWarnings("ConfusingMainMethod")
+        public CommentRunner main(final String className) {
+            final Method method = findMain(className);
+
+            return (comment, counter, input) -> {
+                counter.format("Running test %02d: java %s %s%n", counter.getTestNo(), method.getDeclaringClass().getName(), comment);
+                final ByteArrayOutputStream out = new ByteArrayOutputStream();
+                @SuppressWarnings("UseOfSystemOutOrSystemErr") final PrintStream oldOut = System.out;
+                try {
+                    System.setOut(new PrintStream(out, false, StandardCharsets.UTF_8));
+                    method.invoke(null, new Object[]{input.toArray(String[]::new)});
+                    final BufferedReader reader = new BufferedReader(new InputStreamReader(new ByteArrayInputStream(out.toByteArray()), StandardCharsets.UTF_8));
+                    final List<String> result = new ArrayList<>();
+                    while (true) {
+                        final String line = reader.readLine();
+                        if (line == null) {
+                            if (result.isEmpty()) {
+                                result.add("");
+                            }
+                            return result;
+                        }
+                        result.add(line.trim());
+                    }
+                } catch (final InvocationTargetException e) {
+                    final Throwable cause = e.getCause();
+                    throw Asserts.error("main thrown exception %s: %s", cause.getClass().getSimpleName(), cause);
+                } catch (final Exception e) {
+                    throw Asserts.error("Cannot invoke main: %s: %s", e.getClass().getSimpleName(), e.getMessage());
+                } finally {
+                    System.setOut(oldOut);
+                }
+            };
+        }
+
+        private Method findMain(final String className) {
+            try {
+                final URL url = new File(".").toURI().toURL();
+                final List<String> candidates = packages.stream()
+                        .flatMap(pkg -> {
+                            final String prefix = pkg.isEmpty() ? pkg : pkg + ".";
+                            return Stream.of(prefix + className, prefix + "$" + className);
+                        })
+                        .toList();
+
+                //noinspection ClassLoaderInstantiation,resource,IOResourceOpenedButNotSafelyClosed
+                final URLClassLoader classLoader = new URLClassLoader(new URL[]{url});
+                for (final String candidate : candidates) {
+                    try {
+                        final Class<?> loaded = classLoader.loadClass(candidate);
+                        if (!Modifier.isPublic(loaded.getModifiers())) {
+                            throw Asserts.error("Class %s is not public", candidate);
+                        }
+                        final Method main = loaded.getMethod("main", String[].class);
+                        if (!Modifier.isPublic(main.getModifiers()) || !Modifier.isStatic(main.getModifiers())) {
+                            throw Asserts.error("Method main of class %s should be public and static", candidate);
+                        }
+                        return main;
+                    } catch (final ClassNotFoundException e) {
+                        // Ignore
+                    } catch (final NoSuchMethodException e) {
+                        throw Asserts.error("Could not find method main(String[]) in class %s", candidate, e);
+                    }
+                }
+                throw Asserts.error("Could not find neither of classes %s", candidates);
+            } catch (final MalformedURLException e) {
+                throw Asserts.error("Invalid path", e);
+            }
+        }
+
+        private static String getClassName(final String pkg, final String className) {
+            return pkg.isEmpty() ? className : pkg + "." + className;
+        }
+
+        public Runner args(final String className) {
+            final CommentRunner main = main(className);
+//            final AtomicReference<String> prev = new AtomicReference<>("");
+            return (counter, input) -> {
+                final int total = input.stream().mapToInt(String::length).sum() + input.size() * 3;
+                final String comment = total <= 300
+                        ? input.stream().collect(Collectors.joining("\" \"", "\"", "\""))
+                        : input.size() <= 100
+                        ? String.format("[%d arguments, sizes: %s]", input.size(), input.stream()
+                            .mapToInt(String::length)
+                            .mapToObj(String::valueOf)
+                            .collect(Collectors.joining(", ")))
+                        : String.format("[%d arguments, total size: %d]", input.size(), total);
+//                assert comment.length() <= 5 || !prev.get().equals(comment) : "Duplicate tests " + comment;
+//                prev.set(comment);
+                return main.run(comment, counter, input);
+            };
+        }
+
+        public Runner files(final String className) {
+            final Runner args = args(className);
+            return (counter, input) -> counter.call("io", () -> {
+                final Path inf = counter.getFile("in");
+                final Path ouf = counter.getFile("out");
+                Files.write(inf, input);
+                args.run(counter, List.of(inf.toString(), ouf.toString()));
+                final List<String> output = Files.readAllLines(ouf);
+                Files.delete(inf);
+                Files.delete(ouf);
+                return output;
+            });
+        }
+    }
+}

java/base/Selector.java

@@ -0,0 +1,143 @@
+package base;
+
+import java.util.*;
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class Selector {
+    private final Class<?> owner;
+    private final List<String> modes;
+    private final Set<String> variantNames = new LinkedHashSet<>();
+    private final Map<String, Consumer<TestCounter>> variants = new LinkedHashMap<>();
+
+    public Selector(final Class<?> owner, final String... modes) {
+        this.owner = owner;
+        this.modes = List.of(modes);
+    }
+
+    public Selector variant(final String name, final Consumer<TestCounter> operations) {
+        Asserts.assertTrue("Duplicate variant " + name, variants.put(name.toLowerCase(), operations) == null);
+        variantNames.add(name);
+        return this;
+    }
+
+    private static void check(final boolean condition, final String format, final Object... args) {
+        if (!condition) {
+            throw new IllegalArgumentException(String.format(format, args));
+        }
+    }
+
+    @SuppressWarnings("UseOfSystemOutOrSystemErr")
+    public void main(final String... args) {
+        try {
+            final String mode;
+            if (modes.isEmpty()) {
+                check(args.length >= 1, "At least one argument expected, found %s", args.length);
+                mode = "";
+            } else {
+                check(args.length >= 2, "At least two arguments expected, found %s", args.length);
+                mode = args[0];
+            }
+
+            final List<String> vars = Arrays.stream(args).skip(modes.isEmpty() ? 0 : 1)
+                    .flatMap(arg -> Arrays.stream(arg.split("[ +]+")))
+                    .toList();
+
+            test(mode, vars);
+        } catch (final IllegalArgumentException e) {
+            System.err.println("ERROR: " + e.getMessage());
+            if (modes.isEmpty()) {
+                System.err.println("Usage: " + owner.getName() + " VARIANT...");
+            } else {
+                System.err.println("Usage: " + owner.getName() + " MODE VARIANT...");
+                System.err.println("Modes: " + String.join(", ", modes));
+            }
+            System.err.println("Variants: " + String.join(", ", variantNames));
+            System.exit(1);
+        }
+    }
+
+    public void test(final String mode, List<String> vars) {
+        final int modeNo = modes.isEmpty() ? -1 : modes.indexOf(mode) ;
+        check(modes.isEmpty() || modeNo >= 0, "Unknown mode '%s'", mode);
+        if (variantNames.contains("Base") && !vars.contains("Base")) {
+            vars = new ArrayList<>(vars);
+            vars.add(0, "Base");
+        }
+
+        vars.forEach(variant -> check(variants.containsKey(variant.toLowerCase()), "Unknown variant '%s'", variant));
+
+        final Map<String, String> properties = modes.isEmpty()
+                                               ? Map.of("variant", String.join("+", vars))
+                                               : Map.of("variant", String.join("+", vars), "mode", mode);
+        final TestCounter counter = new TestCounter(owner, modeNo, properties);
+        counter.printHead();
+        vars.forEach(variant -> counter.scope("Testing " + variant, () -> variants.get(variant.toLowerCase()).accept(counter)));
+        counter.printStatus();
+    }
+
+    public static <V extends Tester> Composite<V> composite(final Class<?> owner, final Function<TestCounter, V> factory, final String... modes) {
+        return new Composite<>(owner, factory, (tester, counter) -> tester.test(), modes);
+    }
+
+    public static <V> Composite<V> composite(final Class<?> owner, final Function<TestCounter, V> factory, final BiConsumer<V, TestCounter> tester, final String... modes) {
+        return new Composite<>(owner, factory, tester, modes);
+    }
+
+    public List<String> getModes() {
+        return modes.isEmpty() ? List.of("~") : modes;
+    }
+
+    public List<String> getVariants() {
+        return List.copyOf(variants.keySet());
+    }
+
+    /**
+     * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+     */
+    public static final class Composite<V> {
+        private final Selector selector;
+        private final Function<TestCounter, V> factory;
+        private final BiConsumer<V, TestCounter> tester;
+        private List<Consumer<? super V>> base;
+
+        private Composite(final Class<?> owner, final Function<TestCounter, V> factory, final BiConsumer<V, TestCounter> tester, final String... modes) {
+            selector = new Selector(owner, modes);
+            this.factory = factory;
+            this.tester = tester;
+        }
+
+        @SafeVarargs
+        public final Composite<V> variant(final String name, final Consumer<? super V>... parts) {
+            if ("Base".equalsIgnoreCase(name)) {
+                base = List.of(parts);
+                return v(name.toLowerCase());
+            } else {
+                return v(name, parts);
+            }
+        }
+
+        @SafeVarargs
+        private Composite<V> v(final String name, final Consumer<? super V>... parts) {
+            selector.variant(name, counter -> {
+                final V variant = factory.apply(counter);
+                for (final Consumer<? super V> part : base) {
+                    part.accept(variant);
+                }
+                for (final Consumer<? super V> part : parts) {
+                    part.accept(variant);
+                }
+                tester.accept(variant, counter);
+            });
+            return this;
+        }
+
+        public Selector selector() {
+            return selector;
+        }
+    }
+}

java/base/TestCounter.java

@@ -0,0 +1,184 @@
+package base;
+
+import java.io.File;
+import java.io.IOException;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.Locale;
+import java.util.Map;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.jar.JarEntry;
+import java.util.jar.JarFile;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class TestCounter extends Log {
+    public static final int DENOMINATOR = Integer.getInteger("base.denominator", 1);
+    public static final int DENOMINATOR2 = (int) Math.round(Math.sqrt(DENOMINATOR));
+
+    private static final String JAR_EXT = ".jar";
+    private static final SimpleDateFormat DATE_FORMAT = new SimpleDateFormat("dd.MM.yyyy HH:mm:ss");
+
+    private final Class<?> owner;
+    private final int mode;
+    private final Map<String, ?> properties;
+    private final ExtendedRandom random;
+
+    private final long start = System.currentTimeMillis();
+    private int passed;
+
+    public TestCounter(final Class<?> owner, final int mode, final Map<String, ?> properties) {
+        Locale.setDefault(Locale.US);
+        Asserts.checkAssert(getClass());
+
+        this.owner = owner;
+        this.mode = mode;
+        this.properties = properties;
+        random = new ExtendedRandom(owner);
+    }
+
+    public int mode() {
+        return mode;
+    }
+
+    public int getTestNo() {
+        return passed + 1;
+    }
+
+    public void test(final Runnable action) {
+        testV(() -> {
+            action.run();
+            return null;
+        });
+    }
+
+    public <T> void testForEach(final Iterable<? extends T> items, final Consumer<? super T> action) {
+        for (final T item : items) {
+            test(() -> action.accept(item));
+        }
+    }
+
+    public <T> T testV(final Supplier<T> action) {
+        return silentScope("Test " + getTestNo(), () -> {
+            final T result = action.get();
+            passed++;
+            return result;
+        });
+    }
+
+    private String getLine() {
+        return getIndent() == 0 ? "=" : "-";
+    }
+
+    public void printHead() {
+        println("=== " + getTitle());
+    }
+
+    public void printStatus() {
+        format("%s%n%s%n", getLine().repeat(30), getTitle());
+        format("%d tests passed in %dms%n", passed, System.currentTimeMillis() - start);
+        println("Version: " + getVersion(owner));
+        println("");
+    }
+
+    private String getTitle() {
+        return String.format("%s %s", owner.getSimpleName(), properties.isEmpty() ? "" : properties);
+    }
+
+    private static String getVersion(final Class<?> clazz) {
+        try {
+            final ClassLoader cl = clazz.getClassLoader();
+            final URL url = cl.getResource(clazz.getName().replace('.', '/') + ".class");
+            if (url == null) {
+                return "(no manifest)";
+            }
+
+            final String path = url.getPath();
+            final int index = path.indexOf(JAR_EXT);
+            if (index == -1) {
+                return DATE_FORMAT.format(new Date(new File(path).lastModified()));
+            }
+
+            final String jarPath = path.substring(0, index + JAR_EXT.length());
+            try (final JarFile jarFile = new JarFile(new File(new URI(jarPath)))) {
+                final JarEntry entry = jarFile.getJarEntry("META-INF/MANIFEST.MF");
+                return DATE_FORMAT.format(new Date(entry.getTime()));
+            }
+        } catch (final IOException | URISyntaxException e) {
+            return "error: " + e;
+        }
+    }
+
+    public <T> T call(final String message, final SupplierE<T> supplier) {
+        return get(supplier).either(e -> fail(e, "%s", message), Function.identity());
+    }
+
+    public void shouldFail(final String message, @SuppressWarnings("TypeMayBeWeakened") final RunnableE action) {
+        test(() -> get(action).either(e -> null, v -> fail("%s", message)));
+    }
+
+    public <T> T fail(final String format, final Object... args) {
+        return fail(Asserts.error(format, args));
+    }
+
+    public <T> T fail(final Throwable throwable) {
+        return fail(throwable, "%s: %s", throwable.getClass().getSimpleName(), throwable.getMessage());
+    }
+
+    public <T> T fail(final Throwable throwable, final String format, final Object... args) {
+        final String message = String.format(format, args);
+        println("ERROR: " + message);
+        throw throwable instanceof Error ? (Error) throwable : new AssertionError(throwable);
+    }
+
+    public void checkTrue(final boolean condition, final String message, final Object... args) {
+        if (!condition) {
+            fail(message, args);
+        }
+    }
+
+    public static <T> Either<Exception, T> get(final SupplierE<T> supplier) {
+        return supplier.get();
+    }
+
+    public Path getFile(final String suffix) {
+        return Paths.get(String.format("test%d.%s", getTestNo(), suffix));
+    }
+
+    public ExtendedRandom random() {
+        return random;
+    }
+
+    @FunctionalInterface
+    public interface SupplierE<T> extends Supplier<Either<Exception, T>> {
+        T getE() throws Exception;
+
+        @Override
+        default Either<Exception, T> get() {
+            try {
+                return Either.right(getE());
+            } catch (final Exception e) {
+                return Either.left(e);
+            }
+        }
+    }
+
+    @FunctionalInterface
+    public interface RunnableE extends SupplierE<Void> {
+        void run() throws Exception;
+
+        @Override
+        default Void getE() throws Exception {
+            run();
+            return null;
+        }
+    }
+}

java/base/Tester.java

@@ -0,0 +1,18 @@
+package base;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public abstract class Tester extends BaseChecker {
+    protected Tester(final TestCounter counter) {
+        super(counter);
+    }
+
+    public abstract void test();
+
+    public void run(final Class<?> test, final String... args) {
+        System.out.println("=== Testing " + test.getSimpleName() + " " + String.join(" ", args));
+        test();
+        counter.printStatus();
+    }
+}

java/base/Unit.java

@@ -0,0 +1,15 @@
+package base;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class Unit {
+    public static final Unit INSTANCE = new Unit();
+
+    private Unit() { }
+
+    @Override
+    public String toString() {
+        return "unit";
+    }
+}

java/base/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Common homeworks test classes
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package base;

java/expression/BigDecimalListExpression.java

@@ -0,0 +1,202 @@
+package expression;
+
+import base.Asserts;
+import base.Pair;
+import base.TestCounter;
+import expression.common.ExpressionKind;
+import expression.common.Type;
+
+import java.math.BigDecimal;
+import java.util.List;
+import java.util.stream.IntStream;
+
+/**
+ * One-argument arithmetic expression over {@link BigDecimal}s.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@FunctionalInterface
+@SuppressWarnings("ClassReferencesSubclass")
+public interface BigDecimalListExpression extends ToMiniString {
+    BigDecimal evaluateBd(List<BigDecimal> variables);
+
+    // Tests follow. You may temporarily remove everything til the end.
+
+    Add EXAMPLE = new Add(
+            new Subtract(new Variable(0), new Const(BigDecimal.ONE)),
+            new Multiply(new Variable(1), new Const(BigDecimal.TEN))
+    );
+
+    Type<BigDecimal> TYPE = new Type<>(
+            v -> new BigDecimal(v + ".000"),
+            random -> BigDecimal.valueOf(random.getRandom().nextGaussian()),
+            BigDecimal.class
+    );
+    ExpressionKind<BigDecimalListExpression, BigDecimal> KIND = new ExpressionKind<>(
+            TYPE,
+            BigDecimalListExpression.class,
+            (r, c) -> IntStream.range(0, c)
+                    .mapToObj(name -> Pair.<String, BigDecimalListExpression>of("$" + name, new Variable(name)))
+                    .toList(),
+            (expr, variables, values) -> expr.evaluateBd(values)
+    );
+
+    @SuppressWarnings("BigDecimalMethodWithoutRoundingCalled")
+    static ExpressionTester<?, ?> tester(final TestCounter counter) {
+        Asserts.assertEquals("Example toString()", "(($0 - 1) + ($1 * 10))", EXAMPLE.toString());
+        Asserts.assertEquals(
+                EXAMPLE + " at (2, 3)",
+                BigDecimal.valueOf(31),
+                EXAMPLE.evaluateBd(List.of(BigDecimal.valueOf(2), BigDecimal.valueOf(3)))
+        );
+
+        final Variable vx = new Variable(0);
+        final Variable vy = new Variable(1);
+
+        return new ExpressionTester<>(
+                counter, KIND, c -> v -> c,
+                (op, a, b) -> v -> op.apply(a.evaluateBd(v), b.evaluateBd(v)),
+                BigDecimal::add, BigDecimal::subtract, BigDecimal::multiply, BigDecimal::divide
+        )
+                .basic("10", "10", v -> v(10), c(10))
+                .basic("$x", "$x", BigDecimalListExpression::x, vx)
+                .basic("$y", "$y", BigDecimalListExpression::y, vy)
+                .basic("($x + $y)", "$x + $y", v -> x(v).add(y(v)), new Add(vx, vy))
+                .basic("($x + 2)", "$x + 2", v -> x(v).add(v(2)), new Add(vx, c(2)))
+                .basic("(2 - $x)", "2 - $x", v -> v(2).subtract(x(v)), new Subtract(c(2), vx))
+                .basic("(3 * $x)", "3 * $x", v -> v(3).multiply(x(v)), new Multiply(c(3), vx))
+                .basic("($x + $x)", "$x + $x", v -> x(v).add(x(v)), new Add(vx, vx))
+                .basic("($x / -2)", "$x / -2", v -> x(v).divide(v(-2)), new Divide(vx, c(-2)))
+                .basic("(2 + $x)", "2 + $x", v -> v(2).add(x(v)), new Add(c(2), vx))
+                .basic("((1 + 2) + 3)", "1 + 2 + 3", v -> v(6), new Add(new Add(c(1), c(2)), c(3)))
+                .basic("(1 + (2 * 3))", "1 + 2 * 3", v -> v(7), new Add(c(1), new Multiply(c(2), c(3))))
+                .basic("(1 - (2 * 3))", "1 - 2 * 3", v -> v(-5), new Subtract(c(1), new Multiply(c(2), c(3))))
+                .basic("(1 + (2 + 3))", "1 + 2 + 3", v -> v(6), new Add(c(1), new Add(c(2), c(3))))
+                .basic("((1 - 2) - 3)", "1 - 2 - 3", v -> v(-4), new Subtract(new Subtract(c(1), c(2)), c(3)))
+                .basic("(1 - (2 - 3))", "1 - (2 - 3)", v -> v(2), new Subtract(c(1), new Subtract(c(2), c(3))))
+                .basic("((1 * 2) * 3)", "1 * 2 * 3", v -> v(6), new Multiply(new Multiply(c(1), c(2)), c(3)))
+                .basic("(1 * (2 * 3))", "1 * 2 * 3", v -> v(6), new Multiply(c(1), new Multiply(c(2), c(3))))
+                .basic("((10 / 2) / 3)", "10 / 2 / 3", v -> v(10).divide(v(2)).divide(v(3)), new Divide(new Divide(c(10), c(2)), c(3)))
+                .basic("(10 / (3 / 2))", "10 / (3 / 2)", v -> v(10).divide(v(3).divide(v(2))), new Divide(c(10), new Divide(c(3), c(2))))
+                .basic("(($x * $x) + (($x - 1) / 10))",
+                        "$x * $x + ($x - 1) / 10",
+                        v -> x(v).multiply(x(v)).add(x(v).subtract(v(1)).divide(v(10))),
+                        new Add(new Multiply(vx, vx), new Divide(new Subtract(vx, c(1)), c(10)))
+                )
+                .basic("($x * -1000000000)", "$x * -1000000000", v -> x(v).multiply(v(-1_000_000_000)), new Multiply(vx, c(-1_000_000_000)))
+                .basic("($x * -1000000000000000)", "$x * -1000000000000000", v -> x(v).multiply(v(-1_000_000_000_000_000L)), new Multiply(vx, c(-1_000_000_000_000_000L)))
+                .basic("(10 / $x)", "10 / $x", v -> v(10).divide(x(v)), new Divide(c(10), vx))
+                .basic("($x / $x)", "$x / $x", v -> x(v).divide(x(v)), new Divide(vx, vx))
+
+                .advanced("(2 + 1)", "2 + 1", v -> v(2 + 1), new Add(c(2), c(1)))
+                .advanced("($x - 1)", "$x - 1", v -> x(v).subtract(v(1)), new Subtract(vx, c(1)))
+                .advanced("(1 * 2)", "1 * 2", v -> v(1 * 2), new Multiply(c(1), c(2)))
+                .advanced("($x / 1)", "$x / 1", v -> x(v).divide(v(1)), new Divide(vx, c(1)))
+                .advanced("(1 + (2 + 1))", "1 + 2 + 1", v -> v(1 + 2 + 1), new Add(c(1), new Add(c(2), c(1))))
+                .advanced("($x - ($x - 1))", "$x - ($x - 1)", v -> x(v).subtract(x(v).subtract(v(1))), new Subtract(vx, new Subtract(vx, c(1))))
+                .advanced("(2 * ($x / 1))", "2 * ($x / 1)", v -> v(2).multiply(x(v).divide(v(1))), new Multiply(c(2), new Divide(vx, c(1))))
+                .advanced("(2 / ($x - 1))", "2 / ($x - 1)", v -> v(2).divide(x(v).subtract(v(1))), new Divide(c(2), new Subtract(vx, c(1))))
+                .advanced("((1 * 2) + $x)", "1 * 2 + $x", v -> v(1 * 2).add(x(v)), new Add(new Multiply(c(1), c(2)), vx))
+                .advanced("(($x - 1) - 2)", "$x - 1 - 2", v -> x(v).subtract(v(3)), new Subtract(new Subtract(vx, c(1)), c(2)))
+                .advanced("(($x / 1) * 2)", "$x / 1 * 2", v -> x(v).multiply(v(2)), new Multiply(new Divide(vx, c(1)), c(2)))
+                .advanced("((2 + 1) / 1)", "(2 + 1) / 1", v -> v(3), new Divide(new Add(c(2), c(1)), c(1)))
+                .advanced(
+                        "(1 + (1 + (2 + 1)))",
+                        "1 + 1 + 2 + 1",
+                        v -> v(1 + 1 + 2 + 1),
+                        new Add(c(1), new Add(c(1), new Add(c(2), c(1))))
+                )
+                .advanced(
+                        "($x - ((1 * 2) + $x))",
+                        "$x - (1 * 2 + $x)",
+                        v -> x(v).subtract(v(1 * 2).add(x(v))),
+                        new Subtract(vx, new Add(new Multiply(c(1), c(2)), vx))
+                )
+                .advanced(
+                        "($x * (2 / ($x - 1)))",
+                        "$x * (2 / ($x - 1))",
+                        v -> x(v).multiply(v(2).divide(x(v).subtract(v(1)))),
+                        new Multiply(vx, new Divide(c(2), new Subtract(vx, c(1))))
+                )
+                .advanced(
+                        "($x / (1 + (2 + 1)))",
+                        "$x / (1 + 2 + 1)",
+                        v -> x(v).divide(v(1 + 2 + 1)),
+                        new Divide(vx, new Add(c(1), new Add(c(2), c(1))))
+                )
+                .advanced(
+                        "((1 * 2) + (2 + 1))",
+                        "1 * 2 + 2 + 1",
+                        v -> v(1 * 2 + 2 + 1),
+                        new Add(new Multiply(c(1), c(2)), new Add(c(2), c(1)))
+                )
+                .advanced(
+                        "((2 + 1) - (2 + 1))",
+                        "2 + 1 - (2 + 1)",
+                        v -> v(2 + 1 - (2 + 1)),
+                        new Subtract(new Add(c(2), c(1)), new Add(c(2), c(1)))
+                )
+                .advanced(
+                        "(($x - 1) * ($x / 1))",
+                        "($x - 1) * ($x / 1)",
+                        v -> x(v).subtract(v(1)).multiply(x(v).divide(v(1))),
+                        new Multiply(new Subtract(vx, c(1)), new Divide(vx, c(1)))
+                )
+                .advanced(
+                        "(($x - 1) / (1 * 2))",
+                        "($x - 1) / (1 * 2)",
+                        v -> x(v).subtract(v(1)).divide(v(2)),
+                        new Divide(new Subtract(vx, c(1)), new Multiply(c(1), c(2)))
+                )
+                .advanced(
+                        "((($x - 1) - 2) + $x)",
+                        "$x - 1 - 2 + $x",
+                        v -> x(v).subtract(v(3)).add(x(v)),
+                        new Add(new Subtract(new Subtract(vx, c(1)), c(2)), vx)
+                )
+                .advanced(
+                        "(((1 * 2) + $x) - 1)",
+                        "1 * 2 + $x - 1",
+                        v -> v(1).add(x(v)),
+                        new Subtract(new Add(new Multiply(c(1), c(2)), vx), c(1))
+                )
+                .advanced(
+                        "(((2 + 1) / 1) * $x)",
+                        "(2 + 1) / 1 * $x",
+                        v -> v(3).multiply(x(v)),
+                        new Multiply(new Divide(new Add(c(2), c(1)), c(1)), vx)
+                )
+                .advanced(
+                        "((2 / ($x - 1)) / 2)",
+                        "2 / ($x - 1) / 2",
+                        v -> v(2).divide(x(v).subtract(v(1))).divide(v(2)),
+                        new Divide(new Divide(c(2), new Subtract(vx, c(1))), c(2))
+                );
+    }
+
+    private static BigDecimal x(final List<BigDecimal> vars) {
+        return vars.get(0);
+    }
+
+    private static BigDecimal y(final List<BigDecimal> vars) {
+        return vars.get(1);
+    }
+
+    private static Const c(final BigDecimal v) {
+        return TYPE.constant(v);
+    }
+
+    private static Const c(final long v) {
+        return TYPE.constant(v(v));
+    }
+
+    private static BigDecimal v(final long v) {
+        return BigDecimal.valueOf(v);
+    }
+
+    static void main(final String... args) {
+        TripleExpression.SELECTOR
+                .variant("BigDecimalList", ExpressionTest.v(BigDecimalListExpression::tester))
+                .main(args);
+    }
+}

java/expression/BigIntegerListExpression.java

@@ -0,0 +1,192 @@
+package expression;
+
+import base.Asserts;
+import base.Pair;
+import base.TestCounter;
+import expression.common.ExpressionKind;
+import expression.common.Type;
+
+import java.math.BigInteger;
+import java.util.List;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@FunctionalInterface
+@SuppressWarnings("ClassReferencesSubclass")
+public interface BigIntegerListExpression extends ToMiniString {
+    BigInteger evaluateBi(List<BigInteger> variables);
+
+    // Tests follow. You may temporarily remove everything til the end.
+
+    Add EXAMPLE = new Add(
+            new Subtract(new Variable(0), new Const(BigInteger.ONE)),
+            new Multiply(new Variable(1), new Const(BigInteger.TEN))
+    );
+
+    Type<BigInteger> TYPE = new Type<>(BigInteger::valueOf, random -> v(random.getRandom().nextLong()), BigInteger.class);
+    ExpressionKind<BigIntegerListExpression, BigInteger> KIND = new ExpressionKind<>(
+            TYPE,
+            BigIntegerListExpression.class,
+            (r, c) -> IntStream.range(0, c)
+                    .mapToObj(name -> Pair.<String, BigIntegerListExpression>of("$" + name, new Variable(name)))
+                    .toList(),
+            (expr, variables, values) -> expr.evaluateBi(values)
+    );
+
+    @SuppressWarnings("PointlessArithmeticExpression")
+    static ExpressionTester<?, ?> tester(final TestCounter counter) {
+        Asserts.assertEquals("Example toString()", "(($0 - 1) + ($1 * 10))", EXAMPLE.toString());
+        Asserts.assertEquals(
+                EXAMPLE + " at (2, 3)",
+                BigInteger.valueOf(31),
+                EXAMPLE.evaluateBi(List.of(BigInteger.valueOf(2), BigInteger.valueOf(3)))
+        );
+
+        final Variable vx = new Variable(0);
+        final Variable vy = new Variable(1);
+
+        return new ExpressionTester<>(
+                counter, KIND, c -> v -> c,
+                (op, a, b) -> v -> op.apply(a.evaluateBi(v), b.evaluateBi(v)),
+                BigInteger::add, BigInteger::subtract, BigInteger::multiply, BigInteger::divide
+        )
+                .basic("10", "10", v -> v(10), c(10))
+                .basic("$x", "$x", BigIntegerListExpression::x, vx)
+                .basic("$y", "$y", BigIntegerListExpression::y, vy)
+                .basic("($x + $y)", "$x + $y", v -> x(v).add(y(v)), new Add(vx, vy))
+                .basic("($x + 2)", "$x + 2", v -> x(v).add(v(2)), new Add(vx, c(2)))
+                .basic("(2 - $x)", "2 - $x", v -> v(2).subtract(x(v)), new Subtract(c(2), vx))
+                .basic("(3 * $x)", "3 * $x", v -> v(3).multiply(x(v)), new Multiply(c(3), vx))
+                .basic("($x + $x)", "$x + $x", v -> x(v).add(x(v)), new Add(vx, vx))
+                .basic("($x / -2)", "$x / -2", v -> x(v).divide(v(-2)), new Divide(vx, c(-2)))
+                .basic("(2 + $x)", "2 + $x", v -> v(2).add(x(v)), new Add(c(2), vx))
+                .basic("((1 + 2) + 3)", "1 + 2 + 3", v -> v(6), new Add(new Add(c(1), c(2)), c(3)))
+                .basic("(1 + (2 * 3))", "1 + 2 * 3", v -> v(7), new Add(c(1), new Multiply(c(2), c(3))))
+                .basic("(1 - (2 * 3))", "1 - 2 * 3", v -> v(-5), new Subtract(c(1), new Multiply(c(2), c(3))))
+                .basic("(1 + (2 + 3))", "1 + 2 + 3", v -> v(6), new Add(c(1), new Add(c(2), c(3))))
+                .basic("((1 - 2) - 3)", "1 - 2 - 3", v -> v(-4), new Subtract(new Subtract(c(1), c(2)), c(3)))
+                .basic("(1 - (2 - 3))", "1 - (2 - 3)", v -> v(2), new Subtract(c(1), new Subtract(c(2), c(3))))
+                .basic("((1 * 2) * 3)", "1 * 2 * 3", v -> v(6), new Multiply(new Multiply(c(1), c(2)), c(3)))
+                .basic("(1 * (2 * 3))", "1 * 2 * 3", v -> v(6), new Multiply(c(1), new Multiply(c(2), c(3))))
+                .basic("((10 / 2) / 3)", "10 / 2 / 3", v -> v(10 / 2 / 3), new Divide(new Divide(c(10), c(2)), c(3)))
+                .basic("(10 / (3 / 2))", "10 / (3 / 2)", v -> v(10 / (3 / 2)), new Divide(c(10), new Divide(c(3), c(2))))
+                .basic("(($x * $x) + (($x - 1) / 10))",
+                        "$x * $x + ($x - 1) / 10",
+                        v -> x(v).multiply(x(v)).add(x(v).subtract(v(1)).divide(v(10))),
+                        new Add(new Multiply(vx, vx), new Divide(new Subtract(vx, c(1)), c(10)))
+                )
+                .basic("($x * -1000000000)", "$x * -1000000000", v -> x(v).multiply(v(-1_000_000_000)), new Multiply(vx, c(-1_000_000_000)))
+                .basic("($x * -1000000000000000)", "$x * -1000000000000000", v -> x(v).multiply(v(-1_000_000_000_000_000L)), new Multiply(vx, c(-1_000_000_000_000_000L)))
+                .basic("(10 / $x)", "10 / $x", v -> v(10).divide(x(v)), new Divide(c(10), vx))
+                .basic("($x / $x)", "$x / $x", v -> x(v).divide(x(v)), new Divide(vx, vx))
+
+                .advanced("(2 + 1)", "2 + 1", v -> v(2 + 1), new Add(c(2), c(1)))
+                .advanced("($x - 1)", "$x - 1", v -> x(v).subtract(v(1)), new Subtract(vx, c(1)))
+                .advanced("(1 * 2)", "1 * 2", v -> v(1 * 2), new Multiply(c(1), c(2)))
+                .advanced("($x / 1)", "$x / 1", v -> x(v).divide(v(1)), new Divide(vx, c(1)))
+                .advanced("(1 + (2 + 1))", "1 + 2 + 1", v -> v(1 + 2 + 1), new Add(c(1), new Add(c(2), c(1))))
+                .advanced("($x - ($x - 1))", "$x - ($x - 1)", v -> x(v).subtract(x(v).subtract(v(1))), new Subtract(vx, new Subtract(vx, c(1))))
+                .advanced("(2 * ($x / 1))", "2 * ($x / 1)", v -> v(2).multiply(x(v).divide(v(1))), new Multiply(c(2), new Divide(vx, c(1))))
+                .advanced("(2 / ($x - 1))", "2 / ($x - 1)", v -> v(2).divide(x(v).subtract(v(1))), new Divide(c(2), new Subtract(vx, c(1))))
+                .advanced("((1 * 2) + $x)", "1 * 2 + $x", v -> v(1 * 2).add(x(v)), new Add(new Multiply(c(1), c(2)), vx))
+                .advanced("(($x - 1) - 2)", "$x - 1 - 2", v -> x(v).subtract(v(3)), new Subtract(new Subtract(vx, c(1)), c(2)))
+                .advanced("(($x / 1) * 2)", "$x / 1 * 2", v -> x(v).multiply(v(2)), new Multiply(new Divide(vx, c(1)), c(2)))
+                .advanced("((2 + 1) / 1)", "(2 + 1) / 1", v -> v(3), new Divide(new Add(c(2), c(1)), c(1)))
+                .advanced(
+                        "(1 + (1 + (2 + 1)))",
+                        "1 + 1 + 2 + 1",
+                        v -> v(1 + 1 + 2 + 1),
+                        new Add(c(1), new Add(c(1), new Add(c(2), c(1))))
+                )
+                .advanced(
+                        "($x - ((1 * 2) + $x))",
+                        "$x - (1 * 2 + $x)",
+                        v -> x(v).subtract(v(1 * 2).add(x(v))),
+                        new Subtract(vx, new Add(new Multiply(c(1), c(2)), vx))
+                )
+                .advanced(
+                        "($x * (2 / ($x - 1)))",
+                        "$x * (2 / ($x - 1))",
+                        v -> x(v).multiply(v(2).divide(x(v).subtract(v(1)))),
+                        new Multiply(vx, new Divide(c(2), new Subtract(vx, c(1))))
+                )
+                .advanced(
+                        "($x / (1 + (2 + 1)))",
+                        "$x / (1 + 2 + 1)",
+                        v -> x(v).divide(v(1 + 2 + 1)),
+                        new Divide(vx, new Add(c(1), new Add(c(2), c(1))))
+                )
+                .advanced(
+                        "((1 * 2) + (2 + 1))",
+                        "1 * 2 + 2 + 1",
+                        v -> v(1 * 2 + 2 + 1),
+                        new Add(new Multiply(c(1), c(2)), new Add(c(2), c(1)))
+                )
+                .advanced(
+                        "((2 + 1) - (2 + 1))",
+                        "2 + 1 - (2 + 1)",
+                        v -> v(2 + 1 - (2 + 1)),
+                        new Subtract(new Add(c(2), c(1)), new Add(c(2), c(1)))
+                )
+                .advanced(
+                        "(($x - 1) * ($x / 1))",
+                        "($x - 1) * ($x / 1)",
+                        v -> x(v).subtract(v(1)).multiply(x(v).divide(v(1))),
+                        new Multiply(new Subtract(vx, c(1)), new Divide(vx, c(1)))
+                )
+                .advanced(
+                        "(($x - 1) / (1 * 2))",
+                        "($x - 1) / (1 * 2)",
+                        v -> x(v).subtract(v(1)).divide(v(2)),
+                        new Divide(new Subtract(vx, c(1)), new Multiply(c(1), c(2)))
+                )
+                .advanced(
+                        "((($x - 1) - 2) + $x)",
+                        "$x - 1 - 2 + $x",
+                        v -> x(v).subtract(v(3)).add(x(v)),
+                        new Add(new Subtract(new Subtract(vx, c(1)), c(2)), vx)
+                )
+                .advanced(
+                        "(((1 * 2) + $x) - 1)",
+                        "1 * 2 + $x - 1",
+                        v -> v(1).add(x(v)),
+                        new Subtract(new Add(new Multiply(c(1), c(2)), vx), c(1))
+                )
+                .advanced(
+                        "(((2 + 1) / 1) * $x)",
+                        "(2 + 1) / 1 * $x",
+                        v -> v(3).multiply(x(v)),
+                        new Multiply(new Divide(new Add(c(2), c(1)), c(1)), vx)
+                )
+                .advanced(
+                        "((2 / ($x - 1)) / 2)",
+                        "2 / ($x - 1) / 2",
+                        v -> v(2).divide(x(v).subtract(v(1))).divide(v(2)),
+                        new Divide(new Divide(c(2), new Subtract(vx, c(1))), c(2))
+                );
+    }
+
+    private static BigInteger x(final List<BigInteger> vars) {
+        return vars.get(0);
+    }
+
+    private static BigInteger y(final List<BigInteger> vars) {
+        return vars.get(1);
+    }
+
+    private static Const c(final long v) {
+        return TYPE.constant(v(v));
+    }
+
+    private static BigInteger v(final long v) {
+        return BigInteger.valueOf(v);
+    }
+
+    static void main(final String... args) {
+        TripleExpression.SELECTOR
+                .variant("BigIntegerList", ExpressionTest.v(BigIntegerListExpression::tester))
+                .main(args);
+    }
+}

java/expression/Expression.java

@@ -0,0 +1,112 @@
+package expression;
+
+import base.Asserts;
+import base.ExtendedRandom;
+import base.Pair;
+import base.TestCounter;
+import expression.common.ExpressionKind;
+import expression.common.Type;
+
+import java.util.List;
+
+/**
+ * One-argument arithmetic expression over integers.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@FunctionalInterface
+@SuppressWarnings("ClassReferencesSubclass")
+public interface Expression extends ToMiniString {
+    int evaluate(int x);
+
+    // Tests follow. You may temporarily remove everything til the end.
+
+    Subtract EXAMPLE = new Subtract(
+            new Multiply(new Const(2), new Variable("x")),
+            new Const(3)
+    );
+
+    Type<Integer> TYPE = new Type<>(a -> a, ExtendedRandom::nextInt, int.class);
+    ExpressionKind<Expression, Integer> KIND = new ExpressionKind<>(
+            TYPE,
+            Expression.class,
+            List.of(Pair.of("x", new Variable("x"))),
+            (expr, variables, values) -> expr.evaluate(values.get(0))
+    );
+
+    private static Const c(final int c) {
+        return new Const(c);
+    }
+
+    @SuppressWarnings({"PointlessArithmeticExpression", "Convert2MethodRef"})
+    static ExpressionTester<?, ?> tester(final TestCounter counter) {
+        Asserts.assertEquals("Example toString()", "((2 * x) - 3)", EXAMPLE.toString());
+        Asserts.assertEquals("Example at 5", 7, EXAMPLE.evaluate(5));
+        Asserts.assertTrue("Example equals 1",
+                new Multiply(new Const(2), new Variable("x"))
+                        .equals(new Multiply(new Const(2), new Variable("x"))));
+        Asserts.assertTrue("Example equals 2",
+                !new Multiply(new Const(2), new Variable("x"))
+                        .equals(new Multiply(new Variable("x"), new Const(2))));
+
+        final Variable vx = new Variable("x");
+        final Const c1 = c(1);
+        final Const c2 = c(2);
+
+        return new ExpressionTester<>(
+                counter, KIND, c -> x -> c,
+                (op, a, b) -> x -> op.apply(a.evaluate(x), b.evaluate(x)),
+                (a, b) -> a + b, (a, b) -> a - b, (a, b) -> a * b, (a, b) -> a / b
+        )
+                .basic("10", "10", x -> 10, c(10))
+                .basic("x", "x", x -> x, vx)
+                .basic("(x + 2)", "x + 2", x -> x + 2, new Add(vx, c(2)))
+                .basic("(2 - x)", "2 - x", x -> 2 - x, new Subtract(c(2), vx))
+                .basic("(3 * x)", "3 * x", x -> 3*x, new Multiply(c(3), vx))
+                .basic("(x + x)", "x + x", x -> x + x, new Add(vx, vx))
+                .basic("(x / -2)", "x / -2", x -> -x / 2, new Divide(vx, c(-2)))
+                .basic("(2 + x)", "2 + x", x -> 2 + x, new Add(c(2), vx))
+                .basic("((1 + 2) + 3)", "1 + 2 + 3", x -> 6, new Add(new Add(c(1), c(2)), c(3)))
+                .basic("(1 + (2 + 3))", "1 + 2 + 3", x -> 6, new Add(c(1), new Add(c(2), c(3))))
+                .basic("((1 - 2) - 3)", "1 - 2 - 3", x -> -4, new Subtract(new Subtract(c(1), c(2)), c(3)))
+                .basic("(1 - (2 - 3))", "1 - (2 - 3)", x -> 2, new Subtract(c(1), new Subtract(c(2), c(3))))
+                .basic("((1 * 2) * 3)", "1 * 2 * 3", x -> 6, new Multiply(new Multiply(c(1), c(2)), c(3)))
+                .basic("(1 * (2 * 3))", "1 * 2 * 3", x -> 6, new Multiply(c(1), new Multiply(c(2), c(3))))
+                .basic("((10 / 2) / 3)", "10 / 2 / 3", x -> 10 / 2 / 3, new Divide(new Divide(c(10), c(2)), c(3)))
+                .basic("(10 / (3 / 2))", "10 / (3 / 2)", x -> 10 / (3 / 2), new Divide(c(10), new Divide(c(3), c(2))))
+                .basic("(10 * (3 / 2))", "10 * (3 / 2)", x -> 10 * (3 / 2), new Multiply(c(10), new Divide(c(3), c(2))))
+                .basic("(10 + (3 - 2))", "10 + 3 - 2", x -> 10 + (3 - 2), new Add(c(10), new Subtract(c(3), c(2))))
+                .basic("((x * x) + ((x - 1) / 10))", "x * x + (x - 1) / 10", x -> x * x + (x - 1) / 10, new Add(
+                        new Multiply(vx, vx),
+                        new Divide(new Subtract(vx, c(1)), c(10))
+                ))
+                .basic("(x * -1000000000)", "x * -1000000000", x -> x * -1_000_000_000, new Multiply(vx, c(-1_000_000_000)))
+                .basic("(10 / x)", "10 / x", x -> 10 / x, new Divide(c(10), vx))
+                .basic("(x / x)", "x / x", x -> x / x, new Divide(vx, vx))
+
+                .advanced("(2 + 1)", "2 + 1", x -> 2 + 1, new Add(c2, c1))
+                .advanced("(x - 1)", "x - 1", x -> x - 1, new Subtract(vx, c1))
+                .advanced("(1 * 2)", "1 * 2", x -> 1 * 2, new Multiply(c1, c2))
+                .advanced("(x / 1)", "x / 1", x -> x / 1, new Divide(vx, c1))
+                .advanced("(1 + (2 + 1))", "1 + 2 + 1", x -> 1 + 2 + 1, new Add(c1, new Add(c2, c1)))
+                .advanced("(x - (x - 1))", "x - (x - 1)", x -> x - (x - 1), new Subtract(vx, new Subtract(vx, c1)))
+                .advanced("(2 * (x / 1))", "2 * (x / 1)", x -> 2 * (x / 1), new Multiply(c2, new Divide(vx, c1)))
+                .advanced("(2 / (x - 1))", "2 / (x - 1)", x -> 2 / (x - 1), new Divide(c2, new Subtract(vx, c1)))
+                .advanced("((1 * 2) + x)", "1 * 2 + x", x -> 1 * 2 + x, new Add(new Multiply(c1, c2), vx))
+                .advanced("((x - 1) - 2)", "x - 1 - 2", x -> x - 1 - 2, new Subtract(new Subtract(vx, c1), c2))
+                .advanced("((x / 1) * 2)", "x / 1 * 2", x -> x / 1 * 2, new Multiply(new Divide(vx, c1), c2))
+                .advanced("((2 + 1) / 1)", "(2 + 1) / 1", x -> (2 + 1) / 1, new Divide(new Add(c2, c1), c1))
+                .advanced("(1 + (1 + (2 + 1)))", "1 + 1 + 2 + 1", x -> 1 + 1 + 2 + 1, new Add(c1, new Add(c1, new Add(c2, c1))))
+                .advanced("(x - ((1 * 2) + x))", "x - (1 * 2 + x)", x -> x - (1 * 2 + x), new Subtract(vx, new Add(new Multiply(c1, c2), vx)))
+                .advanced("(x * (2 / (x - 1)))", "x * (2 / (x - 1))", x -> x * (2 / (x - 1)), new Multiply(vx, new Divide(c2, new Subtract(vx, c1))))
+                .advanced("(x / (1 + (2 + 1)))", "x / (1 + 2 + 1)", x -> x / (1 + 2 + 1), new Divide(vx, new Add(c1, new Add(c2, c1))))
+                .advanced("((1 * 2) + (2 + 1))", "1 * 2 + 2 + 1", x -> 1 * 2 + 2 + 1, new Add(new Multiply(c1, c2), new Add(c2, c1)))
+                .advanced("((2 + 1) - (2 + 1))", "2 + 1 - (2 + 1)", x -> 2 + 1 - (2 + 1), new Subtract(new Add(c2, c1), new Add(c2, c1)))
+                .advanced("((x - 1) * (x / 1))", "(x - 1) * (x / 1)", x -> (x - 1) * (x / 1), new Multiply(new Subtract(vx, c1), new Divide(vx, c1)))
+                .advanced("((x - 1) / (1 * 2))", "(x - 1) / (1 * 2)", x -> (x - 1) / (1 * 2), new Divide(new Subtract(vx, c1), new Multiply(c1, c2)))
+                .advanced("(((x - 1) - 2) + x)", "x - 1 - 2 + x", x -> x - 1 - 2 + x, new Add(new Subtract(new Subtract(vx, c1), c2), vx))
+                .advanced("(((1 * 2) + x) - 1)", "1 * 2 + x - 1", x -> 1 * 2 + x - 1, new Subtract(new Add(new Multiply(c1, c2), vx), c1))
+                .advanced("(((2 + 1) / 1) * x)", "(2 + 1) / 1 * x", x -> (2 + 1) / 1 * x, new Multiply(new Divide(new Add(c2, c1), c1), vx))
+                .advanced("((2 / (x - 1)) / 2)", "2 / (x - 1) / 2", x -> 2 / (x - 1) / 2, new Divide(new Divide(c2, new Subtract(vx, c1)), c2));
+    }
+}

java/expression/ExpressionTest.java

@@ -0,0 +1,26 @@
+package expression;
+
+import base.Selector;
+import base.TestCounter;
+
+import java.util.function.Consumer;
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class ExpressionTest {
+    public static final Selector SELECTOR = new Selector(ExpressionTest.class, "easy", "hard")
+            .variant("Base", v(Expression::tester));
+
+    private ExpressionTest() {
+    }
+
+    public static Consumer<TestCounter> v(final Function<TestCounter, ? extends ExpressionTester<?, ?>> tester) {
+        return t -> tester.apply(t).test();
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/expression/ExpressionTester.java

@@ -0,0 +1,288 @@
+package expression;
+
+import base.*;
+import expression.common.*;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Modifier;
+import java.util.*;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+import static base.Asserts.assertTrue;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ExpressionTester<E extends ToMiniString, C> extends Tester {
+    private final List<Integer> VALUES = IntStream.rangeClosed(-10, 10).boxed().toList();
+    private final ExpressionKind<E, C> kind;
+
+    private final List<Test> basic = new ArrayList<>();
+    private final List<Test> advanced = new ArrayList<>();
+    private final Set<String> used = new HashSet<>();
+    private final GeneratorBuilder generator;
+
+    private final List<Pair<ToMiniString, String>> prev = new ArrayList<>();
+    private final Map<String, C> mappings;
+
+    protected ExpressionTester(
+            final TestCounter counter,
+            final ExpressionKind<E, C> kind,
+            final Function<C, E> expectedConstant,
+            final Binary<C, E> binary,
+            final BinaryOperator<C> add,
+            final BinaryOperator<C> sub,
+            final BinaryOperator<C> mul,
+            final BinaryOperator<C> div,
+            final Map<String, C> mappings
+    ) {
+        super(counter);
+        this.kind = kind;
+        this.mappings = mappings;
+
+        generator = new GeneratorBuilder(expectedConstant, kind::constant, binary, kind::randomValue);
+        generator.binary("+",  1600, add, Add.class);
+        generator.binary("-", 1602, sub, Subtract.class);
+        generator.binary("*",  2001, mul, Multiply.class);
+        generator.binary("/", 2002, div, Divide.class);
+    }
+
+    protected ExpressionTester(
+            final TestCounter counter,
+            final ExpressionKind<E, C> kind,
+            final Function<C, E> expectedConstant,
+            final Binary<C, E> binary,
+            final BinaryOperator<C> add,
+            final BinaryOperator<C> sub,
+            final BinaryOperator<C> mul,
+            final BinaryOperator<C> div
+    ) {
+        this(counter, kind, expectedConstant, binary, add, sub, mul, div, Map.of());
+    }
+
+    @Override
+    public String toString() {
+        return kind.getName();
+    }
+
+    @Override
+    public void test() {
+        counter.scope("Basic tests", () -> basic.forEach(Test::test));
+        counter.scope("Advanced tests", () -> advanced.forEach(Test::test));
+        counter.scope("Random tests", generator::testRandom);
+    }
+
+    @SuppressWarnings({"ConstantValue", "EqualsWithItself"})
+    private void checkEqualsAndToString(final String full, final String mini, final ToMiniString expression, final ToMiniString copy) {
+        checkToString("toString", full, expression.toString());
+        if (mode() > 0) {
+            checkToString("toMiniString", mini, expression.toMiniString());
+        }
+
+        counter.test(() -> {
+            assertTrue("Equals to this", expression.equals(expression));
+            assertTrue("Equals to copy", expression.equals(copy));
+            assertTrue("Equals to null", !expression.equals(null));
+            assertTrue("Copy equals to null", !copy.equals(null));
+        });
+
+        final String expressionToString = Objects.requireNonNull(expression.toString());
+        for (final Pair<ToMiniString, String> pair : prev) {
+            counter.test(() -> {
+                final ToMiniString prev = pair.first();
+                final String prevToString = pair.second();
+                final boolean equals = prevToString.equals(expressionToString);
+                assertTrue("Equals to " + prevToString, prev.equals(expression) == equals);
+                assertTrue("Equals to " + prevToString, expression.equals(prev) == equals);
+                assertTrue("Inconsistent hashCode for " + prev + " and " + expression, (prev.hashCode() == expression.hashCode()) == equals);
+            });
+        }
+    }
+
+    private void checkToString(final String method, final String expected, final String actual) {
+        counter.test(() -> assertTrue(String.format("Invalid %s\n     expected: %s\n       actual: %s", method, expected, actual), expected.equals(actual)));
+    }
+
+    private void check(
+            final String full,
+            final E expected,
+            final E actual,
+            final List<String> variables,
+            final List<C> values
+    ) {
+        final String vars = IntStream.range(0, variables.size())
+                .mapToObj(i -> variables.get(i) + "=" + values.get(i))
+                .collect(Collectors.joining(","));
+        counter.test(() -> {
+            final Object expectedResult = evaluate(expected, variables, values);
+            final Object actualResult = evaluate(actual, variables, values);
+            final String reason = String.format(
+                    "%s:%n     expected `%s`,%n       actual `%s`",
+                    String.format("f(%s)\nwhere f is %s", vars, full),
+                    Asserts.toString(expectedResult),
+                    Asserts.toString(actualResult)
+            );
+            if (
+                    expectedResult != null && actualResult != null &&
+                    expectedResult.getClass() == actualResult.getClass()
+                            && (expectedResult.getClass() == Double.class || expectedResult.getClass() == Float.class)
+            ) {
+                final double expectedValue = ((Number) expectedResult).doubleValue();
+                final double actualValue = ((Number) actualResult).doubleValue();
+                Asserts.assertEquals(reason, expectedValue, actualValue, 1e-6);
+            } else {
+                assertTrue(reason, Objects.deepEquals(expectedResult, actualResult));
+            }
+        });
+    }
+
+    private Object evaluate(final E expression, final List<String> variables, final List<C> values) {
+        try {
+            return kind.evaluate(expression, variables, values);
+        } catch (final Exception e) {
+            return e.getClass().getName();
+        }
+    }
+
+    protected ExpressionTester<E, C> basic(final String full, final String mini, final E expected, final E actual) {
+        return basicF(full, mini, expected, vars -> actual);
+    }
+
+    protected ExpressionTester<E, C> basicF(final String full, final String mini, final E expected, final Function<List<String>, E> actual) {
+        return basic(new Test(full, mini, expected, actual));
+    }
+
+    private ExpressionTester<E, C> basic(final Test test) {
+        Asserts.assertTrue(test.full, used.add(test.full));
+        basic.add(test);
+        return this;
+    }
+
+    protected ExpressionTester<E, C> advanced(final String full, final String mini, final E expected, final E actual) {
+        return advancedF(full, mini, expected, vars -> actual);
+    }
+
+    protected ExpressionTester<E, C> advancedF(final String full, final String mini, final E expected, final Function<List<String>, E> actual) {
+        Asserts.assertTrue(full, used.add(full));
+        advanced.add(new Test(full, mini, expected, actual));
+        return this;
+    }
+
+    protected static <E> Named<E> variable(final String name, final E expected) {
+        return Named.of(name, expected);
+    }
+
+    @FunctionalInterface
+    public interface Binary<C, E> {
+        E apply(BinaryOperator<C> op, E a, E b);
+    }
+
+    private final class Test {
+        private final String full;
+        private final String mini;
+        private final E expected;
+        private final Function<List<String>, E> actual;
+
+        private Test(final String full, final String mini, final E expected, final Function<List<String>, E> actual) {
+            this.full = full;
+            this.mini = mini;
+            this.expected = expected;
+            this.actual = actual;
+        }
+
+        private void test() {
+            final List<Pair<String, E>> variables = kind.variables().generate(random(), 3);
+            final List<String> names = Functional.map(variables, Pair::first);
+            final E actual = kind.cast(this.actual.apply(names));
+            final String full = mangle(this.full, names);
+            final String mini = mangle(this.mini, names);
+
+            counter.test(() -> {
+                kind.allValues(variables.size(), VALUES).forEach(values -> check(mini, expected, actual, names, values));
+                checkEqualsAndToString(full, mini, actual, actual);
+                prev.add(Pair.of(actual, full));
+            });
+        }
+
+        private String mangle(String string, final List<String> names) {
+            for (int i = 0; i < names.size(); i++) {
+                string = string.replace("$" + (char) ('x' + i), names.get(i));
+            }
+            for (final Map.Entry<String, C> mapping : mappings.entrySet()) {
+                string = string.replace(mapping.getKey(), mapping.getValue().toString());
+            }
+            return string;
+        }
+    }
+
+    private final class GeneratorBuilder {
+        private final Generator.Builder<C> generator;
+        private final NodeRendererBuilder<C> renderer = new NodeRendererBuilder<>(random());
+        private final Renderer.Builder<C, Unit, E> expected;
+        private final Renderer.Builder<C, Unit, E> actual;
+        private final Renderer.Builder<C, Unit, E> copy;
+        private final Binary<C, E> binary;
+
+        private GeneratorBuilder(
+                final Function<C, E> expectedConstant,
+                final Function<? super C, E> actualConstant,
+                final Binary<C, E> binary,
+                final Function<ExtendedRandom, C> randomValue
+        ) {
+            generator = Generator.builder(() -> randomValue.apply(random()), random());
+            expected = Renderer.builder(expectedConstant::apply);
+            actual = Renderer.builder(actualConstant::apply);
+            copy = Renderer.builder(actualConstant::apply);
+
+            this.binary = binary;
+        }
+
+        private void binary(final String name, final int priority, final BinaryOperator<C> op, final Class<?> type) {
+            generator.add(name, 2);
+            renderer.binary(name, priority);
+
+            expected.binary(name, (unit, a, b) -> binary.apply(op, a, b));
+
+            @SuppressWarnings("unchecked") final Constructor<? extends E> constructor = (Constructor<? extends E>) Arrays.stream(type.getConstructors())
+                    .filter(cons -> Modifier.isPublic(cons.getModifiers()))
+                    .filter(cons -> cons.getParameterCount() == 2)
+                    .findFirst()
+                    .orElseGet(() -> counter.fail("%s(..., ...) constructor not found", type.getSimpleName()));
+            final Renderer.BinaryOperator<Unit, E> actual = (unit, a, b) -> {
+                try {
+                    return constructor.newInstance(a, b);
+                } catch (final Exception e) {
+                    return counter.fail(e);
+                }
+            };
+            this.actual.binary(name, actual);
+            copy.binary(name, actual);
+        }
+
+        private void testRandom() {
+            final NodeRenderer<C> renderer = this.renderer.build();
+            final Renderer<C, Unit, E> expectedRenderer = this.expected.build();
+            final Renderer<C, Unit, E> actualRenderer = this.actual.build();
+            final expression.common.Generator<C, E> generator = this.generator.build(kind.variables(), List.of());
+            generator.testRandom(counter, 1, expr -> {
+                final String full = renderer.render(expr, NodeRenderer.FULL);
+                final String mini = renderer.render(expr, NodeRenderer.MINI);
+                final E expected = expectedRenderer.render(expr, Unit.INSTANCE);
+                final E actual = actualRenderer.render(expr, Unit.INSTANCE);
+
+                final List<Pair<String, E>> variables = expr.variables();
+                final List<String> names = Functional.map(variables, Pair::first);
+                final List<C> values = Stream.generate(() -> kind.randomValue(random()))
+                        .limit(variables.size())
+                        .toList();
+
+                checkEqualsAndToString(full, mini, actual, copy.build().render(expr, Unit.INSTANCE));
+                check(full, expected, actual, names, values);
+            });
+        }
+    }
+}

java/expression/ListExpression.java

@@ -0,0 +1,211 @@
+package expression;
+
+import base.Asserts;
+import base.ExtendedRandom;
+import base.Pair;
+import base.TestCounter;
+import expression.common.ExpressionKind;
+import expression.common.Type;
+
+import java.util.List;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings("ClassReferencesSubclass")
+@FunctionalInterface
+public interface ListExpression extends ToMiniString {
+    int evaluate(List<Integer> variables);
+
+    // Tests follow. You may temporarily remove everything til the end.
+
+    Add EXAMPLE = new Add(
+            new Subtract(new Variable(0), new Const(1)),
+            new Multiply(new Variable(1), new Const(10))
+    );
+
+    Type<Integer> TYPE = new Type<>(a -> a, ExtendedRandom::nextInt, int.class);
+    ExpressionKind<ListExpression, Integer> KIND = new ExpressionKind<>(
+            TYPE,
+            ListExpression.class,
+            (r, c) -> IntStream.range(0, c)
+                    .mapToObj(name -> Pair.<String, ListExpression>of("$" + name, new Variable(name)))
+                    .toList(),
+            (expr, variables, values) -> expr.evaluate(values)
+    );
+
+    private static Const c(final Integer c) {
+        return TYPE.constant(c);
+    }
+
+    @SuppressWarnings({"PointlessArithmeticExpression", "Convert2MethodRef"})
+    static ExpressionTester<?, ?> tester(final TestCounter counter) {
+        Asserts.assertEquals("Example toString()", "(($0 - 1) + ($1 * 10))", EXAMPLE.toString());
+        Asserts.assertEquals(EXAMPLE + " at (2, 3)", 31, EXAMPLE.evaluate(List.of(2, 3)));
+
+        final Variable vx = new Variable(0);
+        return new ExpressionTester<>(
+                counter, KIND, c -> vars -> c,
+                (op, a, b) -> vars -> op.apply(a.evaluate(vars), b.evaluate(vars)),
+                (a, b) -> a + b, (a, b) -> a - b, (a, b) -> a * b, (a, b) -> a / b
+        )
+                .basic("10", "10", vars -> 10, c(10))
+                .basic("$x", "$x", ListExpression::x, vx)
+                .basic("($x + 2)", "$x + 2", vars -> x(vars) + 2, new Add(vx, c(2)))
+                .basic("(2 - $x)", "2 - $x", vars -> 2 - x(vars), new Subtract(c(2), vx))
+                .basic("(3 * $x)", "3 * $x", vars -> 3 * x(vars), new Multiply(c(3), vx))
+                .basic("($x + $x)", "$x + $x", vars -> x(vars) + x(vars), new Add(vx, vx))
+                .basic("($x / -2)", "$x / -2", vars -> -x(vars) / 2, new Divide(vx, c(-2)))
+                .basic("(2 + $x)", "2 + $x", vars -> 2 + x(vars), new Add(c(2), vx))
+                .basic("((1 + 2) + 3)", "1 + 2 + 3", vars -> 6, new Add(new Add(c(1), c(2)), c(3)))
+                .basic("(1 + (2 + 3))", "1 + 2 + 3", vars -> 6, new Add(c(1), new Add(c(2), c(3))))
+                .basic("((1 - 2) - 3)", "1 - 2 - 3", vars -> -4, new Subtract(new Subtract(c(1), c(2)), c(3)))
+                .basic("(1 - (2 - 3))", "1 - (2 - 3)", vars -> 2, new Subtract(c(1), new Subtract(c(2), c(3))))
+                .basic("((1 * 2) * 3)", "1 * 2 * 3", vars -> 6, new Multiply(new Multiply(c(1), c(2)), c(3)))
+                .basic("(1 * (2 * 3))", "1 * 2 * 3", vars -> 6, new Multiply(c(1), new Multiply(c(2), c(3))))
+                .basic("((10 / 2) / 3)", "10 / 2 / 3", vars -> 10 / 2 / 3, new Divide(new Divide(c(10), c(2)), c(3)))
+                .basic("(10 / (3 / 2))", "10 / (3 / 2)", vars -> 10 / (3 / 2), new Divide(c(10), new Divide(c(3), c(2))))
+                .basic("(10 * (3 / 2))", "10 * (3 / 2)", vars -> 10 * (3 / 2), new Multiply(c(10), new Divide(c(3), c(2))))
+                .basic("(10 + (3 - 2))", "10 + 3 - 2", vars -> 10 + (3 - 2), new Add(c(10), new Subtract(c(3), c(2))))
+                .basic(
+                        "(($x * $x) + (($x - 1) / 10))",
+                        "$x * $x + ($x - 1) / 10",
+                        vars -> x(vars) * x(vars) + (x(vars) - 1) / 10,
+                        new Add(new Multiply(vx, vx), new Divide(new Subtract(vx, c(1)), c(10)))
+                )
+                .basic(
+                        "($x * -1000000000)",
+                        "$x * -1000000000",
+                        vars -> x(vars) * -1_000_000_000,
+                        new Multiply(vx, c(-1_000_000_000))
+                )
+                .basic("(10 / $x)", "10 / $x", vars -> 10 / x(vars), new Divide(c(10), vx))
+                .basic("($x / $x)", "$x / $x", vars -> x(vars) / x(vars), new Divide(vx, vx))
+
+                .advanced("(2 + 1)", "2 + 1", vars -> 2 + 1, new Add(c(2), c(1)))
+                .advanced("($x - 1)", "$x - 1", vars -> x(vars) - 1, new Subtract(vx, c(1)))
+                .advanced("(1 * 2)", "1 * 2", vars -> 1 * 2, new Multiply(c(1), c(2)))
+                .advanced("($x / 1)", "$x / 1", vars -> x(vars) / 1, new Divide(vx, c(1)))
+                .advanced("(1 + (2 + 1))", "1 + 2 + 1", vars -> 1 + 2 + 1, new Add(c(1), new Add(c(2), c(1))))
+                .advanced(
+                        "($x - ($x - 1))",
+                        "$x - ($x - 1)",
+                        vars -> x(vars) - (x(vars) - 1),
+                        new Subtract(vx, new Subtract(vx, c(1)))
+                )
+                .advanced(
+                        "(2 * ($x / 1))",
+                        "2 * ($x / 1)",
+                        vars -> 2 * (x(vars) / 1),
+                        new Multiply(c(2), new Divide(vx, c(1)))
+                )
+                .advanced(
+                        "(2 / ($x - 1))",
+                        "2 / ($x - 1)",
+                        vars -> 2 / (x(vars) - 1),
+                        new Divide(c(2), new Subtract(vx, c(1)))
+                )
+                .advanced(
+                        "((1 * 2) + $x)",
+                        "1 * 2 + $x",
+                        vars -> 1 * 2 + x(vars),
+                        new Add(new Multiply(c(1), c(2)), vx)
+                )
+                .advanced(
+                        "(($x - 1) - 2)",
+                        "$x - 1 - 2",
+                        vars -> x(vars) - 1 - 2,
+                        new Subtract(new Subtract(vx, c(1)), c(2))
+                )
+                .advanced(
+                        "(($x / 1) * 2)",
+                        "$x / 1 * 2",
+                        vars -> x(vars) / 1 * 2,
+                        new Multiply(new Divide(vx, c(1)), c(2))
+                )
+                .advanced("((2 + 1) / 1)", "(2 + 1) / 1", vars -> (2 + 1) / 1, new Divide(new Add(c(2), c(1)), c(1)))
+                .advanced(
+                        "(1 + (1 + (2 + 1)))",
+                        "1 + 1 + 2 + 1",
+                        vars -> 1 + 1 + 2 + 1,
+                        new Add(c(1), new Add(c(1), new Add(c(2), c(1))))
+                )
+                .advanced(
+                        "($x - ((1 * 2) + $x))",
+                        "$x - (1 * 2 + $x)",
+                        vars -> x(vars) - (1 * 2 + x(vars)),
+                        new Subtract(vx, new Add(new Multiply(c(1), c(2)), vx))
+                )
+                .advanced(
+                        "($x * (2 / ($x - 1)))",
+                        "$x * (2 / ($x - 1))",
+                        vars -> x(vars) * (2 / (x(vars) - 1)),
+                        new Multiply(vx, new Divide(c(2), new Subtract(vx, c(1))))
+                )
+                .advanced(
+                        "($x / (1 + (2 + 1)))",
+                        "$x / (1 + 2 + 1)",
+                        vars -> x(vars) / (1 + 2 + 1),
+                        new Divide(vx, new Add(c(1), new Add(c(2), c(1))))
+                )
+                .advanced(
+                        "((1 * 2) + (2 + 1))",
+                        "1 * 2 + 2 + 1",
+                        vars -> 1 * 2 + 2 + 1,
+                        new Add(new Multiply(c(1), c(2)), new Add(c(2), c(1)))
+                )
+                .advanced(
+                        "((2 + 1) - (2 + 1))",
+                        "2 + 1 - (2 + 1)",
+                        vars -> 2 + 1 - (2 + 1),
+                        new Subtract(new Add(c(2), c(1)), new Add(c(2), c(1)))
+                )
+                .advanced(
+                        "(($x - 1) * ($x / 1))",
+                        "($x - 1) * ($x / 1)",
+                        vars -> (x(vars) - 1) * (x(vars) / 1),
+                        new Multiply(new Subtract(vx, c(1)), new Divide(vx, c(1)))
+                )
+                .advanced(
+                        "(($x - 1) / (1 * 2))",
+                        "($x - 1) / (1 * 2)",
+                        vars -> (x(vars) - 1) / (1 * 2),
+                        new Divide(new Subtract(vx, c(1)), new Multiply(c(1), c(2)))
+                )
+                .advanced(
+                        "((($x - 1) - 2) + $x)",
+                        "$x - 1 - 2 + $x",
+                        vars -> x(vars) - 1 - 2 + x(vars),
+                        new Add(new Subtract(new Subtract(vx, c(1)), c(2)), vx)
+                )
+                .advanced(
+                        "(((1 * 2) + $x) - 1)",
+                        "1 * 2 + $x - 1",
+                        vars -> 1 * 2 + x(vars) - 1,
+                        new Subtract(new Add(new Multiply(c(1), c(2)), vx), c(1))
+                )
+                .advanced(
+                        "(((2 + 1) / 1) * $x)",
+                        "(2 + 1) / 1 * $x",
+                        vars -> (2 + 1) / 1 * x(vars),
+                        new Multiply(new Divide(new Add(c(2), c(1)), c(1)), vx)
+                )
+                .advanced(
+                        "((2 / ($x - 1)) / 2)",
+                        "2 / ($x - 1) / 2",
+                        vars -> 2 / (x(vars) - 1) / 2,
+                        new Divide(new Divide(c(2), new Subtract(vx, c(1))), c(2))
+                );
+    }
+
+    private static Integer x(final List<Integer> vars) {
+        return vars.get(0);
+    }
+
+    static void main(final String... args) {
+        TripleExpression.SELECTOR
+                .variant("List", ExpressionTest.v(ListExpression::tester))
+                .main(args);
+    }
+}

java/expression/ToMiniString.java

@@ -0,0 +1,10 @@
+package expression;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public interface ToMiniString {
+    default String toMiniString() {
+        return toString();
+    }
+}

java/expression/TripleExpression.java

@@ -0,0 +1,190 @@
+package expression;
+
+import base.ExtendedRandom;
+import base.Pair;
+import base.Selector;
+import base.TestCounter;
+import expression.common.ExpressionKind;
+import expression.common.Type;
+
+import java.util.List;
+
+/**
+ * Three-argument arithmetic expression over integers.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@FunctionalInterface
+@SuppressWarnings("ClassReferencesSubclass")
+public interface TripleExpression extends ToMiniString {
+    int evaluate(int x, int y, int z);
+
+    // Tests follow. You may temporarily remove everything til the end.
+
+    Type<Integer> TYPE = new Type<>(a -> a, ExtendedRandom::nextInt, int.class);
+    ExpressionKind<TripleExpression, Integer> KIND = new ExpressionKind<>(
+            TYPE,
+            TripleExpression.class,
+            List.of(
+                    Pair.of("x", new Variable("x")),
+                    Pair.of("y", new Variable("y")),
+                    Pair.of("z", new Variable("z"))
+            ),
+            (expr, variables, values) -> expr.evaluate(values.get(0), values.get(1), values.get(2))
+    );
+
+    @SuppressWarnings("PointlessArithmeticExpression")
+    static ExpressionTester<?, ?> tester(final TestCounter counter) {
+        final Variable vx = new Variable("x");
+        final Variable vy = new Variable("y");
+        final Variable vz = new Variable("z");
+
+        return new ExpressionTester<>(
+                counter, KIND, c -> (x, y, z) -> c,
+                (op, a, b) -> (x, y, z) -> op.apply(a.evaluate(x, y, z), b.evaluate(x, y, z)),
+                Integer::sum, (a, b) -> a - b, (a, b) -> a * b, (a, b) -> a / b
+        )
+                .basic("10", "10", (x, y, z) -> 10, c(10))
+                .basic("x", "x", (x, y, z) -> x, vx)
+                .basic("y", "y", (x, y, z) -> y, vy)
+                .basic("z", "z", (x, y, z) -> z, vz)
+                .basic("(x + 2)", "x + 2", (x, y, z) -> x + 2, new Add(vx, c(2)))
+                .basic("(2 - y)", "2 - y", (x, y, z) -> 2 - y, new Subtract(c(2), vy))
+                .basic("(3 * z)", "3 * z", (x, y, z) -> 3 * z, new Multiply(c(3), vz))
+                .basic("(x / -2)", "x / -2", (x, y, z) -> -x / 2, new Divide(vx, c(-2)))
+                .basic("((1 + 2) + 3)", "1 + 2 + 3", (x, y, z) -> 6, new Add(new Add(c(1), c(2)), c(3)))
+                .basic("(1 + (2 + 3))", "1 + 2 + 3", (x, y, z) -> 6, new Add(c(1), new Add(c(2), c(3))))
+                .basic("((1 - 2) - 3)", "1 - 2 - 3", (x, y, z) -> -4, new Subtract(new Subtract(c(1), c(2)), c(3)))
+                .basic("(1 - (2 - 3))", "1 - (2 - 3)", (x, y, z) -> 2, new Subtract(c(1), new Subtract(c(2), c(3))))
+                .basic("((1 * 2) * 3)", "1 * 2 * 3", (x, y, z) -> 6, new Multiply(new Multiply(c(1), c(2)), c(3)))
+                .basic("(1 * (2 * 3))", "1 * 2 * 3", (x, y, z) -> 6, new Multiply(c(1), new Multiply(c(2), c(3))))
+                .basic("((10 / 2) / 3)", "10 / 2 / 3", (x, y, z) -> 10 / 2 / 3, new Divide(new Divide(c(10), c(2)), c(3)))
+                .basic("(10 / (3 / 2))", "10 / (3 / 2)", (x, y, z) -> 10, new Divide(c(10), new Divide(c(3), c(2))))
+                .basic("((x * y) + ((z - 1) / 10))", "x * y + (z - 1) / 10", (x, y, z) -> x * y + (z - 1) / 10, new Add(
+                        new Multiply(vx, vy),
+                        new Divide(new Subtract(vz, c(1)), c(10))
+                ))
+                .basic("(x + y)", "x + y", (x, y, z) -> x + y, new Add(vx, vy))
+                .basic("(y + x)", "y + x", (x, y, z) -> y + x, new Add(vy, vx))
+
+                .advanced("(1 + 1)", "1 + 1", (x, y, z) -> 1 + 1, new Add(c(1), c(1)))
+                .advanced("(y - x)", "y - x", (x, y, z) -> y - x, new Subtract(vy, vx))
+                .advanced("(2 * x)", "2 * x", (x, y, z) -> 2 * x, new Multiply(c(2), vx))
+                .advanced("(2 / x)", "2 / x", (x, y, z) -> 2 / x, new Divide(c(2), vx))
+                .advanced("(z + (1 + 1))", "z + 1 + 1", (x, y, z) -> z + 1 + 1, new Add(vz, new Add(c(1), c(1))))
+                .advanced(
+                        "(2 - (y - x))",
+                        "2 - (y - x)",
+                        (x, y, z) -> 2 - (y - x),
+                        new Subtract(c(2), new Subtract(vy, vx))
+                )
+                .advanced(
+                        "(z * (2 / x))",
+                        "z * (2 / x)",
+                        (x, y, z) -> z * (2 / x),
+                        new Multiply(vz, new Divide(c(2), vx))
+                )
+                .advanced(
+                        "(z / (y - x))",
+                        "z / (y - x)",
+                        (x, y, z) -> z / (y - x),
+                        new Divide(vz, new Subtract(vy, vx))
+                )
+                .advanced("((2 * x) + y)", "2 * x + y", (x, y, z) -> 2 * x + y, new Add(new Multiply(c(2), vx), vy))
+                .advanced(
+                        "((y - x) - 2)",
+                        "y - x - 2",
+                        (x, y, z) -> y - x - 2,
+                        new Subtract(new Subtract(vy, vx), c(2))
+                )
+                .advanced("((2 / x) * y)", "2 / x * y", (x, y, z) -> 2 / x * y, new Multiply(new Divide(c(2), vx), vy))
+                .advanced("((1 + 1) / x)", "(1 + 1) / x", (x, y, z) -> (1 + 1) / x, new Divide(new Add(c(1), c(1)), vx))
+                .advanced("(1 + (2 * 3))", "1 + 2 * 3", (x, y, z) -> 7, new Add(c(1), new Multiply(c(2), c(3))))
+                .advanced("(1 - (2 * 3))", "1 - 2 * 3", (x, y, z) -> -5, new Subtract(c(1), new Multiply(c(2), c(3))))
+                .advanced("(1 + (2 / 3))", "1 + 2 / 3", (x, y, z) -> 1, new Add(c(1), new Divide(c(2), c(3))))
+                .advanced("(1 - (2 / 3))", "1 - 2 / 3", (x, y, z) -> 1, new Subtract(c(1), new Divide(c(2), c(3))))
+                .advanced(
+                        "(2 + (z + (1 + 1)))",
+                        "2 + z + 1 + 1",
+                        (x, y, z) -> 2 + z + 1 + 1,
+                        new Add(c(2), new Add(vz, new Add(c(1), c(1))))
+                )
+                .advanced(
+                        "(1 - ((2 * x) + y))",
+                        "1 - (2 * x + y)",
+                        (x, y, z) -> 1 - (2 * x + y),
+                        new Subtract(c(1), new Add(new Multiply(c(2), vx), vy))
+                )
+                .advanced(
+                        "(1 * (z / (y - x)))",
+                        "1 * (z / (y - x))",
+                        (x, y, z) -> 1 * (z / (y - x)),
+                        new Multiply(c(1), new Divide(vz, new Subtract(vy, vx)))
+                )
+                .advanced(
+                        "(z / (z + (1 + 1)))",
+                        "z / (z + 1 + 1)",
+                        (x, y, z) -> z / (z + 1 + 1),
+                        new Divide(vz, new Add(vz, new Add(c(1), c(1))))
+                )
+                .advanced(
+                        "((2 * x) + (1 + 1))",
+                        "2 * x + 1 + 1",
+                        (x, y, z) -> 2 * x + 1 + 1,
+                        new Add(new Multiply(c(2), vx), new Add(c(1), c(1)))
+                )
+                .advanced(
+                        "((1 + 1) - (1 + 1))",
+                        "1 + 1 - (1 + 1)",
+                        (x, y, z) -> 1 + 1 - (1 + 1),
+                        new Subtract(new Add(c(1), c(1)), new Add(c(1), c(1)))
+                )
+                .advanced(
+                        "((y - x) * (2 / x))",
+                        "(y - x) * (2 / x)",
+                        (x, y, z) -> (y - x) * (2 / x),
+                        new Multiply(new Subtract(vy, vx), new Divide(c(2), vx))
+                )
+                .advanced(
+                        "((y - x) / (2 * x))",
+                        "(y - x) / (2 * x)",
+                        (x, y, z) -> (y - x) / (2 * x),
+                        new Divide(new Subtract(vy, vx), new Multiply(c(2), vx))
+                )
+                .advanced(
+                        "(((y - x) - 2) + 1)",
+                        "y - x - 2 + 1",
+                        (x, y, z) -> y - x - 2 + 1,
+                        new Add(new Subtract(new Subtract(vy, vx), c(2)), c(1))
+                )
+                .advanced(
+                        "(((2 * x) + y) - z)",
+                        "2 * x + y - z",
+                        (x, y, z) -> 2 * x + y - z,
+                        new Subtract(new Add(new Multiply(c(2), vx), vy), vz)
+                )
+                .advanced(
+                        "(((1 + 1) / x) * 2)",
+                        "(1 + 1) / x * 2",
+                        (x, y, z) -> (1 + 1) / x * 2,
+                        new Multiply(new Divide(new Add(c(1), c(1)), vx), c(2))
+                )
+                .advanced(
+                        "((z / (y - x)) / x)",
+                        "z / (y - x) / x",
+                        (x, y, z) -> z / (y - x) / x,
+                        new Divide(new Divide(vz, new Subtract(vy, vx)), vx)
+                );
+    }
+
+    private static Const c(final Integer c) {
+        return TYPE.constant(c);
+    }
+
+    Selector SELECTOR = ExpressionTest.SELECTOR
+            .variant("Triple", ExpressionTest.v(TripleExpression::tester));
+
+    static void main(final String... args) {
+        TripleExpression.SELECTOR.main(args);
+    }
+}

java/expression/common/Expr.java

@@ -0,0 +1,32 @@
+package expression.common;
+
+import base.Functional;
+import base.Pair;
+
+import java.util.List;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public record Expr<C, V>(Node<C> node, List<Pair<String, V>> variables) {
+    public <T> List<Pair<String, T>> variables(final BiFunction<String, V, T> f) {
+        return Functional.map(
+                variables,
+                variable -> variable.second(f.apply(variable.first(), variable.second()))
+        );
+    }
+
+    public <T> Expr<C, T> convert(final BiFunction<String, V, T> f) {
+        return of(node, variables(f));
+    }
+
+    public Expr<C, V> node(final Function<Node<C>, Node<C>> f) {
+        return of(f.apply(node), variables);
+    }
+
+    public static <C, V> Expr<C, V> of(final Node<C> node, final List<Pair<String, V>> variables) {
+        return new Expr<>(node, variables);
+    }
+}

java/expression/common/ExpressionKind.java

@@ -0,0 +1,94 @@
+package expression.common;
+
+import base.ExtendedRandom;
+import base.Functional;
+import base.Pair;
+import expression.ToMiniString;
+
+import java.util.List;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ExpressionKind<E extends ToMiniString, C> {
+    private final Type<C> type;
+    private final Class<E> kind;
+    private final Variables<E> variables;
+    private final Evaluator<E, C> evaluator;
+
+    public ExpressionKind(
+            final Type<C> type,
+            final Class<E> kind,
+            final Variables<E> variables,
+            final Evaluator<E, C> evaluator
+    ) {
+        this.type = type;
+        this.kind = kind;
+        this.variables = variables;
+        this.evaluator = evaluator;
+    }
+
+    public ExpressionKind(
+            final Type<C> type,
+            final Class<E> kind,
+            final List<Pair<String, E>> variables,
+            final Evaluator<E, C> evaluator
+    ) {
+        this(type, kind, (r, c) -> variables, evaluator);
+    }
+
+    public C evaluate(final E expression, final List<String> variables, final List<C> values) throws Exception {
+        return evaluator.evaluate(expression, variables, values);
+    }
+
+    public E cast(final Object expression) {
+        return kind.cast(expression);
+    }
+
+    public String getName() {
+        return kind.getSimpleName();
+    }
+
+    public E constant(final C value) {
+        return cast(type.constant(value));
+    }
+
+    public C randomValue(final ExtendedRandom random) {
+        return type.randomValue(random);
+    }
+
+    public List<List<C>> allValues(final int length, final List<Integer> values) {
+        return Functional.allValues(fromInts(values), length);
+    }
+
+    public List<C> fromInts(final List<Integer> values) {
+        return Functional.map(values, this::fromInt);
+    }
+
+    public C fromInt(final int value) {
+        return type.fromInt(value);
+    }
+
+    @Override
+    public String toString() {
+        return kind.getName();
+    }
+
+    public ExpressionKind<E, C> withVariables(final Variables<E> variables) {
+        return new ExpressionKind<>(type, kind, variables, evaluator);
+    }
+
+    public Variables<E> variables() {
+        return variables;
+    }
+
+    @FunctionalInterface
+    public interface Variables<E> {
+        List<Pair<String, E>> generate(final ExtendedRandom random, final int count);
+    }
+
+    @FunctionalInterface
+    public interface Evaluator<E, R> {
+        R evaluate(final E expression, final List<String> vars, final List<R> values) throws Exception;
+    }
+}

java/expression/common/Generator.java

@@ -0,0 +1,173 @@
+package expression.common;
+
+import base.*;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+public class Generator<C, E> {
+    private final Supplier<C> constant;
+    private final List<Named<Integer>> ops;
+    private final ExpressionKind.Variables<E> variables;
+    private final Set<String> forbidden;
+    private final ExtendedRandom random;
+    private final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests;
+
+    public Generator(
+            final Supplier<C> constant,
+            final List<Named<Integer>> ops,
+            final ExpressionKind.Variables<E> variables,
+            final Set<String> forbidden,
+            final ExtendedRandom random,
+            final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests
+    ) {
+        this.constant = constant;
+        this.ops = List.copyOf(ops);
+        this.variables = variables;
+        this.forbidden = Set.copyOf(forbidden);
+        this.random = random;
+        this.basicTests = List.copyOf(basicTests);
+    }
+
+    public static <C> Builder<C> builder(final Supplier<C> constant, final ExtendedRandom random) {
+        return new Builder<>(random, constant);
+    }
+
+    public void testRandom(
+            final TestCounter counter,
+            final int denominator,
+            final Consumer<Expr<C, E>> consumer
+    ) {
+        final int d = Math.max(TestCounter.DENOMINATOR, denominator);
+        testRandom(counter, consumer, 1, 100, 100 / d, (vars, depth) -> generateFullDepth(vars, Math.min(depth, 3)));
+        testRandom(counter, consumer, 2, 1000 / d, 1, this::generateSize);
+        testRandom(counter, consumer, 3, 12, 100 / d, this::generateFullDepth);
+        testRandom(counter, consumer, 4, 777 / d, 1, this::generatePartialDepth);
+    }
+
+    private void testRandom(
+            final TestCounter counter,
+            final Consumer<Expr<C, E>> consumer,
+            final int seq,
+            final int levels,
+            final int perLevel,
+            final BiFunction<List<Node<C>>, Integer, Node<C>> generator
+    ) {
+        counter.scope("Random tests #" + seq, () -> {
+            final int total = levels * perLevel;
+            int generated = 0;
+            for (int level = 0; level < levels; level++) {
+                for (int j = 0; j < perLevel; j++) {
+                    if (generated % 100 == 0) {
+                        progress(counter, total, generated);
+                    }
+                    generated++;
+
+                    final List<Pair<String, E>> vars = variables(random.nextInt(10) + 1);
+                    consumer.accept(Expr.of(generator.apply(Functional.map(vars, v -> Node.op(v.first())), level), vars));
+                }
+            }
+            progress(counter, generated, total);
+        });
+    }
+
+    private static void progress(final TestCounter counter, final int total, final int generated) {
+        counter.format("Completed %4d out of %d%n", generated, total);
+    }
+
+    private Node<C> generate(
+            final List<Node<C>> variables,
+            final boolean nullary,
+            final Supplier<Node<C>> unary,
+            final Supplier<Pair<Node<C>, Node<C>>> binary
+    ) {
+        if (nullary || ops.isEmpty()) {
+            return random.nextBoolean() ? random.randomItem(variables) : Node.constant(constant.get());
+        } else {
+            final Named<Integer> op = random.randomItem(ops);
+            if (Math.abs(op.value()) == 1) {
+                return Node.op(op.name(), (op.value() + 1) >> 1, unary.get());
+            } else {
+                final Pair<Node<C>, Node<C>> pair = binary.get();
+                return Node.op(op.name(), pair.first(), pair.second());
+            }
+        }
+    }
+
+    private Node<C> generate(final List<Node<C>> variables, final boolean nullary, final Supplier<Node<C>> child) {
+        return generate(variables, nullary, child, () -> Pair.of(child.get(), child.get()));
+    }
+
+    private Node<C> generateFullDepth(final List<Node<C>> variables, final int depth) {
+        return generate(variables, depth == 0, () -> generateFullDepth(variables, depth - 1));
+    }
+
+    private Node<C> generatePartialDepth(final List<Node<C>> variables, final int depth) {
+        return generate(variables, depth == 0, () -> generatePartialDepth(variables, random.nextInt(depth)));
+    }
+
+    private Node<C> generateSize(final List<Node<C>> variables, final int size) {
+        final int first = size <= 1 ? 0 : random.nextInt(size);
+        return generate(
+                variables,
+                size == 0,
+                () -> generateSize(variables, size - 1),
+                () -> Pair.of(
+                        generateSize(variables, first),
+                        generateSize(variables, size - 1 - first)
+                )
+        );
+    }
+
+    public void testBasic(final Consumer<Expr<C, E>> consumer) {
+        basicTests.forEach(test -> {
+            final List<Pair<String, E>> vars = variables(random.nextInt(5) + 3);
+            test.apply(Functional.map(vars, v -> Node.op(v.first())))
+                    .map(node -> Expr.of(node, vars))
+                    .forEachOrdered(consumer);
+        });
+    }
+
+    public List<Pair<String, E>> variables(final int count) {
+        List<Pair<String, E>> vars;
+        do {
+            vars = variables.generate(random, count);
+        } while (vars.stream().map(Pair::first).anyMatch(forbidden::contains));
+        return vars;
+    }
+
+    /**
+     * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+     */
+    public static final class Builder<C> {
+        private final ExtendedRandom random;
+        private final Supplier<C> constant;
+
+        private final List<Named<Integer>> ops = new ArrayList<>();
+        private final Set<String> forbidden = new HashSet<>();
+
+        private Builder(final ExtendedRandom random, final Supplier<C> constant) {
+            this.random = random;
+            this.constant = constant;
+        }
+
+        public void add(final String name, final int arity) {
+            ops.add(Named.of(name, arity));
+            forbidden.add(name);
+        }
+
+        public <E> Generator<C, E> build(
+                final ExpressionKind.Variables<E> variables,
+                final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests
+        ) {
+            return new Generator<>(constant, ops, variables, forbidden, random, basicTests);
+        }
+    }
+}

java/expression/common/Node.java

@@ -0,0 +1,106 @@
+package expression.common;
+
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public abstract class Node<T> {
+    private Node() {
+    }
+
+    public abstract <R> R get(Const<T, R> con, Nullary<R> nul, Unary<Node<T>, R> un, Binary<Node<T>, R> bin);
+    public abstract <R> R cata(Const<T, R> con, Nullary<R> nul, Unary<R, R> un, Binary<R, R> bin);
+
+    public final String toPolish() {
+        return cata(
+                T::toString,
+                name -> name,
+                (name, priority, a) -> a + " " + name + ":1",
+                (name, a1, a2) -> a1 + " " + a2 + " " + name + ":2"
+        );
+    }
+
+    @Override
+    public final String toString() {
+        return cata(
+                T::toString,
+                name -> name,
+                (name, priority, a) -> name.equals("[") ? "[" + a + "]" :
+                        (priority & 1) == 1 ? "(" + name + " " + a + ")" : "(" + a + " " + name + ")",
+                (name, a1, a2) -> "(" + a1 + " " + name + " " + a2 + ")"
+        );
+    }
+
+    public static <T> Node<T> constant(final T value) {
+        return new Node<>() {
+            @Override
+            public <R> R get(final Const<T, R> con, final Nullary<R> nul, final Unary<Node<T>, R> un, final Binary<Node<T>, R> bin) {
+                return con.apply(value);
+            }
+
+            @Override
+            public <R> R cata(final Const<T, R> con, final Nullary<R> nul, final Unary<R, R> un, final Binary<R, R> bin) {
+                return con.apply(value);
+            }
+        };
+    }
+
+    public static <T> Node<T> op(final String name) {
+        return new Node<>() {
+            @Override
+            public <R> R get(final Const<T, R> con, final Nullary<R> nul, final Unary<Node<T>, R> un, final Binary<Node<T>, R> bin) {
+                return nul.apply(name);
+            }
+
+            @Override
+            public <R> R cata(final Const<T, R> con, final Nullary<R> nul, final Unary<R, R> un, final Binary<R, R> bin) {
+                return nul.apply(name);
+            }
+        };
+    }
+
+    public static <T> Node<T> op(final String name, final int priority, final Node<T> arg) {
+        return new Node<>() {
+            @Override
+            public <R> R get(final Const<T, R> con, final Nullary<R> nul, final Unary<Node<T>, R> un, final Binary<Node<T>, R> bin) {
+                return un.apply(name, priority, arg);
+            }
+
+            @Override
+            public <R> R cata(final Const<T, R> con, final Nullary<R> nul, final Unary<R, R> un, final Binary<R, R> bin) {
+                return un.apply(name, priority, arg.cata(con, nul, un, bin));
+            }
+        };
+    }
+
+    public static <T> Node<T> op(final String name, final Node<T> arg1, final Node<T> arg2) {
+        return new Node<>() {
+            @Override
+            public <R> R get(final Const<T, R> con, final Nullary<R> nul, final Unary<Node<T>, R> un, final Binary<Node<T>, R> bin) {
+                return bin.apply(name, arg1, arg2);
+            }
+
+            @Override
+            public <R> R cata(final Const<T, R> con, final Nullary<R> nul, final Unary<R, R> un, final Binary<R, R> bin) {
+                return bin.apply(name, arg1.cata(con, nul, un, bin), arg2.cata(con, nul, un, bin));
+            }
+        };
+    }
+
+    @FunctionalInterface
+    public interface Const<T, R> extends Function<T, R> {}
+
+    @FunctionalInterface
+    public interface Nullary<R> extends Function<String, R> {}
+
+    @FunctionalInterface
+    public interface Unary<T, R> {
+        R apply(String name, int priority, T arg);
+    }
+
+    @FunctionalInterface
+    public interface Binary<T, R> {
+        R apply(String name, T arg1, T arg2);
+    }
+}

java/expression/common/NodeRenderer.java

@@ -0,0 +1,96 @@
+package expression.common;
+
+import base.ExtendedRandom;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class NodeRenderer<C> {
+    public static final String PAREN = "[";
+    public static final List<Paren> DEFAULT_PARENS = List.of(paren("(", ")"));
+
+    public static final Mode MINI_MODE = Mode.SIMPLE_MINI; // Replace by TRUE_MINI for some challenge;
+    public static final Settings FULL = Mode.FULL.settings(0);
+    public static final Settings FULL_EXTRA = Mode.FULL.settings(Integer.MAX_VALUE / 4);
+    public static final Settings SAME = Mode.SAME.settings(0);
+    public static final Settings MINI = MINI_MODE.settings(0);
+    public static final Settings TRUE_MINI = Mode.TRUE_MINI.settings(0);
+
+    private final Renderer<C, Settings, Node<C>> renderer;
+    private final Map<String, String> brackets;
+    private final ExtendedRandom random;
+
+    public NodeRenderer(
+            final Renderer<C, Settings, Node<C>> renderer,
+            final Map<String, String> brackets,
+            final ExtendedRandom random
+    ) {
+        this.renderer = renderer;
+        this.brackets = Map.copyOf(brackets);
+        this.random = random;
+    }
+
+    public static <C> Node<C> paren(final boolean condition, final Node<C> node) {
+        return condition ? Node.op(PAREN, 1, node) : node;
+    }
+
+    public static Paren paren(final String open, final String close) {
+        return new Paren(open, close);
+    }
+
+    public Node<C> renderToNode(final Settings settings, final Expr<C, ?> expr) {
+        final Expr<C, Node<C>> convert = expr.convert((name, variable) -> Node.op(name));
+        return renderer.render(convert, settings);
+    }
+
+    public String render(final Node<C> node, final List<Paren> parens) {
+        return node.cata(
+                String::valueOf,
+                name -> name,
+                (name, priority, arg) ->
+                        name == PAREN ? random.randomItem(parens).apply(arg) :
+                        priority == Integer.MAX_VALUE ? name + arg + brackets.get(name) :
+                        (priority & 1) == 1 ? name + arg :
+                        arg + name,
+                (name, a, b) -> a + " " + name + " " + b
+        );
+    }
+
+    public String render(final Expr<C, ?> expr, final Settings settings) {
+        return render(renderToNode(settings, expr), settings.parens());
+    }
+
+    public enum Mode {
+        FULL, SAME, TRUE_MINI, SIMPLE_MINI;
+
+        public Settings settings(final int limit) {
+            return new Settings(this, limit);
+        }
+    }
+
+    public record Paren(String open, String close) {
+        String apply(final String expression) {
+            return open() + expression + close();
+        }
+    }
+
+    public record Settings(Mode mode, int limit, List<Paren> parens) {
+        public Settings(final Mode mode, final int limit) {
+            this(mode, limit, DEFAULT_PARENS);
+        }
+
+        public <C> Node<C> extra(Node<C> node, final ExtendedRandom random) {
+            while (random.nextInt(Integer.MAX_VALUE) < limit) {
+                node = paren(true, node);
+            }
+            return node;
+        }
+
+        public Settings withParens(final List<Paren> parens) {
+            return this.parens.equals(parens) ? this : new Settings(mode, limit, List.copyOf(parens));
+        }
+    }
+}

java/expression/common/NodeRendererBuilder.java

@@ -0,0 +1,145 @@
+package expression.common;
+
+import base.ExtendedRandom;
+import base.Functional;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.Supplier;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@SuppressWarnings("StaticMethodOnlyUsedInOneClass")
+public class NodeRendererBuilder<C> {
+    private final Renderer.Builder<C, NodeRenderer.Settings, Node<C>> nodeRenderer = Renderer.builder(Node::constant);
+    private final Map<String, Priority> priorities = new HashMap<>();
+    private final Map<String, String> brackets = new HashMap<>();
+    private final ExtendedRandom random;
+
+    public NodeRendererBuilder(final ExtendedRandom random) {
+        this.random = random;
+        nodeRenderer.unary(NodeRenderer.PAREN, (mode, arg) -> NodeRenderer.paren(true, arg));
+    }
+
+    public void unary(final String name, final int priority) {
+        final String space = name.equals("-") || Character.isLetter(name.charAt(0)) ? " " : "";
+        nodeRenderer.unary(
+                name,
+                (settings, arg) -> settings.extra(Node.op(name, priority, inner(settings, priority, arg, space)), random)
+        );
+    }
+
+    public void unary(final String left, final String right) {
+        brackets.put(left, right);
+        nodeRenderer.unary(
+                left,
+                (settings, arg) -> settings.extra(Node.op(left, Integer.MAX_VALUE, arg), random)
+        );
+    }
+
+    private Node<C> inner(final NodeRenderer.Settings settings, final int priority, final Node<C> arg, final String space) {
+        if (settings.mode() == NodeRenderer.Mode.FULL) {
+            return NodeRenderer.paren(true, arg);
+        } else {
+            final String op = arg.get(
+                    c -> space,
+                    n -> space,
+                    (n, p, a) ->
+                            priority > unaryPriority(arg) ? NodeRenderer.PAREN :
+                            NodeRenderer.PAREN.equals(n) ? "" :
+                            space,
+                    (n, a, b) -> NodeRenderer.PAREN
+            );
+            return op.isEmpty() ? arg : Node.op(op, Priority.MAX.priority | 1, arg);
+        }
+    }
+
+    private static <C> Integer unaryPriority(final Node<C> node) {
+        return node.get(c -> Integer.MAX_VALUE, n -> Integer.MAX_VALUE, (n, p, a) -> p, (n, a, b) -> Integer.MIN_VALUE);
+    }
+
+    public void binary(final String name, final int priority) {
+        final Priority mp = new Priority(name, priority);
+        priorities.put(name, mp);
+
+        nodeRenderer.binary(name, (settings, l, r) -> settings.extra(process(settings, mp, l, r), random));
+    }
+
+    private Node<C> process(final NodeRenderer.Settings settings, final Priority mp, final Node<C> l, final Node<C> r) {
+        if (settings.mode() == NodeRenderer.Mode.FULL) {
+            return NodeRenderer.paren(true, op(mp, l, r));
+        }
+
+        final Priority lp = priority(l);
+        final Priority rp = priority(r);
+
+        final int rc = rp.compareLevels(mp);
+
+        // :NOTE: Especially ugly code, do not replicate
+        final boolean advanced = settings.mode() == NodeRenderer.Mode.SAME
+                || mp.has(2)
+                || mp.has(1) && (mp != rp || (settings.mode() == NodeRenderer.Mode.TRUE_MINI && hasOther(r, rp)));
+
+        final Node<C> al = NodeRenderer.paren(lp.compareLevels(mp) < 0, l);
+        if (rc == 0 && !advanced) {
+            return get(r, null, (n, a, b) -> rp.op(mp.op(al, a), b));
+        } else {
+            return mp.op(al, NodeRenderer.paren(rc == 0 && advanced || rc < 0, r));
+        }
+    }
+
+    private boolean hasOther(final Node<C> node, final Priority priority) {
+        return get(node, () -> false, (name, l, r) -> {
+            final Priority p = Functional.get(priorities, name);
+            if (p.compareLevels(priority) != 0) {
+                return false;
+            }
+            return p != priority || hasOther(l, priority);
+        });
+    }
+
+    private Node<C> op(final Priority mp, final Node<C> l, final Node<C> r) {
+        return mp.op(l, r);
+    }
+
+    private Priority priority(final Node<C> node) {
+        return get(node, () -> Priority.MAX, (n, a, b) -> Functional.get(priorities, n));
+    }
+
+    private <R> R get(final Node<C> node, final Supplier<R> common, final Node.Binary<Node<C>, R> binary) {
+        return node.get(
+                c -> common.get(),
+                n -> common.get(),
+                (n, p, a) -> common.get(),
+                binary
+        );
+    }
+
+    public NodeRenderer<C> build() {
+        return new NodeRenderer<>(nodeRenderer.build(), brackets, random);
+    }
+
+    // :NOTE: Especially ugly bit-fiddling, do not replicate
+    private record Priority(String op, int priority) {
+        private static final int Q = 3;
+        private static final Priority MAX = new Priority("MAX", Integer.MAX_VALUE - Q);
+
+        private int compareLevels(final Priority that) {
+            return (priority | Q) - (that.priority | Q);
+        }
+
+        @Override
+        public String toString() {
+            return String.format("Priority(%s, %d, %d)", op, priority | Q, priority & Q);
+        }
+
+        public <C> Node<C> op(final Node<C> l, final Node<C> r) {
+            return Node.op(op, l, r);
+        }
+
+        private boolean has(final int value) {
+            return (priority & Q) == value;
+        }
+    }
+}

java/expression/common/Reason.java

@@ -0,0 +1,60 @@
+package expression.common;
+
+import base.Either;
+
+import java.util.function.LongUnaryOperator;
+import java.util.function.Supplier;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class Reason {
+    public static final Reason OVERFLOW = new Reason("Overflow");
+    public static final Reason DBZ = new Reason("Division by zero");
+
+    private final String description;
+
+    public Reason(final String description) {
+        this.description = description;
+    }
+
+    public static <T> Either<Reason, T> eval(final Supplier<T> action) {
+        try {
+            return Either.right(action.get());
+        } catch (final ReasonException e) {
+            return Either.left(e.reason);
+        }
+    }
+
+    public static int overflow(final long value) {
+        return value < Integer.MIN_VALUE || Integer.MAX_VALUE < value
+                ? OVERFLOW.error()
+                : (int) value;
+    }
+
+    public <T> T error() {
+        throw new ReasonException(this);
+    }
+
+    public LongUnaryOperator less(final long limit, final LongUnaryOperator op) {
+        return a -> a < limit ? error() : op.applyAsLong(a);
+    }
+
+    public LongUnaryOperator greater(final int limit, final LongUnaryOperator op) {
+        return a -> a > limit ? error() : op.applyAsLong(a);
+    }
+
+    private static class ReasonException extends RuntimeException {
+        private final Reason reason;
+
+        public ReasonException(final Reason reason) {
+            super(reason.description);
+            this.reason = reason;
+        }
+    }
+
+    @Override
+    public String toString() {
+        return String.format("Reason(%s)", description);
+    }
+}

java/expression/common/Renderer.java

@@ -0,0 +1,60 @@
+package expression.common;
+
+import base.Functional;
+import base.Pair;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public interface Renderer<C, S, R> {
+    static <C, S, R> Builder<C, S, R> builder(final Node.Const<C, R> constant) {
+        return new Builder<>(constant);
+    }
+
+    R render(final Expr<C, R> expr, final S settings);
+
+    @FunctionalInterface
+    interface UnaryOperator<S, R> {
+        R apply(S settings, R arg);
+    }
+
+    @FunctionalInterface
+    interface BinaryOperator<S, R> {
+        R apply(S settings, R arg1, R arg2);
+    }
+
+    final class Builder<C, S, R> {
+        private final Node.Const<C, R> constant;
+        private final Map<String, UnaryOperator<S, R>> unary = new HashMap<>();
+        private final Map<String, BinaryOperator<S, R>> binary = new HashMap<>();
+
+        private Builder(final Node.Const<C, R> constant) {
+            this.constant = constant;
+        }
+
+        public void unary(final String name, final UnaryOperator<S, R> op) {
+            unary.put(name, op);
+        }
+
+        public void binary(final String name, final BinaryOperator<S, R> op) {
+            binary.put(name, op);
+        }
+
+        public Renderer<C, S, R> build() {
+            return (expr, settings) -> {
+                final Map<String, R> vars = expr.variables().stream()
+                        .collect(Collectors.toMap(Pair::first, Pair::second));
+                return expr.node().cata(
+                        constant,
+                        name -> Functional.get(vars, name),
+                        (name, p, arg) -> Functional.get(unary, name).apply(settings, arg),
+                        (name, arg1, arg2) -> Functional.get(binary, name).apply(settings, arg1, arg2)
+                );
+            };
+        }
+    }
+}

java/expression/common/TestGenerator.java

@@ -0,0 +1,56 @@
+package expression.common;
+
+import base.Pair;
+import base.TestCounter;
+import expression.ToMiniString;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Consumer;
+
+public class TestGenerator<C, E extends ToMiniString> {
+    private final Generator<C, E> generator;
+    private final NodeRenderer<C> renderer;
+
+    public TestGenerator(final Generator<C, E> generator, final NodeRenderer<C> renderer) {
+        this.generator = generator;
+        this.renderer = renderer;
+    }
+
+    public void testBasic(final Consumer<Test<C, E>> test) {
+        generator.testBasic(consumer(test));
+    }
+
+    public void testRandom(final TestCounter counter, final int denominator, final Consumer<Test<C, E>> test) {
+        generator.testRandom(counter, denominator, consumer(test));
+    }
+
+    private Consumer<Expr<C, E>> consumer(final Consumer<TestGenerator.Test<C, E>> consumer) {
+        return expr -> consumer.accept(new TestGenerator.Test<>(expr, renderer));
+    }
+
+
+    public List<Pair<String, E>> variables(final int count) {
+        return generator.variables(count);
+    }
+
+    public String render(final Expr<C, ?> expr, final NodeRenderer.Settings settings) {
+        return renderer.render(expr, settings);
+    }
+
+    public static class Test<C, E> {
+        public final Expr<C, E> expr;
+        private final Map<NodeRenderer.Settings, String> rendered = new HashMap<>();
+        private final NodeRenderer<C> renderer;
+
+        public Test(final Expr<C, E> expr, final NodeRenderer<C> renderer) {
+            this.expr = expr;
+            this.renderer = renderer;
+        }
+
+        public String render(final NodeRenderer.Settings settings) {
+            return rendered.computeIfAbsent(settings, s -> renderer.render(expr, s));
+        }
+    }
+}

java/expression/common/TestGeneratorBuilder.java

@@ -0,0 +1,145 @@
+package expression.common;
+
+import base.ExtendedRandom;
+import base.Functional;
+import expression.ToMiniString;
+import expression.common.ExpressionKind.Variables;
+
+import java.util.*;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class TestGeneratorBuilder<C> {
+    private final ExtendedRandom random;
+
+    private final Generator.Builder<C> generator;
+    private final NodeRendererBuilder<C> renderer;
+
+    private final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests = new ArrayList<>();
+    private final List<Node<C>> consts;
+    private final boolean verbose;
+
+    public TestGeneratorBuilder(
+            final ExtendedRandom random,
+            final Supplier<C> constant,
+            final List<C> constants,
+            final boolean verbose
+    ) {
+        this.random = random;
+        this.verbose = verbose;
+
+        generator = Generator.builder(constant, random);
+        renderer = new NodeRendererBuilder<>(random);
+
+        consts = Functional.map(constants, Node::constant);
+        basicTests.add(vars -> consts.stream());
+        basicTests.add(List::stream);
+    }
+
+    private Node<C> c() {
+        return random.randomItem(consts);
+    }
+
+    private Node<C> v(final List<Node<C>> variables) {
+        return random.randomItem(variables);
+    }
+
+    private static <C> Node<C> f(final String name, final int priority, final Node<C> arg) {
+        return Node.op(name, priority, arg);
+    }
+
+    private static <C> Node<C> f(final String left, final Node<C> arg) {
+        return Node.op(left, Integer.MAX_VALUE, arg);
+    }
+
+    private static <C> Node<C> f(final String name, final Node<C> arg1, final Node<C> arg2) {
+        return Node.op(name, arg1, arg2);
+    }
+
+    @SafeVarargs
+    private void basicTests(final Function<List<Node<C>>, Node<C>>... tests) {
+        Arrays.stream(tests).map(test -> test.andThen(Stream::of)).forEachOrdered(basicTests::add);
+    }
+
+    public void unary(final String name, final int priority) {
+        generator.add(name, (priority & 1) * 2 - 1);
+        renderer.unary(name, priority);
+
+        if (verbose) {
+            basicTests.add(vars -> Stream.concat(consts.stream(), vars.stream()).map(a -> f(name, priority, a)));
+        } else {
+            basicTests(vars -> f(name, priority, c()), vars -> f(name, priority, v(vars)));
+        }
+
+        final Function<List<Node<C>>, Node<C>> p1 = vars -> f(name, priority, f(name, priority, f("+", v(vars), c())));
+        final Function<List<Node<C>>, Node<C>> p2 = vars -> f("*", v(vars), f("*", v(vars), f(name, priority, c())));
+        basicTests(
+                vars -> f(name, priority, f("+", v(vars), v(vars))),
+                vars -> f(name, priority, f(name, priority, v(vars))),
+                vars -> f(name, priority, f("/", f(name, priority, v(vars)), f("+", v(vars), v(vars)))),
+                p1,
+                p2,
+                vars -> f("+", p1.apply(vars), p2.apply(vars))
+        );
+    }
+
+    public void unary(final String left, final String right) {
+        generator.add(left, 1);
+        renderer.unary(left, right);
+
+        if (verbose) {
+            basicTests.add(vars -> Stream.concat(consts.stream(), vars.stream()).map(a -> f(left, a)));
+        } else {
+            basicTests(vars -> f(left, c()), vars -> f(left, v(vars)));
+        }
+
+        final Function<List<Node<C>>, Node<C>> p1 = vars -> f(left, f(left, f("+", v(vars), c())));
+        final Function<List<Node<C>>, Node<C>> p2 = vars -> f("*", v(vars), f("*", v(vars), f(left, c())));
+        basicTests(
+                vars -> f(left, f("+", v(vars), v(vars))),
+                vars -> f(left, f(left, v(vars))),
+                vars -> f(left, f("/", f(left, v(vars)), f("+", v(vars), v(vars)))),
+                p1,
+                p2,
+                vars -> f("+", p1.apply(vars), p2.apply(vars))
+        );
+    }
+
+    public void binary(final String name, final int priority) {
+        generator.add(name, 2);
+        renderer.binary(name, priority);
+
+        if (verbose) {
+            basicTests.add(vars -> Stream.concat(consts.stream(), vars.stream().limit(3))
+                            .flatMap(a -> consts.stream().map(b -> f(name, a, b))));
+        } else {
+            basicTests(
+                    vars -> f(name, c(), c()),
+                    vars -> f(name, v(vars), c()),
+                    vars -> f(name, c(), v(vars)),
+                    vars -> f(name, v(vars), v(vars))
+            );
+        }
+
+        final Function<List<Node<C>>, Node<C>> p1 = vars -> f(name, f(name, f("+", v(vars), c()), v(vars)), v(vars));
+        final Function<List<Node<C>>, Node<C>> p2 = vars -> f("*", v(vars), f("*", v(vars), f(name, c(), v(vars))));
+
+        basicTests(
+                vars -> f(name, f(name, v(vars), v(vars)), v(vars)),
+                vars -> f(name, v(vars), f(name, v(vars), v(vars))),
+                vars -> f(name, f(name, v(vars), v(vars)), f(name, v(vars), v(vars))),
+                vars -> f(name, f("-", f(name, v(vars), v(vars)), c()), f("+", v(vars), v(vars))),
+                p1,
+                p2,
+                vars -> f("+", p1.apply(vars), p2.apply(vars))
+        );
+    }
+
+    public <E extends ToMiniString> TestGenerator<C,E> build(final Variables<E> variables) {
+        return new TestGenerator<>(generator.build(variables, basicTests), renderer.build());
+    }
+}

java/expression/common/Type.java

@@ -0,0 +1,51 @@
+package expression.common;
+
+import base.Asserts;
+import base.ExtendedRandom;
+import expression.Const;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+import java.util.function.Function;
+import java.util.function.IntFunction;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class Type<C> {
+    private final IntFunction<C> fromInt;
+    private final Function<ExtendedRandom, C> random;
+    private final Function<C, Const> constant;
+
+    public Type(final IntFunction<C> fromInt, final Function<ExtendedRandom, C> random, final Class<?> type) {
+        this.fromInt = fromInt;
+        this.random = random;
+
+        try {
+            final MethodHandle constructor = MethodHandles.publicLookup()
+                    .findConstructor(Const.class, MethodType.methodType(void.class, type));
+            constant = c -> {
+                try {
+                    return (Const) constructor.invoke(c);
+                } catch (final Throwable e) {
+                    throw Asserts.error("Cannot create new Const(%s): %s", c, e);
+                }
+            };
+        } catch (final IllegalAccessException | NoSuchMethodException e) {
+            throw Asserts.error("Cannot find constructor Const(%s): %s", type, e);
+        }
+    }
+
+    public Const constant(final C value) {
+        return constant.apply(value);
+    }
+
+    public C fromInt(final int value) {
+        return fromInt.apply(value);
+    }
+
+    public C randomValue(final ExtendedRandom random) {
+        return this.random.apply(random);
+    }
+}

java/expression/common/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Expressions generators for expression-based homeworks
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package expression.common;

java/expression/exceptions/ExceptionsTest.java

@@ -0,0 +1,30 @@
+package expression.exceptions;
+
+import base.Selector;
+import expression.ListExpression;
+import expression.parser.Operations;
+
+import static expression.parser.Operations.*;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class ExceptionsTest {
+    private static final ExpressionParser PARSER = new ExpressionParser();
+    private static final Operations.Operation LIST = kind(ListExpression.KIND, PARSER::parse);
+
+    public static final Selector SELECTOR = Selector.composite(ExceptionsTest.class, ExceptionsTester::new, "easy", "hard")
+            .variant("Base", LIST, ADD, SUBTRACT, MULTIPLY, DIVIDE, NEGATE)
+            .variant("3637", POW, LOG)
+            .variant("3839", POW, LOG, POW_2, LOG_2)
+            .variant("3435", POW_2, LOG_2)
+            .variant("3233", HIGH, LOW)
+            .selector();
+
+    private ExceptionsTest() {
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/expression/exceptions/ExceptionsTestSet.java

@@ -0,0 +1,162 @@
+package expression.exceptions;
+
+import base.Functional;
+import base.Named;
+import base.Pair;
+import expression.ToMiniString;
+import expression.Variable;
+import expression.common.ExpressionKind;
+import expression.parser.ParserTestSet;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.LongBinaryOperator;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ExceptionsTestSet<E extends ToMiniString, C> extends ParserTestSet<E, C> {
+    private static final int D = 5;
+    private static final List<Integer> OVERFLOW_VALUES = new ArrayList<>();
+    private final char[] CHARS = "AZ+-*%()[]<>".toCharArray();
+
+    static {
+        Functional.addRange(OVERFLOW_VALUES, D, Integer.MIN_VALUE + D);
+        Functional.addRange(OVERFLOW_VALUES, D, Integer.MIN_VALUE / 2);
+        Functional.addRange(OVERFLOW_VALUES, D, (int) -Math.sqrt(Integer.MAX_VALUE));
+        Functional.addRange(OVERFLOW_VALUES, D, 0);
+        Functional.addRange(OVERFLOW_VALUES, D, (int) Math.sqrt(Integer.MAX_VALUE));
+        Functional.addRange(OVERFLOW_VALUES, D, Integer.MAX_VALUE / 2);
+        Functional.addRange(OVERFLOW_VALUES, D, Integer.MAX_VALUE - D);
+    }
+
+    private final List<Named<String>> parsingTest;
+
+    public ExceptionsTestSet(final ExceptionsTester tester, final ParsedKind<E, C> kind) {
+        super(tester, kind, false);
+        parsingTest = tester.parsingTest;
+    }
+
+    private void testParsingErrors() {
+        counter.testForEach(parsingTest, op -> {
+            final List<String> names = Functional.map(kind.kind().variables().generate(counter.random(), 3), Pair::first);
+            final String expr = mangle(op.value(), names);
+            try {
+                kind.parse(expr, names);
+                counter.fail("Successfully parsed '%s'", op.value());
+            } catch (final Exception e) {
+                counter.format("%-30s %s%n", op.name(), e.getClass().getSimpleName() + ": " + e.getMessage());
+            }
+        });
+    }
+
+    private void testOverflow() {
+        final List<Pair<String, E>> variables = kind.kind().variables().generate(counter.random(), 3);
+        final List<String> names = Functional.map(variables, Pair::first);
+        final Variable vx = (Variable) variables.get(0).second();
+        final Variable vy = (Variable) variables.get(1).second();
+
+        //noinspection Convert2MethodRef
+        testOverflow(names, (a, b) -> a + b, "+", new CheckedAdd(vx, vy));
+        testOverflow(names, (a, b) -> a - b, "-", new CheckedSubtract(vx, vy));
+        testOverflow(names, (a, b) -> a * b, "*", new CheckedMultiply(vx, vy));
+        testOverflow(names, (a, b) -> b == 0 ? Long.MAX_VALUE : a / b, "/", new CheckedDivide(vx, vy));
+        testOverflow(names, (a, b) -> -b, "<- ignore first argument, unary -", new CheckedNegate(vy));
+    }
+
+    private void testOverflow(final List<String> names, final LongBinaryOperator f, final String op, final Object expression) {
+        final ExpressionKind<E, C> kind = this.kind.kind();
+        for (final int a : OVERFLOW_VALUES) {
+            for (final int b : OVERFLOW_VALUES) {
+                final long expected = f.applyAsLong(a, b);
+                final boolean isInt = Integer.MIN_VALUE <= expected && expected <= Integer.MAX_VALUE;
+                try {
+                    final C actual = kind.evaluate(
+                            kind.cast(expression),
+                            names,
+                            kind.fromInts(List.of(a, b, 0))
+                    );
+                    counter.checkTrue(
+                            isInt && kind.fromInt((int) expected).equals(actual),
+                            "%d %s %d == %d", a, op, b, actual
+                    );
+                } catch (final Exception e) {
+                    if (isInt) {
+                        counter.fail(e, "Unexpected error in %d %s %d", a, op, b);
+                    }
+                }
+            }
+        }
+    }
+
+    @Override
+    protected void test() {
+        counter.scope("Overflow tests", (Runnable) this::testOverflow);
+        super.test();
+        counter.scope("Parsing error tests", this::testParsingErrors);
+    }
+
+
+    @Override
+    protected E parse(final String expression, final List<String> variables, final boolean reparse) {
+        final String expr = expression.strip();
+        if (expr.length() > 10) {
+            for (final char ch : CHARS) {
+                for (int i = 0; i < 10; i++) {
+                    final int index = 1 + tester.random().nextInt(expr.length() - 2);
+                    int pi = index - 1;
+                    while (Character.isWhitespace(expr.charAt(pi))) {
+                        pi--;
+                    }
+                    int ni = index;
+                    while (Character.isWhitespace(expr.charAt(ni))) {
+                        ni++;
+                    }
+                    final char pc = expr.charAt(pi);
+                    final char nc = expr.charAt(ni);
+                    if (
+                            "-([{*∛√²³₂₃!‖⎵⎴⌊⌈=?".indexOf(nc) < 0 &&
+                            (!Character.isLetterOrDigit(pc) || !Character.isLetterOrDigit(ch)) &&
+                            nc != ch && pc != ch &&
+                            !Character.isLetterOrDigit(nc) && nc != '$'
+                    ) {
+                        shouldFail(
+                                variables,
+                                "Parsing error expected for " + insert(expr, index, "<ERROR_INSERTED -->" + ch + "<-- ERROR_INSERTED>"),
+                                insert(expr, index, String.valueOf(ch))
+                        );
+                        break;
+                    }
+                }
+            }
+            parens(variables, expr, '[', ']');
+            parens(variables, expr, '{', '}');
+        }
+
+        return counter.testV(() -> counter.call("parse", () -> kind.parse(expr, variables)));
+    }
+
+    private static String insert(final String expr, final int index, final String value) {
+        return expr.substring(0, index) + value + expr.substring(index);
+    }
+
+    private void parens(final List<String> variables, final String expr, final char open, final char close) {
+        if (expr.indexOf(open) >= 0) {
+            replaces(variables, expr, open, '(');
+            replaces(variables, expr, close, ')');
+            if (expr.indexOf('(') >= 0) {
+                replaces(variables, expr, '(', open);
+                replaces(variables, expr, ')', close);
+            }
+        }
+    }
+
+    private void replaces(final List<String> variables, final String expr, final char what, final char by) {
+        final String input = expr.replace(what, by);
+        shouldFail(variables, "Unmatched parentheses: " + input, input);
+    }
+
+    private void shouldFail(final List<String> variables, final String message, final String input) {
+        counter.shouldFail(message, () -> kind.parse(input, variables));
+    }
+}

java/expression/exceptions/ExceptionsTester.java

@@ -0,0 +1,100 @@
+package expression.exceptions;
+
+import base.Named;
+import base.TestCounter;
+import expression.common.Reason;
+import expression.parser.ParserTestSet;
+import expression.parser.ParserTester;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.BiFunction;
+import java.util.function.LongBinaryOperator;
+import java.util.function.LongToIntFunction;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ExceptionsTester extends ParserTester {
+    /* package-private */ final List<Named<String>> parsingTest = new ArrayList<>(List.of(
+            Named.of("No first argument", "* $y * $z"),
+            Named.of("No middle argument", "$x *  * $z"),
+            Named.of("No last argument", "$x * $y * "),
+            Named.of("No first argument'", "1 + (* $y * $z) + 2"),
+            Named.of("No middle argument'", "1 + ($x *  / 9) + 3"),
+            Named.of("No last argument'", "1 + ($x * $y - ) + 3"),
+            Named.of("No opening parenthesis", "$x * $y)"),
+            Named.of("No closing parenthesis", "($x * $y"),
+            Named.of("Mismatched closing parenthesis", "($x * $y]"),
+            Named.of("Mismatched open parenthesis", "[$x * $y)"),
+            Named.of("Start symbol", "@$x * $y"),
+            Named.of("Middle symbol", "$x @ * $y"),
+            Named.of("End symbol", "$x * $y@"),
+            Named.of("Constant overflow 1", Integer.MIN_VALUE - 1L + ""),
+            Named.of("Constant overflow 2", Integer.MAX_VALUE + 1L + ""),
+            Named.of("Bare +", "+"),
+            Named.of("Bare -", "-"),
+            Named.of("Bare a", "a"),
+            Named.of("(())", "(())"),
+            Named.of("Spaces in numbers", "10 20")
+    ));
+
+    public ExceptionsTester(final TestCounter counter) {
+        super(counter);
+    }
+
+
+    private void parsingTests(final String... tests) {
+        for (final String test : tests) {
+            parsingTest.add(Named.of(test, test));
+        }
+    }
+
+    @Override
+    public void unary(final String name, final int priority, final BiFunction<Long, LongToIntFunction, Long> op) {
+        if (allowed(name)) {
+            parsingTests(name, "1 * " + name, name + " * 1");
+        }
+        parsingTests(name + "()", name + "(1, 2)");
+        if (name.length() > 1) {
+            parsingTests(name + "q");
+        }
+        if (allLetterAndDigit(name)) {
+            parsingTests(name + "1", name + "q");
+        }
+        super.unary(name, priority, op);
+    }
+
+    private static boolean allowed(final String name) {
+        return !"xyz".contains(name.substring(0, 1)) && !"xyz".contains(name.substring(name.length() - 1));
+    }
+
+    @Override
+    public void binary(final String name, final int priority, final LongBinaryOperator op) {
+        if (allowed(name)) {
+            parsingTests(name);
+        }
+        parsingTests("1 " + name, "1 " + name + " * 3");
+        if (!"-".equals(name)) {
+            parsingTests(name + " 1", "1 * " + name + " 2");
+        }
+        if (allLetterAndDigit(name)) {
+            parsingTests("5" + name + "5", "5 " + name + "5", "5 " + name + "5 5", "1" + name + "x 1", "1 " + name + "x 1");
+        }
+        super.binary(name, priority, op);
+    }
+
+    private static boolean allLetterAndDigit(final String name) {
+        return name.chars().allMatch(Character::isLetterOrDigit);
+    }
+
+    @Override
+    protected void test(final ParserTestSet.ParsedKind<?, ?> kind) {
+        new ExceptionsTestSet<>(this, kind).test();
+    }
+
+    @Override
+    protected int cast(final long value) {
+        return Reason.overflow(value);
+    }
+}

java/expression/exceptions/ListParser.java

@@ -0,0 +1,13 @@
+package expression.exceptions;
+
+import expression.ListExpression;
+
+import java.util.List;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@FunctionalInterface
+public interface ListParser {
+    ListExpression parse(String expression, final List<String> variables) throws Exception;
+}

java/expression/exceptions/package-info.java

@@ -0,0 +1,8 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#expressions-exceptions">Expression Error Handling</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package expression.exceptions;

java/expression/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#expressions">Expressions</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package expression;

java/expression/parser/ListParser.java

@@ -0,0 +1,13 @@
+package expression.parser;
+
+import expression.ListExpression;
+
+import java.util.List;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+@FunctionalInterface
+public interface ListParser {
+    ListExpression parse(String expression, List<String> variables);
+}

java/expression/parser/Operations.java

@@ -0,0 +1,148 @@
+package expression.parser;
+
+import expression.ToMiniString;
+import expression.common.ExpressionKind;
+import expression.common.Reason;
+
+import java.math.BigInteger;
+import java.util.function.*;
+import java.util.stream.LongStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class Operations {
+    // === Base
+
+    public static final Operation NEGATE = unary("-", 1, a -> -a);
+    @SuppressWarnings("Convert2MethodRef")
+    public static final Operation ADD       = binary("+", 1600, (a, b) -> a + b);
+    public static final Operation SUBTRACT  = binary("-", 1602, (a, b) -> a - b);
+    public static final Operation MULTIPLY  = binary("*", 2001, (a, b) -> a * b);
+    public static final Operation DIVIDE    = binary("/", 2002, (a, b) -> b == 0 ? Reason.DBZ.error() : a / b);
+
+
+    // === MinMax
+    public static final Operation MIN = binary("min", 401, Math::min);
+    public static final Operation MAX = binary("max", 401, Math::max);
+
+
+    // === Reverse
+
+    private static Operation digits(final String name, final boolean mask, final int r, final LongBinaryOperator q) {
+        return unary(name, 1, v -> LongStream.iterate(mask ? v & 0xffff_ffffL : v, n -> n != 0, n -> n / r)
+                .map(n -> n % r)
+                .reduce(0, q));
+    }
+
+    public static final Operation REVERSE = digits("reverse", false, 10, (a, b) -> a * 10 + b);
+
+
+    // === Digits
+    public static final Operation DIGITS = digits("digits", false, 10, Long::sum);
+
+
+    // === Floor and Ceiling
+
+    private static long floor(final long a) {
+        return (a >= 0 ? a : a - FLOOR_CEILING_STEP + 1) / FLOOR_CEILING_STEP * FLOOR_CEILING_STEP;
+    }
+
+    private static long ceiling(final long a) {
+        return (a >= 0 ? a + FLOOR_CEILING_STEP - 1: a) / FLOOR_CEILING_STEP * FLOOR_CEILING_STEP;
+    }
+
+    public static final int FLOOR_CEILING_STEP = 1000;
+    public static final Operation FLOOR = unary("floor", 1, Operations::floor);
+    public static final Operation CEILING = unary("ceiling", 1, Operations::ceiling);
+
+    // === Set, Clear
+
+    @SuppressWarnings("IntegerMultiplicationImplicitCastToLong")
+    public static final Operation SET = binary("set", 202, (a, b) -> a | (1 << b));
+    @SuppressWarnings("IntegerMultiplicationImplicitCastToLong")
+    public static final Operation CLEAR = binary("clear", 202, (a, b) -> a & ~(1 << b));
+
+
+    // === Pow, Log
+    public static final Operation POW_O = binary("**", 2402, (a, b) ->
+            b < 0 ? 1 : BigInteger.valueOf(a).modPow(BigInteger.valueOf(b), BigInteger.valueOf(1L << 32)).intValue());
+    public static final Operation LOG_O = binary("//", 2402, (a, b) ->
+            a == 0 && b > 0 ? Integer.MIN_VALUE :
+                    a <= 0 || b <= 0 || a == 1 && b == 1 ? 0 :
+                            a > 1 && b == 1 ? Integer.MAX_VALUE
+                                    : LongStream.iterate(b, v -> v <= a, v -> v * b).count()
+    );
+
+    private static final Reason INVALID_POW = new Reason("Invalid power");
+    public static final Operation POW = binary("**", 2402, Operations::powC);
+
+    private static long powC(final long a, final long b) {
+        if (b < 0 || a == 0 && b == 0) {
+            return INVALID_POW.error();
+        }
+        if (Math.abs(a) > 1 && b > 32) {
+            return Reason.OVERFLOW.error();
+        }
+        final BigInteger result = BigInteger.valueOf(a).pow((int) b);
+        if (result.compareTo(BigInteger.valueOf(Integer.MIN_VALUE)) < 0 || BigInteger.valueOf(Integer.MAX_VALUE).compareTo(result) < 0) {
+            return Reason.OVERFLOW.error();
+        }
+        return result.intValue();
+    }
+
+    private static final Reason INVALID_LOG = new Reason("Invalid log");
+    public static final Operation LOG = binary("//", 2402, (a, b) ->
+            a <= 0 || b <= 1 ? INVALID_LOG.error() : (int) (Math.log(a) / Math.log(b)));
+
+
+    // Pow2, Log2
+
+    private static final Reason NEG_LOG = new Reason("Logarithm of negative value");
+    public static final Operation LOG_2
+            = unary("log₂", 1, NEG_LOG.less(1, a-> (long) (Math.log(a) / Math.log(2))));
+
+    private static final Reason NEG_POW = new Reason("Exponentiation to negative power");
+    public static final Operation POW_2
+            = unary("pow₂", 1, NEG_POW.less(0, Reason.OVERFLOW.greater(31, a -> (long) Math.pow(2, a))));
+
+
+    // === High, Low
+    public static final Operation HIGH = unary("high", 1, v -> Integer.highestOneBit((int) v));
+    public static final Operation LOW = unary("low", 1, v -> Integer.lowestOneBit((int) v));
+
+    // === Common
+
+    private Operations() {
+    }
+
+    public static Operation unary(final String name, final int priority, final LongUnaryOperator op) {
+        return unary(name, priority, (a, c) -> op.applyAsLong(a));
+    }
+
+    public static Operation unary(final String left, final String right, final LongUnaryOperator op) {
+        return unary(left, right, (a, c) -> op.applyAsLong(a));
+    }
+
+    public static Operation unary(final String name, final int priority, final BiFunction<Long, LongToIntFunction, Long> op) {
+        return tests -> tests.unary(name, priority, op);
+    }
+
+    public static Operation unary(final String left, final String right, final BiFunction<Long, LongToIntFunction, Long> op) {
+        return tests -> tests.unary(left, right, op);
+    }
+
+    public static Operation binary(final String name, final int priority, final LongBinaryOperator op) {
+        return tests -> tests.binary(name, priority, op);
+    }
+
+    public static <E extends ToMiniString, C> Operation kind(
+            final ExpressionKind<E, C> kind,
+            final ParserTestSet.Parser<E> parser
+    ) {
+        return factory -> factory.kind(kind, parser);
+    }
+
+    @FunctionalInterface
+    public interface Operation extends Consumer<ParserTester> {}
+}

java/expression/parser/ParserTest.java

@@ -0,0 +1,31 @@
+package expression.parser;
+
+import base.Selector;
+import expression.ListExpression;
+
+import static expression.parser.Operations.*;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class ParserTest {
+    private static final ExpressionParser PARSER = new ExpressionParser();
+    private static final Operations.Operation LIST = kind(ListExpression.KIND, PARSER::parse);
+
+    // === Common
+
+    public static final Selector SELECTOR = Selector.composite(ParserTest.class, ParserTester::new, "easy", "hard")
+            .variant("Base", LIST, ADD, SUBTRACT, MULTIPLY, DIVIDE, NEGATE)
+            .variant("3637", MIN, MAX, REVERSE)
+            .variant("3839", MIN, MAX, REVERSE, DIGITS)
+            .variant("3435", FLOOR, CEILING, SET, CLEAR)
+            .variant("3233", FLOOR, CEILING)
+            .selector();
+
+    private ParserTest() {
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/expression/parser/ParserTestSet.java

@@ -0,0 +1,236 @@
+package expression.parser;
+
+import base.*;
+import expression.ToMiniString;
+import expression.common.*;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Consumer;
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ParserTestSet<E extends ToMiniString, C> {
+    private static final int D = 5;
+
+    private static final List<Integer> TEST_VALUES = new ArrayList<>();
+    static {
+        Functional.addRange(TEST_VALUES, D, D);
+        Functional.addRange(TEST_VALUES, D, -D);
+    }
+
+    public static final List<Integer> CONSTS
+            = List.of(0, 1, -1, 4, -4, 10, -10, 30, -30, 100, -100, Integer.MAX_VALUE, Integer.MIN_VALUE);
+
+    protected final ParserTester tester;
+    protected final ParsedKind<E, C> kind;
+    private final boolean safe;
+
+    protected final TestCounter counter;
+
+    public ParserTestSet(final ParserTester tester, final ParsedKind<E, C> kind) {
+        this(tester, kind, true);
+    }
+
+    protected ParserTestSet(final ParserTester tester, final ParsedKind<E, C> kind, final boolean safe) {
+        this.tester = tester;
+        this.kind = kind;
+        this.safe = safe;
+
+        counter = tester.getCounter();
+    }
+
+    private void examples(final TestGenerator<Integer, E> generator) {
+        example(generator, "$x+2", (x, y, z) -> x + 2);
+        example(generator, "2-$y", (x, y, z) -> 2 - y);
+        example(generator, "  3*  $z  ", (x, y, z) -> 3 * z);
+        example(generator, "$x/  -  2", (x, y, z) -> -x / 2);
+        example(generator, "$x*$y+($z-1   )/10", (x, y, z) -> x * y + (int) (z - 1) / 10);
+        example(generator, "-(-(-\t\t-5 + 16   *$x*$y) + 1 * $z) -(((-11)))", (x, y, z) -> -(-(5 + 16 * x * y) + z) + 11);
+        example(generator, "" + Integer.MAX_VALUE, (x, y, z) -> (long) Integer.MAX_VALUE);
+        example(generator, "" + Integer.MIN_VALUE, (x, y, z) -> (long) Integer.MIN_VALUE);
+        example(generator, "$x--$y--$z", (x, y, z) -> x + y + z);
+        example(generator, "((2+2))-0/(--2)*555", (x, y, z) -> 4L);
+        example(generator, "$x-$x+$y-$y+$z-($z)", (x, y, z) -> 0L);
+        example(generator, "(".repeat(300) + "$x + $y + (-10*-$z)" + ")".repeat(300), (x, y, z) -> x + y + 10 * z);
+        example(generator, "$x / $y / $z", (x, y, z) -> y == 0 || z == 0 ? Reason.DBZ.error() : (int) x / (int) y / z);
+    }
+
+    private void example(final TestGenerator<Integer, E> generator, final String expr, final ExampleExpression expression) {
+        final List<String> names = Functional.map(generator.variables(3), Pair::first);
+        final TExpression expected = vars -> expression.evaluate(vars.get(0), vars.get(1), vars.get(2));
+
+        counter.test(() -> {
+            final String mangled = mangle(expr, names);
+            final E parsed = parse(mangled, names, true);
+            Functional.allValues(TEST_VALUES, 3).forEach(values -> check(expected, parsed, names, values, mangled));
+        });
+    }
+
+    protected static String mangle(final String expr, final List<String> names) {
+        return expr
+                .replace("$x", names.get(0))
+                .replace("$y", names.get(1))
+                .replace("$z", names.get(2));
+    }
+
+    protected void test() {
+        final TestGenerator<Integer, E> generator = tester.generator.build(kind.kind.variables());
+        final Renderer<Integer, Unit, TExpression> renderer = tester.renderer.build();
+        final Consumer<TestGenerator.Test<Integer, E>> consumer = test -> test(renderer, test);
+        counter.scope("Basic tests", () -> generator.testBasic(consumer));
+        counter.scope("Handmade tests", () -> examples(generator));
+        counter.scope("Random tests", () -> generator.testRandom(counter, 1, consumer));
+    }
+
+    private void test(final Renderer<Integer, Unit, TExpression> renderer, final TestGenerator.Test<Integer, E> test) {
+        final Expr<Integer, E> expr = test.expr;
+        final List<Pair<String, E>> vars = expr.variables();
+        final List<String> variables = Functional.map(vars, Pair::first);
+        final String full = test.render(NodeRenderer.FULL);
+        final String mini = test.render(NodeRenderer.MINI);
+
+        final E fullParsed = parse(test, variables, NodeRenderer.FULL);
+        final E miniParsed = parse(test, variables, NodeRenderer.MINI);
+        final E safeParsed = parse(test, variables, NodeRenderer.SAME);
+
+        checkToString(full, mini, "base", fullParsed);
+        if (tester.mode() > 0) {
+            counter.test(() -> Asserts.assertEquals("mini.toMiniString", mini, miniParsed.toMiniString()));
+            counter.test(() -> Asserts.assertEquals("safe.toMiniString", mini, safeParsed.toMiniString()));
+        }
+        checkToString(full, mini, "extraParentheses", parse(test, variables, NodeRenderer.FULL_EXTRA));
+        checkToString(full, mini, "noSpaces", parse(removeSpaces(full), variables, false));
+        checkToString(full, mini, "extraSpaces", parse(extraSpaces(full), variables, false));
+
+        final TExpression expected = renderer.render(
+                Expr.of(
+                        expr.node(),
+                        Functional.map(vars, (i, var) -> Pair.of(var.first(), args -> args.get(i)))
+                ),
+                Unit.INSTANCE
+        );
+
+        check(expected, fullParsed, variables, tester.random().random(variables.size(), ExtendedRandom::nextInt), full);
+        if (this.safe) {
+            final String safe = test.render(NodeRenderer.SAME);
+            check(expected, safeParsed, variables, tester.random().random(variables.size(), ExtendedRandom::nextInt), safe);
+        }
+    }
+
+    private E parse(
+            final TestGenerator.Test<Integer, E> test,
+            final List<String> variables,
+            final NodeRenderer.Settings settings
+    ) {
+        return parse(test.render(settings.withParens(tester.parens)), variables, false);
+    }
+
+    private static final String LOOKBEHIND = "(?<![a-zA-Z0-9<>*/+=!-])";
+    private static final String LOOKAHEAD = "(?![a-zA-Z0-9<>*/])";
+    private static final Pattern SPACES = Pattern.compile(LOOKBEHIND + " | " + LOOKAHEAD + "|" + LOOKAHEAD + LOOKBEHIND);
+    private String extraSpaces(final String expression) {
+        return SPACES.matcher(expression).replaceAll(r -> tester.random().randomString(
+                ExtendedRandom.SPACES,
+                tester.random().nextInt(5)
+        ));
+    }
+
+    private static String removeSpaces(final String expression) {
+        return SPACES.matcher(expression).replaceAll("");
+    }
+
+    private void checkToString(final String full, final String mini, final String context, final ToMiniString parsed) {
+        counter.test(() -> {
+            assertEquals(context + ".toString", full, full, parsed.toString());
+            if (tester.mode() > 0) {
+                assertEquals(context + ".toMiniString", full, mini, parsed.toMiniString());
+            }
+        });
+    }
+
+    private static void assertEquals(
+            final String context,
+            final String original,
+            final String expected,
+            final String actual
+    ) {
+        final String message = String.format("%s:%n     original `%s`,%n     expected `%s`,%n       actual `%s`",
+                context, original, expected, actual);
+        Asserts.assertTrue(message, Objects.equals(expected, actual));
+    }
+
+    private Either<Reason, Integer> eval(final TExpression expression, final List<Integer> vars) {
+        return Reason.eval(() -> tester.cast(expression.evaluate(vars)));
+    }
+
+    protected E parse(final String expression, final List<String> variables, final boolean reparse) {
+        return counter.testV(() -> {
+            final E parsed = counter.testV(() -> counter.call("parse",
+                    () -> kind.parse(expression, variables)));
+            if (reparse) {
+                counter.testV(() -> counter.call("parse", () -> kind.parse(parsed.toString(), variables)));
+            }
+            return parsed;
+        });
+    }
+
+    private void check(
+            final TExpression expectedExpression,
+            final E expression,
+            final List<String> variables,
+            final List<Integer> values,
+            final String unparsed
+    ) {
+        counter.test(() -> {
+            final Either<Reason, Integer> answer = eval(expectedExpression, values);
+            final String args = IntStream.range(0, variables.size())
+                    .mapToObj(i -> variables.get(i) + "=" + values.get(i))
+                    .collect(Collectors.joining(", "));
+            final String message = String.format("f(%s)%n\twhere f=%s%n\tyour f=%s", args, unparsed, expression);
+            try {
+                final C actual = kind.kind.evaluate(expression, variables, kind.kind.fromInts(values));
+                counter.checkTrue(answer.isRight(), "Error expected for f(%s)%n\twhere f=%s%n\tyour f=%s", args, unparsed, expression);
+                Asserts.assertEquals(message, answer.getRight(), actual);
+            } catch (final Exception e) {
+                if (answer.isRight()) {
+                    counter.fail(e, "No error expected for %s", message);
+                }
+            }
+        });
+    }
+
+    @FunctionalInterface
+    public interface TExpression {
+        long evaluate(List<Integer> vars);
+    }
+
+    @FunctionalInterface
+    protected interface ExampleExpression {
+        long evaluate(long x, long y, long z);
+    }
+
+    /**
+     * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+     */
+    public record ParsedKind<E extends ToMiniString, C>(ExpressionKind<E, C> kind, Parser<E> parser) {
+        public E parse(final String expression, final List<String> variables) throws Exception {
+            return parser.parse(expression, variables);
+        }
+
+        @Override
+        public String toString() {
+            return kind.toString();
+        }
+    }
+
+    @FunctionalInterface
+    public interface Parser<E> {
+        E parse(final String expression, final List<String> variables) throws Exception;
+    }
+}

java/expression/parser/ParserTester.java

@@ -0,0 +1,76 @@
+package expression.parser;
+
+import base.ExtendedRandom;
+import base.TestCounter;
+import base.Tester;
+import base.Unit;
+import expression.ToMiniString;
+import expression.common.*;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.BiFunction;
+import java.util.function.LongBinaryOperator;
+import java.util.function.LongToIntFunction;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ParserTester extends Tester {
+    /* package-private */ final TestGeneratorBuilder<Integer> generator;
+    /* package-private */ final Renderer.Builder<Integer, Unit, ParserTestSet.TExpression> renderer;
+    private final List<ParserTestSet.ParsedKind<?, ?>> kinds = new ArrayList<>();
+    /* package-private */  final List<NodeRenderer.Paren> parens = new ArrayList<>(List.of(NodeRenderer.paren("(", ")")));
+
+    public ParserTester(final TestCounter counter) {
+        super(counter);
+        renderer = Renderer.builder(c -> vars -> c);
+        final ExtendedRandom random = counter.random();
+        generator = new TestGeneratorBuilder<>(random, random::nextInt, ParserTestSet.CONSTS, true);
+    }
+
+    public void unary(final String name, final int priority, final BiFunction<Long, LongToIntFunction, Long> op) {
+        generator.unary(name, priority);
+        renderer.unary(name, (unit, a) -> vars -> cast(op.apply(a.evaluate(vars), this::cast)));
+    }
+
+    public void unary(final String left, final String right, final BiFunction<Long, LongToIntFunction, Long> op) {
+        generator.unary(left, right);
+        renderer.unary(left, (unit, a) -> vars -> cast(op.apply(a.evaluate(vars), this::cast)));
+    }
+
+    public void binary(final String name, final int priority, final LongBinaryOperator op) {
+        generator.binary(name, priority);
+        renderer.binary(name, (unit, a, b) -> vars -> cast(op.applyAsLong(a.evaluate(vars), b.evaluate(vars))));
+    }
+
+    <E extends ToMiniString, C> void kind(final ExpressionKind<E, C> kind, final ParserTestSet.Parser<E> parser) {
+        kinds.add(new ParserTestSet.ParsedKind<>(kind, parser));
+    }
+
+    @Override
+    public void test() {
+        for (final ParserTestSet.ParsedKind<?, ?> kind : kinds) {
+            counter.scope(kind.toString(), () -> test(kind));
+        }
+    }
+
+    protected void test(final ParserTestSet.ParsedKind<?, ?> kind) {
+        new ParserTestSet<>(this, kind).test();
+    }
+
+    public TestCounter getCounter() {
+        return counter;
+    }
+
+    protected int cast(final long value) {
+        return (int) value;
+    }
+
+    public void parens(final String... parens) {
+        assert parens.length % 2 == 0 : "Parens should come in pairs";
+        for (int i = 0; i < parens.length; i += 2) {
+            this.parens.add(NodeRenderer.paren(parens[i], parens[i + 1]));
+        }
+    }
+}

java/expression/parser/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#expressions-parsing">Expressions Parsing</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package expression.parser;

java/markup/MarkupTester.java

@@ -0,0 +1,71 @@
+package markup;
+
+import base.Asserts;
+import base.BaseChecker;
+import base.TestCounter;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+import java.util.Map;
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class MarkupTester {
+    private final Map<String, String> mapping;
+    private final String toString;
+
+    private MarkupTester(final Map<String, String> mapping, final String toString) {
+        this.mapping = mapping;
+        this.toString = toString;
+    }
+
+    public static Consumer<TestCounter> variant(final Consumer<Checker> checker, final String name, final Map<String, String> mapping) {
+        return counter -> test(checker).accept(new MarkupTester(mapping, "to" + name), counter);
+    }
+
+    public static BiConsumer<MarkupTester, TestCounter> test(final Consumer<Checker> tester) {
+        return (checker, counter) -> tester.accept(checker.new Checker(counter));
+    }
+
+    @Override
+    public String toString() {
+        return toString;
+    }
+
+    public class Checker extends BaseChecker {
+        public Checker(final TestCounter counter) {
+            super(counter);
+        }
+
+        private <T> MethodHandle findMethod(final T value) {
+            try {
+                return MethodHandles.publicLookup().findVirtual(value.getClass(), toString, MethodType.methodType(void.class, StringBuilder.class));
+            } catch (final NoSuchMethodException | IllegalAccessException e) {
+                throw Asserts.error("Cannot find method 'void %s(StringBuilder)' for %s", toString, value.getClass());
+            }
+        }
+
+        public <T> void test(final T value, String expectedTemplate) {
+            final MethodHandle method = findMethod(value);
+            for (final Map.Entry<String, String> entry : mapping.entrySet()) {
+                expectedTemplate = expectedTemplate.replace(entry.getKey(), entry.getValue());
+            }
+
+            final String expected = expectedTemplate;
+            counter.println("Test " + counter.getTestNo());
+            counter.test(() -> {
+                final StringBuilder sb = new StringBuilder();
+                try {
+                    method.invoke(value, sb);
+                } catch (final Throwable e) {
+                    throw Asserts.error("%s(StringBuilder) for %s thrown exception: %s", toString, value.getClass(), e);
+                }
+                Asserts.assertEquals("Result", expected, sb.toString());
+            });
+        }
+    }
+}

java/markup/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#markup">Markup</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package markup;

java/md2html/Md2HtmlTest.java

@@ -0,0 +1,71 @@
+package md2html;
+
+import base.Selector;
+
+import java.util.function.Consumer;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class Md2HtmlTest {
+    // === 3637
+    private static final Consumer<? super Md2HtmlTester> INS = tester -> tester
+            .test("<<вставка>>", "<p><ins>вставка</ins></p>")
+            .test("Это <<вставка>>, вложенная в текст", "<p>Это <ins>вставка</ins>, вложенная в текст</p>")
+            .spoiled("Это не <<вставка>>", "<p>Это не &lt;&lt;вставка&gt;&gt;</p>", "<", ">")
+            .spoiled("Это не <<вставка>> 2", "<p>Это не &lt;&lt;вставка&gt;&gt; 2</p>", "<", ">")
+            .addElement("ins", "<<", ">>");
+    private static final Consumer<? super Md2HtmlTester> DEL = tester -> tester
+            .test("}}удаление{{", "<p><del>удаление</del></p>")
+            .test("Это }}удаление{{, вложенное в текст", "<p>Это <del>удаление</del>, вложенное в текст</p>")
+            .spoiled("Это не }}удаление{{", "<p>Это не }}удаление{{</p>", "{")
+            .spoiled("Это не }}удаление{{ 2", "<p>Это не }}удаление{{ 2</p>", "{")
+            .addElement("del", "}}", "{{");
+
+    // === 3839
+    private static final Consumer<? super Md2HtmlTester> PRE = tester -> tester
+            .test("```код __без__ форматирования```", "<p><pre>код __без__ форматирования</pre></p>")
+            .test(
+                    "Это не `\\``код __без__ форматирования``\\`",
+                    "<p>Это не <code>`</code>код <strong>без</strong> форматирования<code></code>`</p>"
+            )
+            .addElement("pre", "```", (checker, markup, input, output) -> {
+                final String contentS = checker.generateInput(markup).replace("`", "");
+
+                input.append("```").append(contentS).append("```");
+                output.append("<pre>").append(contentS.replace("<", "&lt;").replace(">", "")).append("</pre>");
+            });
+
+    // === 3435
+    private static final Consumer<? super Md2HtmlTester> SAMP = tester -> tester
+            .test("!!пример!!", "<p><samp>пример</samp></p>")
+            .test("Это !!пример!!, вложенный в текст", "<p>Это <samp>пример</samp>, вложенный в текст</p>")
+            .spoiled("Это не !!пример!!", "<p>Это не !!пример!!</p>", "!")
+            .spoiled("Это не !!пример!! 2", "<p>Это не !!пример!! 2</p>", "!")
+            .addElement("samp", "!!");
+
+    // === 3233
+    private static final Consumer<Md2HtmlTester> VAR = tester -> tester
+            .test("%переменная%", "<p><var>переменная</var></p>")
+            .test("Это %переменная%, вложенная в текст", "<p>Это <var>переменная</var>, вложенная в текст</p>")
+            .spoiled("Это не %переменная%", "<p>Это не %переменная%</p>", "%")
+            .spoiled("Это не %переменная% 2", "<p>Это не %переменная% 2</p>", "%")
+            .addElement("var", "%");
+
+    // === Common
+
+    public static final Selector SELECTOR = Selector.composite(Md2HtmlTest.class, c -> new Md2HtmlTester(), Md2HtmlTester::test)
+            .variant("Base")
+            .variant("3637", INS, DEL)
+            .variant("3839", PRE)
+            .variant("3435", SAMP)
+            .variant("3233", VAR)
+            .selector();
+
+    private Md2HtmlTest() {
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/md2html/Md2HtmlTester.java

@@ -0,0 +1,355 @@
+package md2html;
+
+import base.*;
+
+import java.util.*;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class Md2HtmlTester {
+    private static final Map<String, String> ESCAPES = Map.of("<", "&lt;", ">", "&gt;");
+
+    private final Map<String, Generator> elements = new HashMap<>();
+    private final Map<String, List<String>> tags = new LinkedHashMap<>();
+    private final List<Pair<String, String>> tests = new ArrayList<>();
+
+    public Md2HtmlTester() {
+        addElement("em", "*");
+        addElement("em", "_");
+        addElement("strong", "**");
+        addElement("strong", "__");
+        addElement("s", "--");
+        addElement("code", "`");
+
+        test(
+                "# Заголовок первого уровня\n\n",
+                "<h1>Заголовок первого уровня</h1>"
+        );
+        test(
+                "## Второго\n\n",
+                "<h2>Второго</h2>"
+        );
+        test(
+                "### Третьего ## уровня\n\n",
+                "<h3>Третьего ## уровня</h3>"
+        );
+        test(
+                "#### Четвертого\n# Все еще четвертого\n\n",
+                "<h4>Четвертого\n# Все еще четвертого</h4>"
+        );
+        test(
+                "Этот абзац текста\nсодержит две строки.",
+                "<p>Этот абзац текста\nсодержит две строки.</p>"
+        );
+        test(
+                "    # Может показаться, что это заголовок.\nНо нет, это абзац, начинающийся с `#`.\n\n",
+                "<p>    # Может показаться, что это заголовок.\nНо нет, это абзац, начинающийся с <code>#</code>.</p>"
+        );
+        test(
+                "#И это не заголовок.\n\n",
+                "<p>#И это не заголовок.</p>"
+        );
+        test(
+                "###### Заголовки могут быть многострочными\n(и с пропуском заголовков предыдущих уровней)\n\n",
+                "<h6>Заголовки могут быть многострочными\n(и с пропуском заголовков предыдущих уровней)</h6>"
+        );
+        test(
+                "Мы все любим *выделять* текст _разными_ способами.\n**Сильное выделение**, используется гораздо реже,\nно __почему бы и нет__?\nНемного --зачеркивания-- еще никому не вредило.\nКод представляется элементом `code`.\n\n",
+                "<p>Мы все любим <em>выделять</em> текст <em>разными</em> способами.\n<strong>Сильное выделение</strong>, используется гораздо реже,\nно <strong>почему бы и нет</strong>?\nНемного <s>зачеркивания</s> еще никому не вредило.\nКод представляется элементом <code>code</code>.</p>"
+        );
+        test(
+                "Обратите внимание, как экранируются специальные\nHTML-символы, такие как `<`, `>` и `&`.\n\n",
+                "<p>Обратите внимание, как экранируются специальные\nHTML-символы, такие как <code>&lt;</code>, <code>&gt;</code> и <code>&amp;</code>.</p>"
+        );
+        test(
+                "Экранирование должно работать во всех местах: <>&.\n\n",
+                "<p>Экранирование должно работать во всех местах: &lt;&gt;&amp;.</p>"
+        );
+        test(
+                "Знаете ли вы, что в Markdown, одиночные * и _\nне означают выделение?\nОни так же могут быть заэкранированы\nпри помощи обратного слэша: \\*.",
+                "<p>Знаете ли вы, что в Markdown, одиночные * и _\nне означают выделение?\nОни так же могут быть заэкранированы\nпри помощи обратного слэша: *.</p>"
+        );
+        test(
+                "\n\n\nЛишние пустые строки должны игнорироваться.\n\n\n\n",
+                "<p>Лишние пустые строки должны игнорироваться.</p>"
+        );
+        test(
+                "Любите ли вы *вложенные __выделения__* так,\nкак __--люблю--__ их я?",
+                "<p>Любите ли вы <em>вложенные <strong>выделения</strong></em> так,\nкак <strong><s>люблю</s></strong> их я?</p>"
+        );
+
+        test(
+                """
+                # Заголовок первого уровня
+
+                ## Второго
+
+                ### Третьего ## уровня
+
+                #### Четвертого
+                # Все еще четвертого
+
+                Этот абзац текста
+                содержит две строки.
+
+                    # Может показаться, что это заголовок.
+                Но нет, это абзац, начинающийся с `#`.
+
+                #И это не заголовок.
+
+                ###### Заголовки могут быть многострочными
+                (и с пропуском заголовков предыдущих уровней)
+
+                Мы все любим *выделять* текст _разными_ способами.
+                **Сильное выделение**, используется гораздо реже,
+                но __почему бы и нет__?
+                Немного --зачеркивания-- еще никому не вредило.
+                Код представляется элементом `code`.
+
+                Обратите внимание, как экранируются специальные
+                HTML-символы, такие как `<`, `>` и `&`.
+
+                Знаете ли вы, что в Markdown, одиночные * и _
+                не означают выделение?
+                Они так же могут быть заэкранированы
+                при помощи обратного слэша: \\*.
+
+
+
+                Лишние пустые строки должны игнорироваться.
+
+                Любите ли вы *вложенные __выделения__* так,
+                как __--люблю--__ их я?
+                """,
+            """
+                <h1>Заголовок первого уровня</h1>
+                <h2>Второго</h2>
+                <h3>Третьего ## уровня</h3>
+                <h4>Четвертого
+                # Все еще четвертого</h4>
+                <p>Этот абзац текста
+                содержит две строки.</p>
+                <p>    # Может показаться, что это заголовок.
+                Но нет, это абзац, начинающийся с <code>#</code>.</p>
+                <p>#И это не заголовок.</p>
+                <h6>Заголовки могут быть многострочными
+                (и с пропуском заголовков предыдущих уровней)</h6>
+                <p>Мы все любим <em>выделять</em> текст <em>разными</em> способами.
+                <strong>Сильное выделение</strong>, используется гораздо реже,
+                но <strong>почему бы и нет</strong>?
+                Немного <s>зачеркивания</s> еще никому не вредило.
+                Код представляется элементом <code>code</code>.</p>
+                <p>Обратите внимание, как экранируются специальные
+                HTML-символы, такие как <code>&lt;</code>, <code>&gt;</code> и <code>&amp;</code>.</p>
+                <p>Знаете ли вы, что в Markdown, одиночные * и _
+                не означают выделение?
+                Они так же могут быть заэкранированы
+                при помощи обратного слэша: *.</p>
+                <p>Лишние пустые строки должны игнорироваться.</p>
+                <p>Любите ли вы <em>вложенные <strong>выделения</strong></em> так,
+                как <strong><s>люблю</s></strong> их я?</p>
+                """
+        );
+
+        test("# Без перевода строки в конце", "<h1>Без перевода строки в конце</h1>");
+        test("# Один перевод строки в конце\n", "<h1>Один перевод строки в конце</h1>");
+        test("# Два перевода строки в конце\n\n", "<h1>Два перевода строки в конце</h1>");
+        test(
+                "Выделение может *начинаться на одной строке,\n а заканчиваться* на другой",
+                "<p>Выделение может <em>начинаться на одной строке,\n а заканчиваться</em> на другой</p>"
+        );
+        test("# *Выделение* и `код` в заголовках", "<h1><em>Выделение</em> и <code>код</code> в заголовках</h1>");
+    }
+
+    protected void addElement(final String tag, final String markup) {
+        addElement(tag, markup, markup);
+    }
+
+    protected void addElement(final String tag, final String begin, final String end) {
+        addElement(tag, begin, (checker, markup, input, output) -> {
+            checker.space(input, output);
+            input.append(begin);
+            open(output, tag);
+
+            checker.word(input, output);
+            checker.generate(markup, input, output);
+            checker.word(input, output);
+
+            input.append(end);
+            close(output, tag);
+            checker.space(input, output);
+        });
+    }
+
+    public void addElement(final String tag, final String begin, final Generator generator) {
+        Asserts.assertTrue("Duplicate element " + begin, elements.put(begin, generator) == null);
+        tags.computeIfAbsent(tag, k -> new ArrayList<>()).add(begin);
+    }
+
+    private final Runner runner = Runner.packages("md2html").files("Md2Html");
+
+    protected Md2HtmlTester test(final String input, final String output) {
+        tests.add(Pair.of(input, output));
+        return this;
+    }
+
+    protected Md2HtmlTester spoiled(final String input, final String output, final String... spoilers) {
+        for (final String spoiler : spoilers) {
+            final Indexer in = new Indexer(input, spoiler);
+            final Indexer out = new Indexer(output, ESCAPES.getOrDefault(spoiler, spoiler));
+            while (in.next() && out.next()) {
+                tests.add(Pair.of(in.cut(), out.cut()));
+                tests.add(Pair.of(in.escape(), output));
+            }
+        }
+        return this;
+    }
+
+    private static class Indexer {
+        private final String string;
+        private final String spoiler;
+        private int index = - 1;
+
+        public Indexer(final String string, final String spoiler) {
+            this.string = string;
+            this.spoiler = spoiler;
+        }
+
+        public boolean next() {
+            index = string.indexOf(spoiler, index + 1);
+            return index >= 0;
+        }
+
+        public String cut() {
+            return string.substring(0, index) + string.substring(index + spoiler.length());
+        }
+
+        public String escape() {
+            return string.substring(0, index) + "\\" + string.substring(index);
+        }
+    }
+
+    private static void open(final StringBuilder output, final String tag) {
+        output.append("<").append(tag).append(">");
+    }
+
+    private static void close(final StringBuilder output, final String tag) {
+        output.append("</").append(tag).append(">");
+    }
+
+    public void test(final TestCounter counter) {
+        counter.scope("Testing " + String.join(", ", tags.keySet()), () -> new Checker(counter).test());
+    }
+
+    public class Checker extends BaseChecker {
+        public Checker(final TestCounter counter) {
+            super(counter);
+        }
+
+        protected void test() {
+            for (final Pair<String, String> test : tests) {
+                test(test);
+            }
+
+            for (final String markup : elements.keySet()) {
+                randomTest(3, 10, List.of(markup));
+            }
+
+            final int d = TestCounter.DENOMINATOR;
+            for (int i = 0; i < 10; i++) {
+                randomTest(100, 1000, randomMarkup());
+            }
+            randomTest(100, 100_000 / d, randomMarkup());
+        }
+
+        private void test(final Pair<String, String> test) {
+            runner.testEquals(counter, Arrays.asList(test.first().split("\n")), Arrays.asList(test.second().split("\n")));
+        }
+
+        private List<String> randomMarkup() {
+            return Functional.map(tags.values(), random()::randomItem);
+        }
+
+        private void randomTest(final int paragraphs, final int length, final List<String> markup) {
+            final StringBuilder input = new StringBuilder();
+            final StringBuilder output = new StringBuilder();
+            emptyLines(input);
+            final List<String> markupList = new ArrayList<>(markup);
+            for (int i = 0; i < paragraphs; i++) {
+                final StringBuilder inputSB = new StringBuilder();
+                paragraph(length, inputSB, output, markupList);
+                input.append(inputSB);
+                emptyLines(input);
+            }
+            test(Pair.of(input.toString(), output.toString()));
+        }
+
+        private void paragraph(final int length, final StringBuilder input, final StringBuilder output, final List<String> markup) {
+            final int h = random().nextInt(0, 6);
+            final String tag = h == 0 ? "p" : "h" + h;
+            if (h > 0) {
+                input.append(new String(new char[h]).replace('\0', '#')).append(" ");
+            }
+
+            open(output, tag);
+            while (input.length() < length) {
+                generate(markup, input, output);
+                final String middle = random().randomString(ExtendedRandom.ENGLISH);
+                input.append(middle).append("\n");
+                output.append(middle).append("\n");
+            }
+            output.setLength(output.length() - 1);
+            close(output, tag);
+
+            output.append("\n");
+            input.append("\n");
+        }
+
+        private void space(final StringBuilder input, final StringBuilder output) {
+            if (random().nextBoolean()) {
+                final String space = random().nextBoolean() ? " " : "\n";
+                input.append(space);
+                output.append(space);
+            }
+        }
+
+        public void generate(final List<String> markup, final StringBuilder input, final StringBuilder output) {
+            word(input, output);
+            if (markup.isEmpty()) {
+                return;
+            }
+            final String type = random().randomItem(markup);
+
+            markup.remove(type);
+            elements.get(type).generate(this, markup, input, output);
+            markup.add(type);
+        }
+
+        protected void word(final StringBuilder input, final StringBuilder output) {
+            final String word = random().randomString(random().randomItem(ExtendedRandom.ENGLISH, ExtendedRandom.GREEK, ExtendedRandom.RUSSIAN));
+            input.append(word);
+            output.append(word);
+        }
+
+        private void emptyLines(final StringBuilder sb) {
+            while (random().nextBoolean()) {
+                sb.append('\n');
+            }
+        }
+
+        String generateInput(final List<String> markup) {
+            final StringBuilder sb = new StringBuilder();
+            generate(markup, sb, new StringBuilder());
+            return sb.toString()
+                    .replace("<", "")
+                    .replace(">", "")
+                    .replace("]", "");
+        }
+    }
+
+    @FunctionalInterface
+    public interface Generator {
+        void generate(Checker checker, List<String> markup, StringBuilder input, StringBuilder output);
+    }
+}

java/md2html/package-info.java

@@ -0,0 +1,8 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#md2html">Markdown to HTML</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package md2html;

java/reverse/FastReverseTest.java

@@ -0,0 +1,87 @@
+package reverse;
+
+import base.ExtendedRandom;
+import base.Named;
+import base.Selector;
+import base.TestCounter;
+import wordStat.WordStatTest;
+
+import java.util.Arrays;
+import java.util.function.BiFunction;
+import java.util.function.LongBinaryOperator;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class FastReverseTest {
+    // === 3637
+
+    private static final Named<BiFunction<ExtendedRandom, Integer, String>> OCT = Named.of("",
+            (r, i) -> r.nextBoolean() ? Integer.toString(i) : Integer.toOctalString(i) + (r.nextBoolean() ? "o" : "O")
+    );
+    private static final Named<BiFunction<ExtendedRandom, Integer, String>> DEC = Named.of("", (r, i) -> Integer.toString(i));
+
+    private static final Named<String> PUNCT = Named.of(
+            "",
+            IntStream.range(0, Character.MAX_VALUE)
+                    .filter(ch -> ch == ' ' || Character.getType(ch) == Character.START_PUNCTUATION || Character.getType(ch) == Character.END_PUNCTUATION)
+                    .filter(ch -> ch != 13 && ch != 10)
+                    .mapToObj(Character::toString)
+                    .collect(Collectors.joining())
+    );
+
+    public static final Named<ReverseTester.Op> MIN_C = Named.of("MinC", scan2((a, b) -> b));
+    public static final Named<ReverseTester.Op> MIN = Named.of("Min", scan2(Math::min));
+
+    private static ReverseTester.Op scan2(final LongBinaryOperator reduce) {
+        return ints -> {
+            // This code is intentionally obscure
+            final int length = Arrays.stream(ints).mapToInt(r -> r.length).max().orElse(0);
+            final long[] cs = new long[length];
+            final long[] cc = new long[length + 1];
+            Arrays.fill(cs, Integer.MAX_VALUE);
+            Arrays.fill(cc, Integer.MAX_VALUE);
+            //noinspection NestedAssignment
+            final long[][] rows = range(ints.length).mapToObj(i -> {
+                        range(ints[i].length).forEachOrdered(j -> cc[j] = reduce.applyAsLong(
+                                cc[j + 1],
+                                cs[j] = Math.min(cs[j], ints[i][j])
+                        ));
+                        return Arrays.copyOf(cc, ints[i].length);
+                    })
+                    .toArray(long[][]::new);
+            return range(ints.length).mapToObj(i -> rows[i]).toArray(long[][]::new);
+        };
+    }
+
+    private static IntStream range(final int length) {
+        return IntStream.iterate(length - 1, i -> i >= 0, i -> i - 1);
+    }
+    
+
+    // === Common
+
+    public static final int MAX_SIZE = 1_000_000 / TestCounter.DENOMINATOR / TestCounter.DENOMINATOR;
+
+    public static final Selector SELECTOR = new Selector(FastReverseTest.class)
+            .variant("Base", ReverseTester.variant(MAX_SIZE, ReverseTest.REVERSE))
+            .variant("3637", ReverseTester.variant(MAX_SIZE, "", MIN_C, OCT, DEC, PUNCT))
+            .variant("3839", ReverseTester.variant(MAX_SIZE, "", MIN, OCT, DEC, PUNCT))
+            .variant("3435", ReverseTester.variant(MAX_SIZE, ReverseTest.ROTATE, PUNCT))
+            .variant("3233", ReverseTester.variant(MAX_SIZE, ReverseTest.EVEN, PUNCT))
+            .variant("4142", ReverseTester.variant(MAX_SIZE, ReverseTest.AVG, PUNCT))
+            .variant("4749", ReverseTester.variant(MAX_SIZE, ReverseTest.SUM, PUNCT))
+
+            ;
+
+
+    private FastReverseTest() {
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+        WordStatTest.main(args);
+    }
+}

java/reverse/ReverseTest.java

@@ -0,0 +1,165 @@
+package reverse;
+
+import base.Named;
+import base.Selector;
+import base.TestCounter;
+import reverse.ReverseTester.Op;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.function.IntToLongFunction;
+import java.util.function.LongBinaryOperator;
+import java.util.stream.IntStream;
+
+/**
+ * Tests for {@code Reverse} homework.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class ReverseTest {
+    // === Base
+    public static final Named<Op> REVERSE = Named.of("", ReverseTester::transform);
+
+
+    // === Max
+
+    public static final Named<Op> MAX_C = Named.of("MaxC", scan2((a, b) -> b));
+    public static final Named<Op> MAX = Named.of("Max", scan2(Math::max));
+
+    private static Op scan2(final LongBinaryOperator reduce) {
+        return ints -> {
+            // This code is intentionally obscure
+            final int length = Arrays.stream(ints).mapToInt(r -> r.length).max().orElse(0);
+            final long[] cs = new long[length];
+            final long[] cc = new long[length + 1];
+            Arrays.fill(cs, Integer.MIN_VALUE);
+            Arrays.fill(cc, Integer.MIN_VALUE);
+            //noinspection NestedAssignment
+            final long[][] rows = range(ints.length).mapToObj(i -> {
+                        range(ints[i].length).forEachOrdered(j -> cc[j] = reduce.applyAsLong(
+                                cc[j + 1],
+                                cs[j] = Math.max(cs[j], ints[i][j])
+                        ));
+                        return Arrays.copyOf(cc, ints[i].length);
+                    })
+                    .toArray(long[][]::new);
+            return range(ints.length).mapToObj(i -> rows[i]).toArray(long[][]::new);
+        };
+    }
+
+    private static IntStream range(final int length) {
+        return IntStream.iterate(length - 1, i -> i >= 0, i -> i - 1);
+    }
+
+
+    // === Rotate
+    public static final Named<Op> ROTATE = Named.of("Rotate", ints -> {
+        final List<int[]> rows = new ArrayList<>(List.of(ints));
+        return IntStream.range(0, Arrays.stream(ints).mapToInt(r -> r.length).max().orElse(0))
+                .mapToObj(c -> {
+                    rows.removeIf(r -> r.length <= c);
+                    return range(rows.size()).mapToObj(rows::get).mapToLong(r -> r[c]).toArray();
+                })
+                .toArray(long[][]::new);
+    });
+
+
+    // === Even
+    public static final Named<Op> EVEN = Named.of(
+            "Even",
+            ints -> ReverseTester.transform(IntStream.range(0, ints.length)
+                    .mapToObj(i -> IntStream.range(0, ints[i].length)
+                            .filter(j -> (i + j) % 2 == 0)
+                            .map(j -> ints[i][j]))
+                    .map(IntStream::toArray).toArray(int[][]::new))
+    );
+
+    // Sum
+    @FunctionalInterface
+    interface LongTernaryOperator {
+        long applyAsLong(long a, long b, long c);
+    }
+    
+    public static final Named<Op> SUM = cross("Sum", 0, Long::sum, (r, c, v) -> r + c - v);
+
+    private static long[][] cross(
+            final int[][] ints,
+            final IntToLongFunction map,
+            final LongBinaryOperator reduce,
+            final int zero,
+            final LongTernaryOperator get
+    ) {
+        // This code is intentionally obscure
+        final long[] rt = Arrays.stream(ints)
+                .map(Arrays::stream)
+                .mapToLong(row -> row.mapToLong(map).reduce(zero, reduce))
+                .toArray();
+        final long[] ct = new long[Arrays.stream(ints).mapToInt(r -> r.length).max().orElse(0)];
+        Arrays.fill(ct, zero);
+        Arrays.stream(ints).forEach(r -> IntStream.range(0, r.length)
+                .forEach(i -> ct[i] = reduce.applyAsLong(ct[i], map.applyAsLong(r[i]))));
+        return IntStream.range(0, ints.length)
+                .mapToObj(r -> IntStream.range(0, ints[r].length)
+                        .mapToLong(c -> get.applyAsLong(rt[r], ct[c], ints[r][c]))
+                        .toArray())
+                .toArray(long[][]::new);
+    }
+
+    private static Named<Op> cross(
+            final String name,
+            final int zero,
+            final LongBinaryOperator reduce,
+            final LongTernaryOperator get
+    ) {
+        return Named.of(name, ints -> cross(ints, n -> n, reduce, zero, get));
+    }
+
+    public static final Named<Op> AVG = avg(
+            "Avg",
+            ints -> cross(ints, n -> n, Long::sum, 0, (r, c, v) -> r + c - v),
+            ints -> cross(ints, n -> 1, Long::sum, 0, (r1, c1, v1) -> r1 + c1 - 1)
+    );
+
+    private static Named<Op> avg(
+            final String name,
+            final Op fs,
+            final Op fc
+    ) {
+        return Named.of(name, ints -> avg(ints, fs.apply(ints), fc.apply(ints)));
+    }
+
+    private static long[][] avg(final int[][] ints, final long[][] as, final long[][] ac) {
+        return IntStream.range(0, ints.length).mapToObj(i -> IntStream.range(0, ints[i].length)
+                        .mapToLong(j -> as[i][j] / ac[i][j])
+                        .toArray())
+                .toArray(long[][]::new);
+    }
+
+
+    // === Common
+
+    public static final int MAX_SIZE = 10_000 / TestCounter.DENOMINATOR;
+
+    public static final Selector SELECTOR = selector(ReverseTest.class, MAX_SIZE);
+
+    private ReverseTest() {
+        // Utility class
+    }
+
+    public static Selector selector(final Class<?> owner, final int maxSize) {
+        return new Selector(owner)
+                .variant("Base",        ReverseTester.variant(maxSize, REVERSE))
+                .variant("3637",        ReverseTester.variant(maxSize, MAX_C))
+                .variant("3839",        ReverseTester.variant(maxSize, MAX))
+                .variant("3435",        ReverseTester.variant(maxSize, ROTATE))
+                .variant("3233",        ReverseTester.variant(maxSize, EVEN))
+                .variant("4142",        ReverseTester.variant(maxSize, AVG))
+                .variant("4749",        ReverseTester.variant(maxSize, SUM))
+                ;
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/reverse/ReverseTester.java

@@ -0,0 +1,299 @@
+package reverse;
+
+import base.*;
+
+import java.util.*;
+import java.util.function.*;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class ReverseTester {
+
+    public static final Named<Op> TRANSFORM = Named.of("", ReverseTester::transform);
+    public static final Named<String> SPACE = Named.of("", " ");
+
+    @FunctionalInterface
+    public interface Op extends Function<int[][], long[][]> {}
+
+    private static final int[] DIVISORS = {100, 10, 1};
+
+    private final Op transform;
+    private final BiFunction<ExtendedRandom, Integer, String> inputToString;
+    private final BiFunction<ExtendedRandom, Integer, String> outputToString;
+    private final String name;
+    private final String spaces;
+
+    private ReverseTester(final String className, final Op transform, final String spaces) {
+        this(className, transform, spaces, (r, i) -> Integer.toString(i), (r, i) -> Long.toString(i));
+    }
+
+    private ReverseTester(
+            final String className,
+            final Op transform,
+            final String spaces,
+            final BiFunction<ExtendedRandom, Integer, String> inputToString,
+            final BiFunction<ExtendedRandom, Integer, String> outputToString
+    ) {
+        name = className;
+        this.transform = transform;
+        this.spaces = spaces;
+        this.inputToString = inputToString;
+        this.outputToString = outputToString;
+    }
+
+    private static Consumer<TestCounter> variant(final int maxSize, final Supplier<ReverseTester> tester) {
+        return counter -> tester.get().run(counter, maxSize);
+    }
+
+    public static Consumer<TestCounter> variant(final int maxSize, final Named<Op> transform) {
+        return variant(maxSize, transform, SPACE);
+    }
+
+
+    public static Consumer<TestCounter> variant(final int maxSize, final Named<Op> transform, final Named<String> spaces) {
+        Objects.requireNonNull(transform);
+        Objects.requireNonNull(spaces);
+        return variant(
+                maxSize,
+                () -> new ReverseTester("Reverse" + transform.name() + spaces.name(), transform.value(), spaces.value())
+        );
+    }
+
+    public static Consumer<TestCounter> variant(
+            final int maxSize,
+            final String suffix,
+            final Named<BiFunction<ExtendedRandom, Integer, String>> input,
+            final Named<BiFunction<ExtendedRandom, Integer, String>> output
+    ) {
+        return variant(maxSize, suffix, TRANSFORM, input, output);
+    }
+
+    public static Consumer<TestCounter> variant(
+            final int maxSize,
+            final String suffix,
+            final Named<Op> op,
+            final Named<BiFunction<ExtendedRandom, Integer, String>> input,
+            final Named<BiFunction<ExtendedRandom, Integer, String>> output
+    ) {
+        return variant(maxSize, suffix, op, input, output, SPACE);
+    }
+
+    public static Consumer<TestCounter> variant(
+            final int maxSize,
+            final String suffix,
+            final Named<Op> op,
+            final Named<BiFunction<ExtendedRandom, Integer, String>> input,
+            final Named<BiFunction<ExtendedRandom, Integer, String>> output,
+            final Named<String> spaces
+    ) {
+        final String out = input.name().contains(output.name()) ? "" : output.name();
+        return variant(maxSize, () -> new ReverseTester(
+                "Reverse" + op.name() + input.name() + out + suffix + spaces.name(),
+                op.value(),
+                spaces.value(),
+                input.value(),
+                output.value()
+        ));
+    }
+
+    private void run(final TestCounter counter, final int maxSize) {
+        new Checker(counter, maxSize, Runner.packages("", "reverse").std(name), spaces).test();
+    }
+
+    @Override
+    public String toString() {
+        return name;
+    }
+
+    public static long[][] transform(final int[][] ints) {
+        return IntStream.range(1, ints.length + 1)
+                .mapToObj(i -> ints[ints.length - i])
+                .map(is -> IntStream.range(1, is.length + 1).mapToLong(i -> is[is.length - i]).toArray())
+                .toArray(long[][]::new);
+    }
+
+    /**
+     * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+     */
+    private class Checker extends BaseChecker {
+        private final int maxSize;
+        private final Runner runner;
+        private final String spaces;
+        private final Set<String> manualTests = new HashSet<>();
+
+        Checker(final TestCounter counter, final int maxSize, final Runner runner, final String spaces) {
+            super(counter);
+            this.maxSize = maxSize;
+            this.runner = runner;
+            this.spaces = spaces;
+        }
+
+        public void manualTest(final int[][] ints) {
+            for (final List<int[]> permutation : permutations(new ArrayList<>(Arrays.asList(ints)))) {
+                final int[][] input = permutation.toArray(int[][]::new);
+                final String[][] lines = toString(input, inputToString);
+                if (manualTests.add(Arrays.deepToString(lines))) {
+                    test(lines, toString(transform.apply(input), outputToString));
+                }
+            }
+        }
+
+        public void test(final int[][] ints) {
+            test(toString(ints, inputToString), toString(transform.apply(ints), outputToString));
+        }
+
+        public void test(final String[][] input, final String[][] output) {
+            final List<String> inputLines = toLines(input, random().randomString(spaces, 1, 10));
+            final List<String> outputLines = toLines(output, " ");
+            runner.testEquals(counter, inputLines, outputLines);
+        }
+
+        private String[][] toString(final int[][] ints, final BiFunction<ExtendedRandom, Integer, String> toString) {
+            return Arrays.stream(ints)
+                    .map(row -> Arrays.stream(row).mapToObj(i -> toString.apply(random(), i)).toArray(String[]::new))
+                    .toArray(String[][]::new);
+        }
+
+        private String[][] toString(final long[][] ints, final BiFunction<ExtendedRandom, Integer, String> toString) {
+            return Arrays.stream(ints)
+                    .map(row -> Arrays.stream(row).mapToObj(i -> toString.apply(random(), (int) i)).toArray(String[]::new))
+                    .toArray(String[][]::new);
+        }
+
+        private List<String> toLines(final String[][] data, final String delimiter) {
+            if (data.length == 0) {
+                return Collections.singletonList("");
+            }
+            return Arrays.stream(data)
+                    .map(row -> String.join(delimiter, row))
+                    .collect(Collectors.toList());
+        }
+
+        public int[][] random(final int[] profile) {
+            final int col = random().nextInt(Arrays.stream(profile).max().orElse(0));
+            final int row = random().nextInt(profile.length);
+            final int m = random().nextInt(5) - 2;
+            final int[][] ints = Arrays.stream(profile).mapToObj(random().getRandom()::ints).map(IntStream::toArray).toArray(int[][]::new);
+            Arrays.stream(ints).filter(r -> col < r.length).forEach(r -> r[col] = Math.abs(r[col]) / 2 * m);
+            ints[row] = Arrays.stream(ints[row]).map(Math::abs).map(v -> v / 2 * m).toArray();
+            return ints;
+        }
+
+        public void test() {
+            manualTest(new int[][]{
+                    {1}
+            });
+            manualTest(new int[][]{
+                    {1, 2},
+                    {3}
+            });
+            manualTest(new int[][]{
+                    {1, 2, 3},
+                    {4, 5},
+                    {6}
+            });
+            manualTest(new int[][]{
+                    {1, 2, 3},
+                    {},
+                    {4, 5},
+                    {6}
+            });
+            manualTest(new int[][]{
+                    {1, 2, 3},
+                    {-4, -5},
+                    {6}
+            });
+            manualTest(new int[][]{
+                    {1, -2, 3},
+                    {},
+                    {4, -5},
+                    {6}
+            });
+            manualTest(new int[][]{
+                    {1, 2, 0},
+                    {1, 0},
+                    {0},
+            });
+            manualTest(new int[][]{
+                    {1},
+                    {1, 3},
+                    {1, 2, 3},
+            });
+            manualTest(new int[][]{
+                    {-1},
+                    {-1, -2},
+                    {-1, -2, -3},
+            });
+            manualTest(new int[][]{
+                    {},
+            });
+            manualTest(new int[][]{
+                    {},
+                    {},
+                    {},
+            });
+            testRandom(tweakProfile(constProfile(10, 10), new int[][]{}));
+            testRandom(tweakProfile(constProfile(100, 100), new int[][]{}));
+            testRandom(randomProfile(100, maxSize));
+            testRandom(randomProfile(maxSize / 10, maxSize));
+            testRandom(randomProfile(maxSize, maxSize));
+            for (final int d : DIVISORS) {
+                final int size = maxSize / d;
+                testRandom(tweakProfile(constProfile(size / 2, 0), new int[][]{{size / 2, 0}}));
+                testRandom(tweakProfile(randomProfile(size, size / 2), new int[][]{{size / 2, 0}}));
+                testRandom(tweakProfile(constProfile(size / 2, 0), new int[][]{{size / 2, size / 2 - 1}}));
+                testRandom(tweakProfile(constProfile(size / 3, 1), new int[][]{{size / 3, size / 6, size / 3 - 1}}));
+            }
+        }
+
+        private int[] randomProfile(final int length, final int values) {
+            final int[] profile = new int[length];
+            for (int i = 0; i < values; i++) {
+                profile[random().nextInt(0, length - 1)]++;
+            }
+            return profile;
+        }
+
+        private void testRandom(final int[] profile) {
+            test(random(profile));
+        }
+
+        private int[] constProfile(final int length, final int value) {
+            final int[] profile = new int[length];
+            Arrays.fill(profile, value);
+            return profile;
+        }
+
+        private int[] tweakProfile(final int[] profile, final int[][] mods) {
+            for (final int[] mod : mods) {
+                Arrays.stream(mod).skip(1).forEach(i -> profile[i] = mod[0]);
+            }
+            return profile;
+        }
+    }
+
+    private static <T> List<List<T>> permutations(final List<T> elements) {
+        final List<List<T>> result = new ArrayList<>();
+        permutations(new ArrayList<>(elements), result, elements.size() - 1);
+        return result;
+    }
+
+    private static <T> void permutations(final List<T> elements, final List<List<T>> result, final int n) {
+        if (n == 0) {
+            result.add(List.copyOf(elements));
+        } else {
+            for (int i = 0; i < n; i++) {
+                permutations(elements, result, n - 1);
+                if (n % 2 == 1) {
+                    Collections.swap(elements, i, n);
+                } else {
+                    Collections.swap(elements, 0, n);
+                }
+            }
+            permutations(elements, result, n - 1);
+        }
+    }
+}

java/reverse/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#reverse">Reverse</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package reverse;

java/sum/SumTest.java

@@ -0,0 +1,178 @@
+package sum;
+
+import base.*;
+
+import java.math.BigDecimal;
+import java.math.BigInteger;
+import java.util.Locale;
+import java.util.function.*;
+import java.util.stream.Collectors;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class SumTest {
+    // === Base
+
+    @FunctionalInterface
+    /* package-private */ interface Op<T extends Number> extends UnaryOperator<SumTester<T>> {}
+
+    private static final BiConsumer<Number, String> TO_STRING = (expected, out) -> Asserts.assertEquals("Sum", expected.toString(), out);
+
+    private static final Named<Supplier<SumTester<Integer>>> BASE = Named.of("", () -> new SumTester<>(
+            Integer::sum, n -> (int) n, (r, max) -> r.nextInt() % max, TO_STRING,
+            10, 100, Integer.MAX_VALUE
+    ));
+
+    /* package-private */ static <T extends Number> Named<Op<T>> plain() {
+        return Named.of("", test -> test);
+    }
+
+
+    // === DoubleHex
+
+    private static BiConsumer<Number, String> approximate(final Function<String, Number> parser, final double precision) {
+        return (expected, out) ->
+                Asserts.assertEquals("Sum", expected.doubleValue(), parser.apply(out).doubleValue(), precision);
+    }
+
+    private static final Named<Supplier<SumTester<Double>>> DOUBLE = Named.of("Double", () -> new SumTester<>(
+            Double::sum, n -> (double) n, (r, max) -> (r.getRandom().nextDouble() - 0.5) * 2 * max,
+            approximate(Double::parseDouble, 1e-10),
+            10.0, 0.01, 1e20, 1e100, Double.MAX_VALUE / 10000)
+            .test(5, "2.5 2.5")
+            .test(0, "1e100 -1e100")
+            .testT(2e100, "1.5e100 0.5e100"));
+
+    private static <T extends Number> Named<Op<T>> hexFull(final Function<T, String> toHex) {
+        final Function<T, String> toHexSpoiled = toHex.andThen(s ->s.chars()
+                .map(ch -> ((ch ^ s.hashCode()) & 1) == 0 ? Character.toLowerCase(ch) : Character.toUpperCase(ch))
+                .mapToObj(Character::toString)
+                .collect(Collectors.joining()));
+        return Named.of("Hex", test -> test
+                .test(toHex, 1)
+                .test(toHex, 0x1a)
+                .test(toHexSpoiled, 0xA2)
+                .test(toHexSpoiled, 0X0, 0X1, 0XF, 0XF, 0x0, 0x1, 0xF, 0xf)
+                .test(toHexSpoiled, 0x12345678)
+                .test(toHexSpoiled, 0x09abcdef)
+                .test(toHexSpoiled, 0x3CafeBab)
+                .test(toHexSpoiled, 0x3DeadBee)
+
+                .test(toHex, Integer.MAX_VALUE)
+                .test(toHex, Integer.MIN_VALUE)
+                .setToString(number -> {
+                    final int hashCode = number.hashCode();
+                    if ((hashCode & 1) == 0) {
+                        return number.toString();
+                    }
+
+                    return toHexSpoiled.apply(number);
+                })
+        );
+    }
+
+    // === Octal
+    private static <T extends Number> Named<Op<T>> octal(final Function<T, String> toOctal) {
+        //noinspection OctalInteger,StringConcatenationMissingWhitespace
+        return Named.of("Octal", test -> test
+                .test(1, "1o")
+                .test(017, "17o")
+                .testSpaces(6, " 1o 2o 3O ")
+                .test(01234567, "1234567O")
+
+                .test(Integer.MIN_VALUE, "-0" + String.valueOf(Integer.MIN_VALUE).substring(1))
+                .test(Integer.MAX_VALUE, "0" + Integer.MAX_VALUE)
+                .test(Integer.MAX_VALUE, Integer.toOctalString(Integer.MAX_VALUE) + "o")
+                .test(Integer.MAX_VALUE, "0" + Integer.toOctalString(Integer.MAX_VALUE) + "O")
+                .setToString(number -> {
+                    final int hashCode = number.hashCode();
+                    if ((hashCode & 1) == 0) {
+                        return number.toString();
+                    }
+
+                    final String lower = toOctal.apply(number).toLowerCase(Locale.ROOT) + "o";
+                    return (hashCode & 2) == 0 ? lower : lower.toUpperCase(Locale.ROOT);
+                })
+        );
+    }
+
+    // === Long
+
+    private static final Named<Supplier<SumTester<Long>>> LONG = Named.of("Long", () -> new SumTester<>(
+            Long::sum, n -> n, (r, max) -> r.getRandom().nextLong() % max, TO_STRING,
+            10L, 100L, (long) Integer.MAX_VALUE, Long.MAX_VALUE)
+            .test(12345678901234567L, " +12345678901234567 ")
+            .test(0L, " +12345678901234567 -12345678901234567")
+            .test(0L, " +12345678901234567 -12345678901234567"));
+
+    // === BigInteger
+
+    private static final Named<Supplier<SumTester<BigInteger>>> BIG_INTEGER = Named.of("BigInteger", () -> new SumTester<>(
+            BigInteger::add, BigInteger::valueOf, (r, max) -> new BigInteger(max.bitLength(), r.getRandom()), TO_STRING,
+            BigInteger.TEN, BigInteger.TEN.pow(10), BigInteger.TEN.pow(100), BigInteger.TWO.pow(1000))
+            .test(0, "10000000000000000000000000000000000000000 -10000000000000000000000000000000000000000"));
+
+
+    // === BigDecimalHex
+
+    @SuppressWarnings("BigDecimalMethodWithoutRoundingCalled")
+    private static final Named<Supplier<SumTester<BigDecimal>>> BIG_DECIMAL = Named.of("BigDecimal", () -> new SumTester<>(
+            BigDecimal::add, BigDecimal::valueOf,
+            (r, max) -> {
+                final BigInteger unscaled = new BigInteger((max.precision() - max.scale() + 2) * 3, r.getRandom());
+                return new BigDecimal(unscaled, 3);
+            },
+            TO_STRING,
+            BigDecimal.TEN, BigDecimal.TEN.pow(10), BigDecimal.TEN.pow(100), BigDecimal.ONE.add(BigDecimal.ONE).pow(1000))
+            .testT(BigDecimal.ZERO.setScale(3), "10000000000000000000000000000000000000000.123 -10000000000000000000000000000000000000000.123"));
+
+    private static String bigDecimalToString(final BigDecimal number) {
+        final int scale = number.scale();
+        return "0x" + number.unscaledValue().toString(16) + (scale == 0 ? "" : "s" + Integer.toString(scale, 16));
+    }
+
+
+    // === Hex
+
+    private static <T extends Number> Named<Op<T>> hex(final Function<T, String> toHex) {
+        return hexFull(v -> "0x" + toHex.apply(v));
+    }
+
+
+    // === Common
+
+    /* package-private */ static <T extends Number> Consumer<TestCounter> variant(
+            final Named<Function<String, Runner>> runner,
+            final Named<Supplier<SumTester<T>>> test,
+            final Named<? extends Function<? super SumTester<T>, ? extends SumTester<?>>> modifier
+    ) {
+        return counter -> modifier.value().apply(test.value().get())
+                .test("Sum" + test.name() + modifier.name() + runner.name(), counter, runner.value());
+    }
+
+    /* package-private */ static final Named<Function<String, Runner>> RUNNER =
+            Named.of("", Runner.packages("", "sum")::args);
+
+    public static final Selector SELECTOR = selector(SumTest.class, RUNNER);
+
+    private SumTest() {
+        // Utility class
+    }
+
+    public static Selector selector(final Class<?> owner, final Named<Function<String, Runner>> runner) {
+        return new Selector(owner)
+                .variant("Base",            variant(runner, BASE, plain()))
+                .variant("3637",            variant(runner, DOUBLE, hexFull(value -> value == value.intValue() && value > 0 ? "0x" + Integer.toHexString(value.intValue()) : Double.toHexString(value))))
+                .variant("3839",            variant(runner, BIG_DECIMAL, hexFull(SumTest::bigDecimalToString)))
+                .variant("3435",            variant(runner, BASE, hex(Integer::toHexString)))
+                .variant("3233",            variant(runner, DOUBLE, plain()))
+                .variant("4749",            variant(runner, LONG, octal(Long::toOctalString)))
+                .variant("4142",            variant(runner, BIG_INTEGER, octal(number -> number.toString(8))))
+                ;
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/sum/SumTester.java

@@ -0,0 +1,189 @@
+package sum;
+
+import base.*;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.List;
+import java.util.function.*;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.LongStream;
+import java.util.stream.Stream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class SumTester<T extends Number> {
+    private static final List<String> SPACES = List.of(
+            " \t\n\u000B\u2029\f",
+            IntStream.rangeClosed(0, Character.MAX_VALUE)
+                    .filter(Character::isWhitespace)
+                    .mapToObj(Character::toString)
+                    .collect(Collectors.joining())
+    );
+
+    private final BinaryOperator<T> add;
+    private final LongFunction<T> fromLong;
+    private BiFunction<ExtendedRandom, T, String> toString;
+    private final BiFunction<ExtendedRandom, T, T> randomValue;
+    private final BiConsumer<Number, String> verifier;
+    private List<String> spaces;
+    private final List<T> limits;
+
+    private final List<Consumer<Checker>> tests = new ArrayList<>();
+
+    @SafeVarargs
+    public SumTester(
+            final BinaryOperator<T> add,
+            final LongFunction<T> fromLong,
+            final BiFunction<ExtendedRandom, T, T> randomValue,
+            final BiConsumer<Number, String> verifier,
+            final T... limits
+    ) {
+        this.add = add;
+        this.fromLong = fromLong;
+        this.randomValue = randomValue;
+        this.verifier = verifier;
+        this.limits = List.of(limits);
+
+        setSpaces(SPACES);
+        setToString(String::valueOf);
+
+        test(1, "1");
+        test(6, "1", "2", "3");
+        test(1, " 1");
+        test(1, "1 ");
+        test(1, " 1 ");
+        test(12345, " 12345 ");
+        test(60, "010", "020", "030");
+        testSpaces(1368, " 123 456 789 ");
+        test(-1, "-1");
+        test(-6, "-1", "-2", "-3");
+        test(-12345, " -12345 ");
+        testSpaces(-1368, " -123 -456 -789 ");
+        test(1, "+1");
+        test(6, "+1", "+2", "+3");
+        test(12345, " +12345 ");
+        testSpaces(1368, " +123 +456 +789 ");
+        test(0);
+        testSpaces(0, " ", "  ");
+    }
+
+    protected SumTester<T> setSpaces(final List<String> spaces) {
+        this.spaces = spaces;
+        return this;
+    }
+
+    protected SumTester<T> addSpaces(final String... spaces) {
+        this.spaces = Stream.concat(this.spaces.stream(), Arrays.stream(spaces)).toList();
+        return this;
+    }
+
+    public SumTester<T> setToString(final Function<T, String> toString) {
+        return setToString((r, n) -> toString.apply(n));
+    }
+
+    public SumTester<T> setToString(final BiFunction<ExtendedRandom, T, String> toString) {
+        this.toString = toString;
+        return this;
+    }
+
+    protected SumTester<T> test(final Function<T, String> toString, final long... input) {
+        return testT(
+                fromLong.apply(LongStream.of(input).sum()),
+                LongStream.of(input).mapToObj(fromLong).map(toString).collect(Collectors.joining(" "))
+        );
+    }
+
+    protected SumTester<T> test(final long result, final String... input) {
+        return testT(fromLong.apply(result), input);
+    }
+
+    protected SumTester<T> testT(final T result, final String... input) {
+        return testT(result, Arrays.asList(input));
+    }
+
+    private SumTester<T> testT(final T result, final List<String> input) {
+        tests.add(checker -> checker.test(result, input));
+        return this;
+    }
+
+    public SumTester<T> testSpaces(final long result, final String... input) {
+        final T res = fromLong.apply(result);
+        tests.add(checker -> spaces.stream()
+                .flatMapToInt(String::chars)
+                .forEach(space -> checker.test(
+                        res,
+                        Functional.map(Arrays.asList(input), s -> s.replace(' ', (char) space))
+                ))
+        );
+        return this;
+    }
+
+    public void test(final String name, final TestCounter counter, final Function<String, Runner> runner) {
+        new Checker(counter, runner.apply(name)).test();
+    }
+
+    private class Checker extends BaseChecker {
+        private final Runner runner;
+
+        public Checker(final TestCounter counter, final Runner runner) {
+            super(counter);
+            this.runner = runner;
+        }
+
+        public void test() {
+            tests.forEach(test -> test.accept(this));
+
+            for (final T limit : limits) {
+                for (int n = 10; n <= 10_000 / TestCounter.DENOMINATOR; n *= 10) {
+                    randomTest(n, limit);
+                }
+            }
+        }
+
+        private void test(final T result, final List<String> input) {
+            counter.test(() -> {
+                final List<String> out = runner.run(counter, input);
+                Asserts.assertEquals("Single line expected", 1, out.size());
+                verifier.accept(result, out.get(0));
+            });
+        }
+
+        private void randomTest(final int numbers, final T max) {
+            for (final String spaces : spaces) {
+                randomTest(numbers, max, spaces);
+            }
+        }
+
+        private void randomTest(final int numbers, final T max, final String spaces) {
+            final List<T> values = new ArrayList<>();
+            for (int i = 0; i < numbers; i++) {
+                values.add(randomValue.apply(random(), max));
+            }
+            testRandom(values.stream().reduce(fromLong.apply(0), add), values, spaces);
+        }
+
+        private void testRandom(final T result, final List<T> args, final String spaces) {
+            final List<String> spaced = args.stream()
+                    .map(n -> toString.apply(random(), n))
+                    .map(value -> randomSpace(spaces) + value + randomSpace(spaces))
+                    .toList();
+            final List<String> argsList = new ArrayList<>();
+            for (final Iterator<String> i = spaced.listIterator(); i.hasNext(); ) {
+                final StringBuilder next = new StringBuilder(i.next());
+                while (i.hasNext() && random().nextBoolean()) {
+                    next.append(randomSpace(spaces)).append(i.next());
+                }
+                argsList.add(next.toString());
+            }
+            test(result, argsList);
+        }
+
+        private String randomSpace(final String spaces) {
+            return random().randomString(spaces);
+        }
+    }
+}

java/sum/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#sum">Sum</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package sum;

java/wordStat/WordStatChecker.java

@@ -0,0 +1,127 @@
+package wordStat;
+
+import base.*;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Locale;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class WordStatChecker extends BaseChecker {
+    public static final String DASH = "-֊־‒–—―⸗⸚⸺〰゠︱︲﹘﹣－'";
+    public static final String SIMPLE_DELIMITERS = " \t";
+    public static final String ADVANCED_DELIMITERS = " \t!\"#%&()*+,./:;<=>?@[\\]^`{|}~ ¡¦§¨©«¬\u00AD®¯°±²³´¶·¸¹»¼½¾¿×÷˂˃˄˅˒˓˔˕˖˗˘˙˚˛˜˝";
+    public static final String ALL = ExtendedRandom.RUSSIAN + ExtendedRandom.ENGLISH + ExtendedRandom.GREEK + DASH;
+    private static final Pattern PATTERN = Pattern.compile("[^\\p{IsLetter}'\\p{Pd}]+");
+    public static final Runner.Packages RUNNER = Runner.packages("", "wordstat", "wspp");
+
+    private final Function<String[][], ? extends List<? extends Pair<?, ?>>> processor;
+
+    private final MainChecker main;
+
+    private WordStatChecker(
+            final String className,
+            final Function<String[][], ? extends List<? extends Pair<?, ?>>> processor,
+            final TestCounter counter
+    ) {
+        super(counter);
+        main = new MainChecker(RUNNER.files(className));
+        this.processor = processor;
+    }
+
+    public static void test(
+            final TestCounter counter,
+            final String className,
+            final Function<String[][], ? extends List<? extends Pair<?, ?>>> processor,
+            final Consumer<WordStatChecker> tests
+    ) {
+        tests.accept(new WordStatChecker(className, processor, counter));
+    }
+
+    public void test(final String... lines) {
+        test(PATTERN, lines);
+    }
+
+    public void test(final Pattern pattern, final String... lines) {
+        final String[][] data = Arrays.stream(lines)
+                .map(line -> Arrays.stream(pattern.split(line)).filter(Predicate.not(String::isEmpty)).toArray(String[]::new))
+                .toArray(String[][]::new);
+        test(lines, processor.apply(data));
+    }
+
+    private void randomTest(
+            final int wordLength,
+            final int totalWords,
+            final int wordsPerLine,
+            final int lines,
+            final String chars,
+            final String delimiters,
+            final Function<String[][], List<? extends Pair<?, ?>>> processor
+    ) {
+        final String[] words = generateWords(wordLength, totalWords, chars);
+        final String[][] text = generateTest(lines, words, wordsPerLine);
+        test(input(text, delimiters), processor.apply(text));
+    }
+
+    public void randomTest(
+            final int wordLength,
+            final int totalWords,
+            final int wordsPerLine,
+            final int lines,
+            final String chars,
+            final String delimiters
+    ) {
+        randomTest(wordLength, totalWords, wordsPerLine, lines, chars, delimiters, processor::apply);
+    }
+
+    private void test(final String[] text, final List<? extends Pair<?, ?>> expected) {
+        final List<String> expectedList = expected.stream()
+                .map(p -> p.first() + " " + p.second())
+                .collect(Collectors.toList());
+        main.testEquals(counter, Arrays.asList(text), expectedList);
+    }
+
+    public void test(final String[][] text, final String delimiters, final List<Pair<String, Integer>> answer) {
+        test(input(text, delimiters), answer);
+    }
+
+    private String[] generateWords(final int wordLength, final int totalWords, final String chars) {
+        final String allChars = chars.chars().anyMatch(Character::isUpperCase)
+                ? chars : chars + chars.toUpperCase(Locale.ROOT);
+        return IntStream.range(0, totalWords)
+                .mapToObj(i -> random().randomString(allChars, wordLength / 2, wordLength))
+                .toArray(String[]::new);
+    }
+
+    private String[][] generateTest(final int lines, final String[] words, final int wordsPerLine) {
+        final String[][] text = new String[lines][];
+        for (int i = 0; i < text.length; i++) {
+            text[i] = new String[random().nextInt(wordsPerLine / 2, wordsPerLine)];
+            for (int j = 0; j < text[i].length; j++) {
+                text[i][j] = random().randomItem(words);
+            }
+        }
+        return text;
+    }
+
+    private String[] input(final String[][] text, final String delimiters) {
+        final String[] input = new String[text.length];
+        for (int i = 0; i < text.length; i++) {
+            final String[] line = text[i];
+            final StringBuilder sb = new StringBuilder(random().randomString(delimiters));
+            for (final String word : line) {
+                sb.append(word).append(random().randomString(delimiters));
+            }
+            input[i] = sb.toString();
+        }
+        return input;
+    }
+}

java/wordStat/WordStatTest.java

@@ -0,0 +1,70 @@
+package wordStat;
+
+import base.Named;
+import base.Selector;
+
+import java.util.Comparator;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#wordstat">Word Statistics</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class WordStatTest {
+    // === Base
+    private static final Named<Function<String, Stream<String>>> ID  = Named.of("", Stream::of);
+    private static final WordStatTester.Variant BASE = new WordStatTester.Variant("", false, Comparator.comparingInt(p -> 0));
+
+
+    // === 3637
+    public static final int SIZE = 3;
+    private static final WordStatTester.Variant LENGTH = new WordStatTester.Variant("Length", false, Comparator.comparingInt(p -> p.first().length()));
+    private static final Named<Function<String, Stream<String>>> MIDDLE =
+            size("Middle", SIZE * 2 + 1, s -> Stream.of(s.substring(SIZE, s.length() - SIZE)));
+
+    static Named<Function<String, Stream<String>>> size(
+            final String name,
+            final int length,
+            final Function<String, Stream<String>> f
+    ) {
+        return Named.of(name, s -> s.length() >= length ? f.apply(s) : Stream.empty());
+    }
+
+    // === 3839
+    private static final Named<Function<String, Stream<String>>> AFFIX = size(
+            "Affix",
+            2,
+            s -> Stream.of(s.substring(0, s.length() / 2), s.substring(s.length() - s.length() / 2))
+    );
+
+    // === 3536
+    private static final Named<Function<String, Stream<String>>> SUFFIX =
+            size("Suffix", 2, s -> Stream.of(s.substring(s.length() - s.length() / 2)));
+
+    // === 4749
+    private static final Named<Function<String, Stream<String>>> PREFIX =
+            size("Prefix", 2, s -> Stream.of(s.substring(0, s.length() / 2)));
+
+    // === Common
+    public static final Selector SELECTOR = new Selector(WordStatTester.class)
+            .variant("Base", BASE.with(ID))
+            .variant("3637", LENGTH.with(MIDDLE))
+            .variant("3839", LENGTH.with(AFFIX))
+            .variant("3435", LENGTH.with(SUFFIX))
+            .variant("3233", LENGTH.with(ID))
+            .variant("4142", LENGTH.with(MIDDLE))
+            .variant("4749", LENGTH.with(PREFIX))
+
+            ;
+
+    private WordStatTest() {
+        // Utility class
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/wordStat/WordStatTester.java

@@ -0,0 +1,100 @@
+package wordStat;
+
+import base.ExtendedRandom;
+import base.Named;
+import base.Pair;
+import base.TestCounter;
+
+import java.util.*;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class WordStatTester {
+    public static final String PRE_LOWER = chars()
+            .filter(s -> s.toLowerCase(Locale.ROOT).length() == 1)
+            .collect(Collectors.joining());
+    public static final String POST_LOWER = chars()
+            .collect(Collectors.joining())
+            .toLowerCase();
+
+    private WordStatTester() {
+    }
+
+    private static Stream<String> chars() {
+        return IntStream.range(' ', Character.MAX_VALUE)
+                .filter(ch -> !Character.isSurrogate((char) ch))
+                .filter(ch -> Character.getType(ch) != Character.NON_SPACING_MARK)
+                .filter(ch -> Character.getType(ch) != Character.DIRECTIONALITY_NONSPACING_MARK)
+                .mapToObj(Character::toString);
+    }
+
+    /* package-private */ record Variant(String name, boolean reverse, Comparator<Pair<String, Integer>> c) {
+        public Consumer<TestCounter> with(final Named<Function<String, Stream<String>>> split) {
+            return counter -> WordStatChecker.test(
+                    counter,
+                    "WordStat" + name + split.name(),
+                    text -> answer(split.value(), text),
+                    checker -> {
+                        checker.test("To be, or not to be, that is the question:");
+                        checker.test("Monday's child is fair of face.", "Tuesday's child is full of grace.");
+                        checker.test("Шалтай-Болтай", "Сидел на стене.", "Шалтай-Болтай", "Свалился во сне.");
+                        checker.test(
+                                "27 октября — 300-й день григорианскому календарю. До конца года остаётся 65 дней.",
+                                "До 15 октября 1582 года — 27 октября по юлианскому календарю, с 15 октября 1582 года — 27 октября по григорианскому календарю.",
+                                "В XX и XXI веках соответствует 14 октября по юлианскому календарю[1].",
+                                "(c) Wikipedia"
+                        );
+                        checker.test("23 октября — Всемирный день психического здоровья", "Тема 2025 года: Психическое здоровье на рабочем месте");
+
+                        checker.randomTest(3, 10, 10, 3, ExtendedRandom.ENGLISH, WordStatChecker.SIMPLE_DELIMITERS);
+                        checker.randomTest(10, 3, 5, 5, ExtendedRandom.RUSSIAN, WordStatChecker.SIMPLE_DELIMITERS);
+                        checker.randomTest(4, 10, 10, 3, ExtendedRandom.GREEK, WordStatChecker.SIMPLE_DELIMITERS);
+                        checker.randomTest(4, 10, 10, 3, WordStatChecker.DASH, WordStatChecker.SIMPLE_DELIMITERS);
+                        checker.randomTest(3, 10, 10, 3, ExtendedRandom.ENGLISH, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(10, 3, 5, 5, ExtendedRandom.RUSSIAN, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(3, 10, 10, 3, ExtendedRandom.GREEK, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(3, 10, 10, 3, WordStatChecker.DASH, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(3, 10, 10, 10, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+
+                        final int d = TestCounter.DENOMINATOR;
+                        final int d2 = TestCounter.DENOMINATOR;
+                        checker.randomTest(10, 10000 / d, 10, 10, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(10, 1, 10, 10, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(10, 1000 / d, 100 / d2, 100 / d2, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(4, 1000 / d, 10, 3000 / d, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(4, 1000 / d, 3000 / d, 10, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(10000 / d, 20, 10, 5, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                        checker.randomTest(1000000 / d, 2, 2, 1, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+
+                        checker.test(PRE_LOWER);
+                        checker.test(POST_LOWER);
+                    }
+            );
+        }
+
+        private List<Pair<String, Integer>> answer(final Function<String, Stream<String>> split, final String[][] text) {
+            final List<String> parts = Arrays.stream(text)
+                    .flatMap(Arrays::stream)
+                    .filter(Predicate.not(String::isEmpty))
+                    .flatMap(split)
+                    .peek(s -> {assert !s.isBlank();})
+                    .collect(Collectors.toList());
+            if (reverse()) {
+                Collections.reverse(parts);
+            }
+            return parts.stream()
+                    .collect(Collectors.toMap(String::toLowerCase, v -> 1, Integer::sum, LinkedHashMap::new))
+                    .entrySet().stream()
+                    .map(Pair::of)
+                    .sorted(c)
+                    .toList();
+        }
+    }
+}

java/wordStat/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#wordstat">Word Statistics</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package wordStat;

java/wspp/WsppTest.java

@@ -0,0 +1,51 @@
+package wspp;
+
+import base.Named;
+import base.Selector;
+
+import java.util.Comparator;
+import java.util.Map;
+import java.util.function.IntFunction;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class WsppTest {
+    // === Base
+    private static final Named<Comparator<Map.Entry<String, Integer>>> INPUT = Named.of("", Comparator.comparingInt(e -> 0));
+    private static final Named<IntFunction<IntStream>> ALL = Named.of("", size -> IntStream.range(0, size));
+    private static final Named<WsppTester.Extractor<Object>> WSPP = Named.of("", (r, l, L, g, G) -> g);
+    private static final Named<String> NONE = Named.of("", "");
+
+    // === 3637
+    private static final Named<Comparator<Map.Entry<String, Integer>>> LENGTH = Named.of("",
+            Map.Entry.comparingByKey(Comparator.comparingInt(String::length)));
+    private static final Named<IntFunction<IntStream>> LAST = Named.of("Last", size -> IntStream.of(size - 1));
+    private static final Named<String> JAVA = Named.of("", "XHB7TmR9JF8=");
+
+    // === 3839
+    private static final Named<IntFunction<IntStream>> MIDDLE = Named.of("Middle", size -> IntStream.of(size / 2));
+
+    // === 3435
+    public static final WsppTester.Extractor<String> POSITION = (r, l, L, g, G) -> r + ":" + (G - g + 1);
+
+
+    // === Common
+    public static final Selector SELECTOR = new Selector(WsppTester.class)
+            .variant("Base",            WsppTester.variant(INPUT, ALL, WSPP, NONE))
+            .variant("3637",            WsppTester.variant(LENGTH, LAST, WSPP, JAVA))
+            .variant("3839",            WsppTester.variant(LENGTH, MIDDLE, WSPP, JAVA))
+            .variant("3435",            WsppTester.variant(LENGTH, ALL, Named.of("Position", POSITION), JAVA))
+            .variant("3233",            WsppTester.variant(INPUT, ALL, Named.of("Pos", POSITION), JAVA))
+            .variant("4142",            WsppTester.variant(LENGTH, LAST, WSPP, JAVA))
+            .variant("4749",            WsppTester.variant(LENGTH, ALL, Named.of("Position", POSITION), JAVA))
+            ;
+
+    private WsppTest() {
+    }
+
+    public static void main(final String... args) {
+        SELECTOR.main(args);
+    }
+}

java/wspp/WsppTester.java

@@ -0,0 +1,139 @@
+package wspp;
+
+import base.ExtendedRandom;
+import base.Named;
+import base.Pair;
+import base.TestCounter;
+import wordStat.WordStatChecker;
+import wordStat.WordStatTester;
+
+import java.nio.charset.StandardCharsets;
+import java.util.*;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.IntFunction;
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public final class WsppTester {
+    private WsppTester() {
+    }
+
+    public static <T> Consumer<TestCounter> variant(
+            final Named<Comparator<Map.Entry<String, Integer>>> comparator,
+            final Named<IntFunction<IntStream>> selector,
+            final Named<Extractor<T>> extractor,
+            final Named<String> extra
+    ) {
+        // Stream "magic" code. You do not expect to understand it
+        return counter -> WordStatChecker.test(
+                counter,
+                "Wspp" + comparator.name() + selector.name() + extractor.name() + extra.name(),
+                text -> {
+                    final Map<String, Integer> totals = Arrays.stream(text)
+                            .flatMap(Arrays::stream)
+                            .map(word -> word.toLowerCase(Locale.ROOT))
+                            .collect(Collectors.toMap(Function.identity(), k -> 1, Integer::sum, LinkedHashMap::new));
+                    final int[] lengths = Arrays.stream(text).mapToInt(a -> a.length).toArray();
+                    final int[] sizes = new int[lengths.length + 1];
+                    int start = 0;
+                    for (int i = 0; i < lengths.length; i++) {
+                        sizes[i] = start;
+                        start += lengths[i];
+                    }
+                    sizes[lengths.length] = start;
+
+                    final Map<String, String> selected = IntStream.range(0, text.length).boxed()
+                            .flatMap(r -> {
+                                     final String[] line = text[r];
+                                     return IntStream.range(0, line.length).boxed()
+                                             .collect(Collectors.groupingBy(
+                                                     w -> line[w].toLowerCase(Locale.ROOT),
+                                                     Collectors.collectingAndThen(
+                                                             Collectors.mapping(
+                                                                     w -> extractor.value().select(
+                                                                             r + 1,
+                                                                             w + 1,
+                                                                             line.length,
+                                                                             sizes[r] + w + 1,
+                                                                             sizes[lengths.length]
+                                                                     ),
+                                                                     Collectors.toUnmodifiableList()
+                                                             ),
+                                                             list -> selector.value()
+                                                                     .apply(list.size())
+                                                                     .mapToObj(list::get)
+                                                                     .toList()
+                                                     )
+                                             ))
+                                             .entrySet().stream();
+                                 }
+                            )
+                            .collect(Collectors.groupingBy(
+                                    Map.Entry::getKey,
+                                    Collectors.flatMapping(
+                                            e -> e.getValue().stream(),
+                                            Collectors.mapping(
+                                                    String::valueOf,
+                                                    Collectors.mapping(" "::concat, Collectors.joining())
+                                            )
+                                    )
+                            ));
+                    return totals.entrySet().stream()
+                            .sorted(comparator.value())
+                            .map(e -> Pair.of(e.getKey(), e.getValue() + selected.get(e.getKey())))
+                            .collect(Collectors.toList());
+                },
+                checker -> {
+                    final Pattern pattern = Pattern.compile(new String(Base64.getDecoder().decode("W15ccHtJc0xldHRlcn0nXHB7UGR9" + extra.value()), StandardCharsets.US_ASCII) + "]+");
+                    final String good = String.join("", pattern.split(WordStatTester.POST_LOWER));
+
+                    checker.test(pattern, "To be, or not to be, that is the question:");
+                    checker.test(
+                            pattern,
+                            "Monday's child is fair of face.",
+                            "Tuesday's child is full of grace."
+                    );
+                    checker.test(
+                            pattern,
+                            "Шалтай-Болтай",
+                            "Сидел на стене.",
+                            "Шалтай-Болтай",
+                            "Свалился во сне."
+                    );
+
+                    checker.randomTest(3, 10, 10, 3, ExtendedRandom.ENGLISH, WordStatChecker.SIMPLE_DELIMITERS);
+                    checker.randomTest(10, 3, 5, 5, ExtendedRandom.RUSSIAN, WordStatChecker.SIMPLE_DELIMITERS);
+                    checker.randomTest(3, 10, 10, 3, ExtendedRandom.GREEK, WordStatChecker.SIMPLE_DELIMITERS);
+                    checker.randomTest(3, 10, 10, 3, WordStatChecker.DASH, WordStatChecker.SIMPLE_DELIMITERS);
+                    checker.randomTest(3, 10, 10, 3, ExtendedRandom.ENGLISH, WordStatChecker.ADVANCED_DELIMITERS);
+                    checker.randomTest(10, 3, 5, 5, ExtendedRandom.RUSSIAN, WordStatChecker.ADVANCED_DELIMITERS);
+                    checker.randomTest(3, 10, 10, 3, ExtendedRandom.GREEK, WordStatChecker.ADVANCED_DELIMITERS);
+                    checker.randomTest(3, 10, 10, 3, WordStatChecker.DASH, WordStatChecker.ADVANCED_DELIMITERS);
+
+                    checker.randomTest(10, 20, 10, 3, good, WordStatChecker.SIMPLE_DELIMITERS);
+                    checker.randomTest(10, 20, 10, 3, good, WordStatChecker.ADVANCED_DELIMITERS);
+
+                    final int d = TestCounter.DENOMINATOR;
+                    final int d2 = TestCounter.DENOMINATOR2;
+                    checker.randomTest(100, 1000 / d, 1000 / d2, 1000 / d2, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                    checker.randomTest(10, 1000 / d, 1000 / d2, 1000 / d2, good, WordStatChecker.ADVANCED_DELIMITERS);
+
+                    checker.randomTest(10000 / d, 20, 10, 5, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+                    checker.randomTest(1000000 / d, 2, 2, 1, WordStatChecker.ALL, WordStatChecker.ADVANCED_DELIMITERS);
+
+                    checker.test(pattern, WordStatTester.PRE_LOWER);
+                    checker.test(pattern, WordStatTester.POST_LOWER);
+                }
+        );
+    }
+
+    @FunctionalInterface
+    public interface Extractor<T> {
+        T select(int l, int li, int lt, int gi, int gt);
+    }
+}

java/wspp/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Tests for <a href="https://www.kgeorgiy.info/courses/prog-intro/homeworks.html#wspp">Word Statistics++</a> homework
+ * of <a href="https://www.kgeorgiy.info/courses/prog-intro/">Introduction to Programming</a> course.
+ *
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+package wspp;