initial commit

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();
+    }
+}