Vector<Vector<T>> combinations(Vector<T> elements, int k) {
return (k == 0)
? Vector.of(Vector.empty())
: elements
.zipWithIndex()
.flatMap(
t ->
combinations(elements.drop(t._2 + 1), (k - 1))
.map((Vector<T> c) -> c.prepend(t._1)));
}
@Override
public Vector<T> slice(int beginIndex, int endIndex) {
if (beginIndex < 0 || beginIndex > endIndex || endIndex > length()) {
throw new IndexOutOfBoundsException(
String.format("slice(%s, %s) on List of length %s", beginIndex, endIndex, length()));
}
if (beginIndex == endIndex) {
return Vector.empty();
}
HashArrayMappedTrie<Integer, T> trie = HashArrayMappedTrie.empty();
for (int i = beginIndex; i < endIndex; i++) {
trie = trie.put(trie.size(), get(i));
}
return trie.isEmpty() ? empty() : new Vector<>(trie);
}
@Override
public Vector<Vector<T>> permutations() {
if (isEmpty()) {
return Vector.empty();
} else {
final Vector<T> tail = tail();
if (tail.isEmpty()) {
return Vector.of(this);
} else {
final Vector<Vector<T>> zero = empty();
// TODO: IntelliJ IDEA 14.1.1 needs a redundant cast here, jdk 1.8.0_40 compiles fine
return distinct()
.foldLeft(
zero,
(xs, x) ->
xs.appendAll(
remove(x)
.permutations()
.map((Function<Vector<T>, Vector<T>>) l -> l.prepend(x))));
}
}
}
@Override
public Vector<T> clear() {
return Vector.empty();
}
@Override
protected <T> Vector<T> empty() {
return Vector.empty();
}