@@ -109,7 +109,8 @@ public final class Vector<T> implements Kind1<Vector<?>, T>, IndexedSeq<T>, Seri
     @SafeVarargs
     @SuppressWarnings("varargs")
     public static <T> Vector<T> of(T... elements) {
-        return ofAll(Iterator.of(elements));
+        Objects.requireNonNull(elements, "elements is null");
+        return ofAll(BitMappedTrie.ofAll(elements));
     }
 
     /**
/*     / \____  _    _  ____   ______  / \ ____  __    _______
 *    /  /    \/ \  / \/    \ /  /\__\/  //    \/  \  //  /\__\   JΛVΛSLΛNG
 *  _/  /  /\  \  \/  /  /\  \\__\\  \  //  /\  \ /\\/ \ /__\ \   Copyright 2014-2017 Javaslang, http://javaslang.io
 * /___/\_/  \_/\____/\_/  \_/\__\/__/\__\_/  \_//  \__/\_____/   Licensed under the Apache License, Version 2.0
 */
package javaslang.collection;

import javaslang.*;
import javaslang.collection.VectorModule.Combinations;
import javaslang.control.Option;

import java.io.Serializable;
import java.util.*;
import java.util.function.*;
import java.util.stream.Collector;

import static javaslang.collection.Collections.areEqual;
import static javaslang.collection.Collections.withSize;

/**
 * Vector is the default Seq implementation that provides effectively constant time access to any element.
 * Many other operations (e.g. `tail`, `drop`, `slice`) are also effectively constant.
 *
 * The implementation is based on a `bit-mapped trie`, a very wide and shallow tree (i.e. depth ≤ 6).
 *
 * @param <T> Component type of the Vector.
 * @author Ruslan Sennov, Pap Lőrinc
 * @since 2.0.0
 */
public final class Vector<T> implements Kind1<Vector<?>, T>, IndexedSeq<T>, Serializable {
    private static final long serialVersionUID = 1L;

    private static final Vector<?> EMPTY = new Vector<>(BitMappedTrie.empty());

    final BitMappedTrie<T> trie;
    private Vector(BitMappedTrie<T> trie) { this.trie = trie; }

    @SuppressWarnings("ObjectEquality")
    private Vector<T> wrap(BitMappedTrie<T> trie) {
        return (trie == this.trie)
               ? this
               : ofAll(trie);
    }

    private static <T> Vector<T> ofAll(BitMappedTrie<T> trie) {
        return (trie.length() == 0)
               ? empty()
               : new Vector<>(trie);
    }

    /**
     * Returns the empty Vector.
     *
     * @param <T> Component type.
     * @return The empty Vector.
     */
    @SuppressWarnings("unchecked")
    public static <T> Vector<T> empty() { return (Vector<T>) EMPTY; }

    /**
     * Returns a {@link Collector} which may be used in conjunction with
     * {@link java.util.stream.Stream#collect(Collector)} to obtain a {@link Vector}.
     *
     * @param <T> Component type of the Vector.
     * @return A javaslang.collection.List Collector.
     */
    public static <T> Collector<T, ArrayList<T>, Vector<T>> collector() {
        final Supplier<ArrayList<T>> supplier = ArrayList::new;
        final BiConsumer<ArrayList<T>, T> accumulator = ArrayList::add;
        final BinaryOperator<ArrayList<T>> combiner = (left, right) -> {
            left.addAll(right);
            return left;
        };
        final Function<ArrayList<T>, Vector<T>> finisher = Vector::ofAll;
        return Collector.of(supplier, accumulator, combiner, finisher);
    }

    /**
     * Narrows a widened {@code Vector<? extends T>} to {@code Vector<T>}
     * by performing a type-safe cast. This is eligible because immutable/read-only
     * collections are covariant.
     *
     * @param vector An {@code Vector}.
     * @param <T>    Component type of the {@code Vector}.
     * @return the given {@code vector} instance as narrowed type {@code Vector<T>}.
     */
    @SuppressWarnings("unchecked")
    public static <T> Vector<T> narrow(Vector<? extends T> vector) { return (Vector<T>) vector; }

    /**
     * Returns a singleton {@code Vector}, i.e. a {@code Vector} of one element.
     *
     * @param element An element.
     * @param <T>     The component type
     * @return A new Vector instance containing the given element
     */
    public static <T> Vector<T> of(T element) {
        return ofAll(Iterator.of(element));
    }

    /**
     * Creates a Vector of the given elements.
     *
     * @param <T>      Component type of the Vector.
     * @param elements Zero or more elements.
     * @return A vector containing the given elements in the same order.
     * @throws NullPointerException if {@code elements} is null
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    public static <T> Vector<T> of(T... elements) {
        return ofAll(Iterator.of(elements));
    }

    /**
     * Returns a Vector containing {@code n} values of a given Function {@code f}
     * over a range of integer values from 0 to {@code n - 1}.
     *
     * @param <T> Component type of the Vector
     * @param n   The number of elements in the Vector
     * @param f   The Function computing element values
     * @return A Vector consisting of elements {@code f(0),f(1), ..., f(n - 1)}
     * @throws NullPointerException if {@code f} is null
     */
    public static <T> Vector<T> tabulate(int n, Function<? super Integer, ? extends T> f) {
        Objects.requireNonNull(f, "f is null");
        return Collections.tabulate(n, f, empty(), Vector::of);
    }

    /**
     * Returns a Vector containing {@code n} values supplied by a given Supplier {@code s}.
     *
     * @param <T> Component type of the Vector
     * @param n   The number of elements in the Vector
     * @param s   The Supplier computing element values
     * @return A Vector of size {@code n}, where each element contains the result supplied by {@code s}.
     * @throws NullPointerException if {@code s} is null
     */
    public static <T> Vector<T> fill(int n, Supplier<? extends T> s) {
        Objects.requireNonNull(s, "s is null");
        return Collections.fill(n, s, empty(), Vector::of);
    }

    /**
     * Creates a Vector of the given elements.
     * <p>
     * The resulting vector has the same iteration order as the given iterable of elements
     * if the iteration order of the elements is stable.
     *
     * @param <T>      Component type of the Vector.
     * @param iterable An Iterable of elements.
     * @return A vector containing the given elements in the same order.
     * @throws NullPointerException if {@code elements} is null
     */
    @SuppressWarnings("unchecked")
    public static <T> Vector<T> ofAll(Iterable<? extends T> iterable) {
        Objects.requireNonNull(iterable, "iterable is null");
        if (iterable instanceof Vector) {
            return (Vector<T>) iterable;
        } else {
            final Object[] values = withSize(iterable).toArray();
            return ofAll(BitMappedTrie.ofAll(values));
        }
    }

    /**
     * Creates a Vector that contains the elements of the given {@link java.util.stream.Stream}.
     *
     * @param javaStream A {@link java.util.stream.Stream}
     * @param <T>        Component type of the Stream.
     * @return A Vector containing the given elements in the same order.
     */
    public static <T> Vector<T> ofAll(java.util.stream.Stream<? extends T> javaStream) {
        Objects.requireNonNull(javaStream, "javaStream is null");
        return ofAll(Iterator.ofAll(javaStream.iterator()));
    }

    /**
     * Creates a Vector from boolean values.
     *
     * @param elements boolean values
     * @return A new Vector of Boolean values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Boolean> ofAll(boolean... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from byte values.
     *
     * @param elements byte values
     * @return A new Vector of Byte values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Byte> ofAll(byte... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from char values.
     *
     * @param elements char values
     * @return A new Vector of Character values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Character> ofAll(char... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from double values.
     *
     * @param elements double values
     * @return A new Vector of Double values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Double> ofAll(double... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from float values.
     *
     * @param elements float values
     * @return A new Vector of Float values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Float> ofAll(float... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from int values.
     *
     * @param elements int values
     * @return A new Vector of Integer values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Integer> ofAll(int... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from long values.
     *
     * @param elements long values
     * @return A new Vector of Long values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Long> ofAll(long... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    /**
     * Creates a Vector from short values.
     *
     * @param elements short values
     * @return A new Vector of Short values
     * @throws NullPointerException if elements is null
     */
    public static Vector<Short> ofAll(short... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(BitMappedTrie.ofAll(elements));
    }

    public static Vector<Character> range(char from, char toExclusive) {
        return ofAll(ArrayType.<char[]> asPrimitives(char.class, Iterator.range(from, toExclusive)));
    }

    public static Vector<Character> rangeBy(char from, char toExclusive, int step) {
        return ofAll(ArrayType.<char[]> asPrimitives(char.class, Iterator.rangeBy(from, toExclusive, step)));
    }

    @GwtIncompatible
    public static Vector<Double> rangeBy(double from, double toExclusive, double step) {
        return ofAll(ArrayType.<double[]> asPrimitives(double.class, Iterator.rangeBy(from, toExclusive, step)));
    }

    /**
     * Creates a Vector of int numbers starting from {@code from}, extending to {@code toExclusive - 1}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.range(0, 0)  // = Vector()
     * Vector.range(2, 0)  // = Vector()
     * Vector.range(-2, 2) // = Vector(-2, -1, 0, 1)
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toExclusive the last number + 1
     * @return a range of int values as specified or the empty range if {@code from >= toExclusive}
     */
    public static Vector<Integer> range(int from, int toExclusive) {
        return ofAll(ArrayType.<int[]> asPrimitives(int.class, Iterator.range(from, toExclusive)));
    }

    /**
     * Creates a Vector of int numbers starting from {@code from}, extending to {@code toExclusive - 1},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.rangeBy(1, 3, 1)  // = Vector(1, 2)
     * Vector.rangeBy(1, 4, 2)  // = Vector(1, 3)
     * Vector.rangeBy(4, 1, -2) // = Vector(4, 2)
     * Vector.rangeBy(4, 1, 2)  // = Vector()
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toExclusive the last number + 1
     * @param step        the step
     * @return a range of long values as specified or the empty range if<br>
     * {@code from >= toInclusive} and {@code step > 0} or<br>
     * {@code from <= toInclusive} and {@code step < 0}
     * @throws IllegalArgumentException if {@code step} is zero
     */
    public static Vector<Integer> rangeBy(int from, int toExclusive, int step) {
        return ofAll(ArrayType.<int[]> asPrimitives(int.class, Iterator.rangeBy(from, toExclusive, step)));
    }

    /**
     * Creates a Vector of long numbers starting from {@code from}, extending to {@code toExclusive - 1}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.range(0L, 0L)  // = Vector()
     * Vector.range(2L, 0L)  // = Vector()
     * Vector.range(-2L, 2L) // = Vector(-2L, -1L, 0L, 1L)
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toExclusive the last number + 1
     * @return a range of long values as specified or the empty range if {@code from >= toExclusive}
     */
    public static Vector<Long> range(long from, long toExclusive) {
        return ofAll(ArrayType.<long[]> asPrimitives(long.class, Iterator.range(from, toExclusive)));
    }

    /**
     * Creates a Vector of long numbers starting from {@code from}, extending to {@code toExclusive - 1},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.rangeBy(1L, 3L, 1L)  // = Vector(1L, 2L)
     * Vector.rangeBy(1L, 4L, 2L)  // = Vector(1L, 3L)
     * Vector.rangeBy(4L, 1L, -2L) // = Vector(4L, 2L)
     * Vector.rangeBy(4L, 1L, 2L)  // = Vector()
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toExclusive the last number + 1
     * @param step        the step
     * @return a range of long values as specified or the empty range if<br>
     * {@code from >= toInclusive} and {@code step > 0} or<br>
     * {@code from <= toInclusive} and {@code step < 0}
     * @throws IllegalArgumentException if {@code step} is zero
     */
    public static Vector<Long> rangeBy(long from, long toExclusive, long step) {
        return ofAll(ArrayType.<long[]> asPrimitives(long.class, Iterator.rangeBy(from, toExclusive, step)));
    }

    public static Vector<Character> rangeClosed(char from, char toInclusive) {
        return ofAll(ArrayType.<char[]> asPrimitives(char.class, Iterator.rangeClosed(from, toInclusive)));
    }

    public static Vector<Character> rangeClosedBy(char from, char toInclusive, int step) {
        return ofAll(ArrayType.<char[]> asPrimitives(char.class, Iterator.rangeClosedBy(from, toInclusive, step)));
    }

    @GwtIncompatible
    public static Vector<Double> rangeClosedBy(double from, double toInclusive, double step) {
        return ofAll(ArrayType.<double[]> asPrimitives(double.class, Iterator.rangeClosedBy(from, toInclusive, step)));
    }

    /**
     * Creates a Vector of int numbers starting from {@code from}, extending to {@code toInclusive}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.rangeClosed(0, 0)  // = Vector(0)
     * Vector.rangeClosed(2, 0)  // = Vector()
     * Vector.rangeClosed(-2, 2) // = Vector(-2, -1, 0, 1, 2)
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toInclusive the last number
     * @return a range of int values as specified or the empty range if {@code from > toInclusive}
     */
    public static Vector<Integer> rangeClosed(int from, int toInclusive) {
        return ofAll(ArrayType.<int[]> asPrimitives(int.class, Iterator.rangeClosed(from, toInclusive)));
    }

    /**
     * Creates a Vector of int numbers starting from {@code from}, extending to {@code toInclusive},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.rangeClosedBy(1, 3, 1)  // = Vector(1, 2, 3)
     * Vector.rangeClosedBy(1, 4, 2)  // = Vector(1, 3)
     * Vector.rangeClosedBy(4, 1, -2) // = Vector(4, 2)
     * Vector.rangeClosedBy(4, 1, 2)  // = Vector()
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toInclusive the last number
     * @param step        the step
     * @return a range of int values as specified or the empty range if<br>
     * {@code from > toInclusive} and {@code step > 0} or<br>
     * {@code from < toInclusive} and {@code step < 0}
     * @throws IllegalArgumentException if {@code step} is zero
     */
    public static Vector<Integer> rangeClosedBy(int from, int toInclusive, int step) {
        return ofAll(ArrayType.<int[]> asPrimitives(int.class, Iterator.rangeClosedBy(from, toInclusive, step)));
    }

    /**
     * Creates a Vector of long numbers starting from {@code from}, extending to {@code toInclusive}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.rangeClosed(0L, 0L)  // = Vector(0L)
     * Vector.rangeClosed(2L, 0L)  // = Vector()
     * Vector.rangeClosed(-2L, 2L) // = Vector(-2L, -1L, 0L, 1L, 2L)
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toInclusive the last number
     * @return a range of long values as specified or the empty range if {@code from > toInclusive}
     */
    public static Vector<Long> rangeClosed(long from, long toInclusive) {
        return ofAll(ArrayType.<long[]> asPrimitives(long.class, Iterator.rangeClosed(from, toInclusive)));
    }

    /**
     * Creates a Vector of long numbers starting from {@code from}, extending to {@code toInclusive},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Vector.rangeClosedBy(1L, 3L, 1L)  // = Vector(1L, 2L, 3L)
     * Vector.rangeClosedBy(1L, 4L, 2L)  // = Vector(1L, 3L)
     * Vector.rangeClosedBy(4L, 1L, -2L) // = Vector(4L, 2L)
     * Vector.rangeClosedBy(4L, 1L, 2L)  // = Vector()
     * </code>
     * </pre>
     *
     * @param from        the first number
     * @param toInclusive the last number
     * @param step        the step
     * @return a range of int values as specified or the empty range if<br>
     * {@code from > toInclusive} and {@code step > 0} or<br>
     * {@code from < toInclusive} and {@code step < 0}
     * @throws IllegalArgumentException if {@code step} is zero
     */
    public static Vector<Long> rangeClosedBy(long from, long toInclusive, long step) {
        return ofAll(ArrayType.<long[]> asPrimitives(long.class, Iterator.rangeClosedBy(from, toInclusive, step)));
    }

    /**
     * Transposes the rows and columns of a {@link Vector} matrix.
     *
     * @param matrix to be transposed.
     * @return a transposed {@link Vector} matrix.
     * @throws IllegalArgumentException if the row lengths of {@code matrix} differ.
     * <p>
     * ex: {@code
     * Vector.transpose(Vector(Vector(1,2,3), Vector(4,5,6))) → Vector(Vector(1,4), Vector(2,5), Vector(3,6))
     * }
     */
    public static <T> Vector<Vector<T>> transpose(Vector<Vector<T>> matrix) {
        return Collections.transpose(matrix, Vector::ofAll, Vector::of);
    }

    /**
     * Creates a Vector from a seed value and a function.
     * The function takes the seed at first.
     * The function should return {@code None} when it's
     * done generating the Vector, otherwise {@code Some} {@code Tuple}
     * of the element for the next call and the value to add to the
     * resulting Vector.
     * <p>
     * Example:
     * <pre>
     * <code>
     * Vector.unfoldRight(10, x -&gt; x == 0
     *             ? Option.none()
     *             : Option.of(new Tuple2&lt;&gt;(x, x-1)));
     * // Vector(10, 9, 8, 7, 6, 5, 4, 3, 2, 1))
     * </code>
     * </pre>
     *
     * @param <T>  type of seeds
     * @param <U>  type of unfolded values
     * @param seed the start value for the iteration
     * @param f    the function to get the next step of the iteration
     * @return a Vector with the values built up by the iteration
     * @throws NullPointerException if {@code f} is null
     */
    public static <T, U> Vector<U> unfoldRight(T seed, Function<? super T, Option<Tuple2<? extends U, ? extends T>>> f) {
        return Iterator.unfoldRight(seed, f).toVector();
    }

    /**
     * Creates a Vector from a seed value and a function.
     * The function takes the seed at first.
     * The function should return {@code None} when it's
     * done generating the Vector, otherwise {@code Some} {@code Tuple}
     * of the value to add to the resulting Vector and
     * the element for the next call.
     * <p>
     * Example:
     * <pre>
     * <code>
     * Vector.unfoldLeft(10, x -&gt; x == 0
     *             ? Option.none()
     *             : Option.of(new Tuple2&lt;&gt;(x-1, x)));
     * // Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
     * </code>
     * </pre>
     *
     * @param <T>  type of seeds
     * @param <U>  type of unfolded values
     * @param seed the start value for the iteration
     * @param f    the function to get the next step of the iteration
     * @return a Vector with the values built up by the iteration
     * @throws NullPointerException if {@code f} is null
     */
    public static <T, U> Vector<U> unfoldLeft(T seed, Function<? super T, Option<Tuple2<? extends T, ? extends U>>> f) {
        return Iterator.unfoldLeft(seed, f).toVector();
    }

    /**
     * Creates a Vector from a seed value and a function.
     * The function takes the seed at first.
     * The function should return {@code None} when it's
     * done generating the Vector, otherwise {@code Some} {@code Tuple}
     * of the value to add to the resulting Vector and
     * the element for the next call.
     * <p>
     * Example:
     * <pre>
     * <code>
     * Vector.unfold(10, x -&gt; x == 0
     *             ? Option.none()
     *             : Option.of(new Tuple2&lt;&gt;(x-1, x)));
     * // Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
     * </code>
     * </pre>
     *
     * @param <T>  type of seeds and unfolded values
     * @param seed the start value for the iteration
     * @param f    the function to get the next step of the iteration
     * @return a Vector with the values built up by the iteration
     * @throws NullPointerException if {@code f} is null
     */
    public static <T> Vector<T> unfold(T seed, Function<? super T, Option<Tuple2<? extends T, ? extends T>>> f) {
        return Iterator.unfold(seed, f).toVector();
    }

    @Override
    public Vector<T> append(T element) { return appendAll(List.of(element)); }

    @Override
    public Vector<T> appendAll(Iterable<? extends T> iterable) {
        Objects.requireNonNull(iterable, "iterable is null");
        return isEmpty()
               ? ofAll(iterable)
               : new Vector<>(trie.appendAll(iterable));
    }

    @Override
    public Vector<Vector<T>> combinations() { return rangeClosed(0, length()).map(this::combinations).flatMap(Function.identity()); }

    @Override
    public Vector<Vector<T>> combinations(int k) { return Combinations.apply(this, Math.max(k, 0)); }

    @Override
    public Iterator<Vector<T>> crossProduct(int power) { return Collections.crossProduct(empty(), this, power); }

    @Override
    public Vector<T> distinct() { return distinctBy(Function.identity()); }

    @Override
    public Vector<T> distinctBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator, "comparator is null");
        final java.util.Set<T> seen = new java.util.TreeSet<>(comparator);
        return filter(seen::add);
    }

    @Override
    public <U> Vector<T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
        final java.util.Set<U> seen = new java.util.HashSet<>(length());
        return filter(t -> seen.add(keyExtractor.apply(t)));
    }

    @Override
    public Vector<T> drop(int n) {
        return wrap(trie.drop(n));
    }

    @Override
    public Vector<T> dropUntil(Predicate<? super T> predicate) {
        return Collections.dropUntil(this, predicate);
    }

    @Override
    public Vector<T> dropWhile(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        return dropUntil(predicate.negate());
    }

    @Override
    public Vector<T> dropRight(int n) {
        return take(length() - n);
    }

    @Override
    public Vector<T> dropRightUntil(Predicate<? super T> predicate) {
        return Collections.dropRightUntil(this, predicate);
    }

    @Override
    public Vector<T> dropRightWhile(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        return dropRightUntil(predicate.negate());
    }

    @Override
    public Vector<T> filter(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        return wrap(trie.filter(predicate));
    }

    @Override
    public <U> Vector<U> flatMap(Function<? super T, ? extends Iterable<? extends U>> mapper) {
        Objects.requireNonNull(mapper, "mapper is null");
        final Iterator<? extends U> results = iterator().flatMap(mapper);
        return ofAll(results);
    }

    @Override
    public T get(int index) {
        if (isValid(index)) {
            return trie.get(index);
        } else {
            throw new IndexOutOfBoundsException("get(" + index + ")");
        }
    }
    private boolean isValid(int index) { return (index >= 0) && (index < length()); }

    @Override
    public T head() {
        if (nonEmpty()) {
            return get(0);
        } else {
            throw new NoSuchElementException("head of empty Vector");
        }
    }

    @Override
    public <C> Map<C, Vector<T>> groupBy(Function<? super T, ? extends C> classifier) { return Collections.groupBy(this, classifier, Vector::ofAll); }

    @Override
    public Iterator<Vector<T>> grouped(int size) { return sliding(size, size); }

    @Override
    public boolean hasDefiniteSize() { return true; }

    @Override
    public int indexOf(T element, int from) {
        for (int i = from; i < length(); i++) {
            if (Objects.equals(get(i), element)) {
                return i;
            }
        }
        return -1;
    }

    @Override
    public Vector<T> init() {
        if (nonEmpty()) {
            return dropRight(1);
        } else {
            throw new UnsupportedOperationException("init of empty Vector");
        }
    }

    @Override
    public Option<Vector<T>> initOption() { return isEmpty() ? Option.none() : Option.some(init()); }

    @Override
    public Vector<T> insert(int index, T element) { return insertAll(index, Iterator.of(element)); }

    @Override
    public Vector<T> insertAll(int index, Iterable<? extends T> elements) {
        if ((index >= 0) && (index <= length())) {
            final Vector<T> begin = take(index).appendAll(elements);
            final Vector<T> end = drop(index);
            return (begin.size() > end.size())
                   ? begin.appendAll(end)
                   : end.prependAll(begin);
        } else {
            throw new IndexOutOfBoundsException("insert(" + index + ", e) on Vector of length " + length());
        }
    }

    @Override
    public Vector<T> intersperse(T element) { return ofAll(iterator().intersperse(element)); }

    @Override
    public boolean isEmpty() { return length() == 0; }

    @Override
    public boolean isTraversableAgain() { return true; }

    @Override
    public Iterator<T> iterator() {
        return isEmpty() ? Iterator.empty()
                         : trie.iterator();
    }

    @Override
    public int lastIndexOf(T element, int end) {
        for (int i = Math.min(end, length() - 1); i >= 0; i--) {
            if (Objects.equals(get(i), element)) {
                return i;
            }
        }
        return -1;
    }

    @Override
    public int length() { return trie.length(); }

    @Override
    public <U> Vector<U> map(Function<? super T, ? extends U> mapper) {
        Objects.requireNonNull(mapper, "mapper is null");
        return ofAll(trie.map(mapper));
    }

    @Override
    public Vector<T> padTo(int length, T element) {
        final int actualLength = length();
        return (length <= actualLength)
               ? this
               : appendAll(Iterator.continually(element)
                .take(length - actualLength));
    }

    @Override
    public Vector<T> leftPadTo(int length, T element) {
        if (length <= length()) {
            return this;
        } else {
            final Iterator<T> prefix = Iterator.continually(element).take(length - length());
            return prependAll(prefix);
        }
    }

    @Override
    public Vector<T> patch(int from, Iterable<? extends T> that, int replaced) {
        from = Math.max(from, 0);
        replaced = Math.max(replaced, 0);

        Vector<T> result = take(from).appendAll(that);
        from += replaced;
        result = result.appendAll(drop(from));
        return result;
    }

    @Override
    public Tuple2<Vector<T>, Vector<T>> partition(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        final ArrayList<T> left = new ArrayList<>(), right = new ArrayList<>();
        for (int i = 0; i < length(); i++) {
            final T t = get(i);
            (predicate.test(t) ? left : right).add(t);
        }
        return Tuple.of(ofAll(left), ofAll(right));
    }

    @Override
    public Vector<T> peek(Consumer<? super T> action) {
        Objects.requireNonNull(action, "action is null");
        if (!isEmpty()) {
            action.accept(head());
        }
        return this;
    }

    @Override
    public Vector<Vector<T>> permutations() {
        if (isEmpty()) {
            return empty();
        } else if (length() == 1) {
            return of(this);
        } else {
            Vector<Vector<T>> results = empty();
            for (T t : distinct()) {
                for (Vector<T> ts : remove(t).permutations()) {
                    results = results.append(of(t).appendAll(ts));
                }
            }
            return results;
        }
    }

    @Override
    public Vector<T> prepend(T element) { return prependAll(List.of(element)); }

    @Override
    public Vector<T> prependAll(Iterable<? extends T> iterable) {
        Objects.requireNonNull(iterable, "iterable is null");
        return isEmpty()
               ? ofAll(iterable)
               : new Vector<>(trie.prependAll(iterable));
    }

    @Override
    public Vector<T> remove(T element) {
        for (int i = 0; i < length(); i++) {
            if (Objects.equals(get(i), element)) {
                return removeAt(i);
            }
        }
        return this;
    }

    @Override
    public Vector<T> removeFirst(Predicate<T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        for (int i = 0; i < length(); i++) {
            if (predicate.test(get(i))) {
                return removeAt(i);
            }
        }
        return this;
    }

    @Override
    public Vector<T> removeLast(Predicate<T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        for (int i = length() - 1; i >= 0; i--) {
            if (predicate.test(get(i))) {
                return removeAt(i);
            }
        }
        return this;
    }

    @Override
    public Vector<T> removeAt(int index) {
        if (isValid(index)) {
            final Vector<T> begin = take(index);
            final Vector<T> end = drop(index + 1);
            return (begin.size() > end.size())
                   ? begin.appendAll(end)
                   : end.prependAll(begin);
        } else {
            throw new IndexOutOfBoundsException("removeAt(" + index + ")");
        }
    }

    @Override
    public Vector<T> removeAll(T element) {
        return Collections.removeAll(this, element);
    }

    @Override
    public Vector<T> removeAll(Iterable<? extends T> elements) {
        return Collections.removeAll(this, elements);
    }

    @Override
    public Vector<T> removeAll(Predicate<? super T> predicate) {
        return Collections.removeAll(this, predicate);
    }

    @Override
    public Vector<T> replace(T currentElement, T newElement) {
        return indexOfOption(currentElement)
                .map(i -> update(i, newElement))
                .getOrElse(this);
    }

    @Override
    public Vector<T> replaceAll(T currentElement, T newElement) {
        Vector<T> result = this;
        int index = 0;
        for (T value : iterator()) {
            if (Objects.equals(value, currentElement)) {
                result = result.update(index, newElement);
            }
            index++;
        }
        return result;
    }

    @Override
    public Vector<T> retainAll(Iterable<? extends T> elements) {
        return Collections.retainAll(this, elements);
    }

    @Override
    public Vector<T> reverse() {
        return (length() <= 1) ? this : ofAll(reverseIterator());
    }

    @Override
    public Vector<T> scan(T zero, BiFunction<? super T, ? super T, ? extends T> operation) {
        return scanLeft(zero, operation);
    }

    @Override
    public <U> Vector<U> scanLeft(U zero, BiFunction<? super U, ? super T, ? extends U> operation) {
        return Collections.scanLeft(this, zero, operation, Iterator::toVector);
    }

    @Override
    public <U> Vector<U> scanRight(U zero, BiFunction<? super T, ? super U, ? extends U> operation) {
        return Collections.scanRight(this, zero, operation, Iterator::toVector);
    }

    @Override
    public Vector<T> shuffle() {
        return Collections.shuffle(this, Vector::ofAll);
    }

    @Override
    public Vector<T> slice(int beginIndex, int endIndex) {
        if ((beginIndex >= endIndex) || (beginIndex >= size()) || isEmpty()) {
            return empty();
        } else if ((beginIndex <= 0) && (endIndex >= length())) {
            return this;
        } else {
            return take(endIndex).drop(beginIndex);
        }
    }

    @Override
    public Iterator<Vector<T>> slideBy(Function<? super T, ?> classifier) {
        return iterator().slideBy(classifier).map(Vector::ofAll);
    }

    @Override
    public Iterator<Vector<T>> sliding(int size) {
        return sliding(size, 1);
    }

    @Override
    public Iterator<Vector<T>> sliding(int size, int step) {
        return iterator().sliding(size, step).map(Vector::ofAll);
    }

    @Override
    public Vector<T> sorted() {
        return isEmpty() ? this : toJavaStream().sorted().collect(collector());
    }

    @Override
    public Vector<T> sorted(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator, "comparator is null");
        return isEmpty() ? this : toJavaStream().sorted(comparator).collect(collector());
    }

    @Override
    public <U extends Comparable<? super U>> Vector<T> sortBy(Function<? super T, ? extends U> mapper) {
        return sortBy(U::compareTo, mapper);
    }

    @Override
    public <U> Vector<T> sortBy(Comparator<? super U> comparator, Function<? super T, ? extends U> mapper) {
        final Function<? super T, ? extends U> domain = Function1.of(mapper::apply).memoized();
        return toJavaStream()
                .sorted((e1, e2) -> comparator.compare(domain.apply(e1), domain.apply(e2)))
                .collect(collector());
    }

    @Override
    public Tuple2<Vector<T>, Vector<T>> span(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        return Tuple.of(takeWhile(predicate), dropWhile(predicate));
    }

    @Override
    public Tuple2<Vector<T>, Vector<T>> splitAt(int n) {
        return Tuple.of(take(n), drop(n));
    }

    @Override
    public Tuple2<Vector<T>, Vector<T>> splitAt(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        final Vector<T> init = takeWhile(predicate.negate());
        return Tuple.of(init, drop(init.size()));
    }

    @Override
    public Tuple2<Vector<T>, Vector<T>> splitAtInclusive(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        for (int i = 0; i < length(); i++) {
            final T value = get(i);
            if (predicate.test(value)) {
                return (i == (length() - 1)) ? Tuple.of(this, empty())
                                             : Tuple.of(take(i + 1), drop(i + 1));
            }
        }
        return Tuple.of(this, empty());
    }

    @Override
    public Spliterator<T> spliterator() {
        return Spliterators.spliterator(iterator(), length(), Spliterator.ORDERED | Spliterator.IMMUTABLE);
    }

    @Override
    public Vector<T> subSequence(int beginIndex) {
        if ((beginIndex >= 0) && (beginIndex <= length())) {
            return drop(beginIndex);
        } else {
            throw new IndexOutOfBoundsException("subSequence(" + beginIndex + ")");
        }
    }

    @Override
    public Vector<T> subSequence(int beginIndex, int endIndex) {
        if ((beginIndex >= 0) && (beginIndex <= endIndex) && (endIndex <= length())) {
            return slice(beginIndex, endIndex);
        } else {
            throw new IndexOutOfBoundsException("subSequence(" + beginIndex + ", " + endIndex + ") on Vector of size " + length());
        }
    }

    @Override
    public Vector<T> tail() {
        if (nonEmpty()) {
            return drop(1);
        } else {
            throw new UnsupportedOperationException("tail of empty Vector");
        }
    }

    @Override
    public Option<Vector<T>> tailOption() { return isEmpty() ? Option.none() : Option.some(tail()); }

    @Override
    public Vector<T> take(int n) {
        return wrap(trie.take(n));
    }

    @Override
    public Vector<T> takeRight(int n) {
        return drop(length() - n);
    }

    @Override
    public Vector<T> takeUntil(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        return takeWhile(predicate.negate());
    }

    @Override
    public Vector<T> takeWhile(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        for (int i = 0; i < length(); i++) {
            final T value = get(i);
            if (!predicate.test(value)) {
                return take(i);
            }
        }
        return this;
    }

    /**
     * Transforms this {@code Vector}.
     *
     * @param f   A transformation
     * @param <U> Type of transformation result
     * @return An instance of type {@code U}
     * @throws NullPointerException if {@code f} is null
     */
    public <U> U transform(Function<? super Vector<T>, ? extends U> f) {
        Objects.requireNonNull(f, "f is null");
        return f.apply(this);
    }

    @Override
    public <U> Vector<U> unit(Iterable<? extends U> iterable) { return ofAll(iterable); }

    @Override
    public <T1, T2> Tuple2<Vector<T1>, Vector<T2>> unzip(Function<? super T, Tuple2<? extends T1, ? extends T2>> unzipper) {
        Objects.requireNonNull(unzipper, "unzipper is null");
        Vector<T1> xs = empty();
        Vector<T2> ys = empty();
        for (int i = 0; i < length(); i++) {
            final Tuple2<? extends T1, ? extends T2> t = unzipper.apply(get(i));
            xs = xs.append(t._1);
            ys = ys.append(t._2);
        }
        return Tuple.of(xs, ys);
    }

    @Override
    public <T1, T2, T3> Tuple3<Vector<T1>, Vector<T2>, Vector<T3>> unzip3(Function<? super T, Tuple3<? extends T1, ? extends T2, ? extends T3>> unzipper) {
        Objects.requireNonNull(unzipper, "unzipper is null");
        Vector<T1> xs = empty();
        Vector<T2> ys = empty();
        Vector<T3> zs = empty();
        for (int i = 0; i < length(); i++) {
            final Tuple3<? extends T1, ? extends T2, ? extends T3> t = unzipper.apply(get(i));
            xs = xs.append(t._1);
            ys = ys.append(t._2);
            zs = zs.append(t._3);
        }
        return Tuple.of(xs, ys, zs);
    }

    @Override
    public Vector<T> update(int index, T element) {
        if (isValid(index)) {
            return wrap(trie.update(index, element));
        } else {
            throw new IndexOutOfBoundsException("update(" + index + ")");
        }
    }

    @Override
    public Vector<T> update(int index, Function<? super T, ? extends T> updater) {
        Objects.requireNonNull(updater, "updater is null");
        return update(index, updater.apply(get(index)));
    }

    @Override
    public <U> Vector<Tuple2<T, U>> zip(Iterable<? extends U> that) {
        return zipWith(that, Tuple::of);
    }

    @Override
    public <U, R> Vector<R> zipWith(Iterable<? extends U> that, BiFunction<? super T, ? super U, ? extends R> mapper) {
        Objects.requireNonNull(that, "that is null");
        Objects.requireNonNull(mapper, "mapper is null");
        return ofAll(iterator().zipWith(that, mapper));
    }

    @Override
    public <U> Vector<Tuple2<T, U>> zipAll(Iterable<? extends U> that, T thisElem, U thatElem) {
        Objects.requireNonNull(that, "that is null");
        return ofAll(iterator().zipAll(that, thisElem, thatElem));
    }

    @Override
    public Vector<Tuple2<T, Integer>> zipWithIndex() {
        return zipWithIndex(Tuple::of);
    }

    @Override
    public <U> Vector<U> zipWithIndex(BiFunction<? super T, ? super Integer, ? extends U> mapper) {
        Objects.requireNonNull(mapper, "mapper is null");
        return ofAll(iterator().zipWithIndex(mapper));
    }

    private Object readResolve() { return isEmpty() ? EMPTY : this; }

    @Override
    public boolean equals(Object that) {
        return (that == this) || ((that instanceof Vector) && areEqual(this, (Vector<?>) that));
    }

    @Override
    public int hashCode() { return Collections.hash(this); }

    @Override
    public String stringPrefix() { return "Vector"; }

    @Override
    public String toString() { return mkString(stringPrefix() + "(", ", ", ")"); }
}

interface VectorModule {
    final class Combinations {
        static <T> Vector<Vector<T>> apply(Vector<T> elements, int k) {
            return (k == 0)
                   ? Vector.of(Vector.empty())
                   : elements.zipWithIndex().flatMap(
                    t -> apply(elements.drop(t._2 + 1), (k - 1)).map((Vector<T> c) -> c.prepend(t._1)));
        }
    }
}

should be optimal for constructing it from array source ... what was I thinking wrapping this in an iterator :/