vavr/collection/Queue.java

/*                        __    __  __  __    __  ___
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 *                        \  \/  /  /\  \  \/  /  /
 *                         \____/__/  \__\____/__/.ɪᴏ
 * ᶜᵒᵖʸʳᶦᵍʰᵗ ᵇʸ ᵛᵃᵛʳ ⁻ ˡᶦᶜᵉⁿˢᵉᵈ ᵘⁿᵈᵉʳ ᵗʰᵉ ᵃᵖᵃᶜʰᵉ ˡᶦᶜᵉⁿˢᵉ ᵛᵉʳˢᶦᵒⁿ ᵗʷᵒ ᵈᵒᵗ ᶻᵉʳᵒ
 */
package io.vavr.collection;

import io.vavr.*;
import io.vavr.control.Option;

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

import static io.vavr.collection.JavaConverters.ChangePolicy.IMMUTABLE;
import static io.vavr.collection.JavaConverters.ChangePolicy.MUTABLE;

/**
 * An immutable {@code Queue} stores elements allowing a first-in-first-out (FIFO) retrieval.
 * <p>
 * Queue API:
 *
 * <ul>
 * <li>{@link #dequeue()}</li>
 * <li>{@link #dequeueOption()}</li>
 * <li>{@link #enqueue(Object)}</li>
 * <li>{@link #enqueue(Object[])}</li>
 * <li>{@link #enqueueAll(Iterable)}</li>
 * <li>{@link #peek()}</li>
 * <li>{@link #peekOption()}</li>
 * </ul>
 *
 * A Queue internally consists of a front List containing the front elements of the Queue in the correct order and a
 * rear List containing the rear elements of the Queue in reverse order.
 * <p>
 * When the front list is empty, front and rear are swapped and rear is reversed. This implies the following queue
 * invariant: {@code front.isEmpty() => rear.isEmpty()}.
 * <p>
 * See Okasaki, Chris: <em>Purely Functional Data Structures</em> (p. 42 ff.). Cambridge, 2003.
 *
 * @param <T> Component type of the Queue
 * @author Daniel Dietrich
 */
public final class Queue<T> extends AbstractQueue<T, Queue<T>> implements LinearSeq<T> {

    private static final long serialVersionUID = 1L;

    private static final Queue<?> EMPTY = new Queue<>(io.vavr.collection.List.empty(), io.vavr.collection.List.empty());

    private final io.vavr.collection.List<T> front;
    private final io.vavr.collection.List<T> rear;

    /**
     * Creates a Queue consisting of a front List and a rear List.
     * <p>
     * For a {@code Queue(front, rear)} the following invariant holds: {@code Queue is empty <=> front is empty}.
     * In other words: If the Queue is not empty, the front List contains at least one element.
     *
     * @param front A List of front elements, in correct order.
     * @param rear  A List of rear elements, in reverse order.
     */
    private Queue(io.vavr.collection.List<T> front, io.vavr.collection.List<T> rear) {
        final boolean frontIsEmpty = front.isEmpty();
        this.front = frontIsEmpty ? rear.reverse() : front;
        this.rear = frontIsEmpty ? front : rear;
    }

    /**
     * Returns a {@link java.util.stream.Collector} which may be used in conjunction with
     * {@link java.util.stream.Stream#collect(java.util.stream.Collector)} to obtain a {@link Queue}
     * .
     *
     * @param <T> Component type of the Queue.
     * @return A io.vavr.collection.Queue Collector.
     */
    public static <T> Collector<T, ArrayList<T>, Queue<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>, Queue<T>> finisher = Queue::ofAll;
        return Collector.of(supplier, accumulator, combiner, finisher);
    }

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

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

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

    /**
     * Creates a Queue of the given elements.
     *
     * @param <T>      Component type of the Queue.
     * @param elements Zero or more elements.
     * @return A queue containing the given elements in the same order.
     * @throws NullPointerException if {@code elements} is null
     */
    @SuppressWarnings("varargs")
    @SafeVarargs
    public static <T> Queue<T> of(T... elements) {
        Objects.requireNonNull(elements, "elements is null");
        return ofAll(io.vavr.collection.List.of(elements));
    }

    /**
     * Creates a Queue of the given elements.
     *
     * @param <T>      Component type of the Queue.
     * @param elements An Iterable of elements.
     * @return A queue containing the given elements in the same order.
     * @throws NullPointerException if {@code elements} is null
     */
    @SuppressWarnings("unchecked")
    public static <T> Queue<T> ofAll(Iterable<? extends T> elements) {
        Objects.requireNonNull(elements, "elements is null");
        if (elements instanceof Queue) {
            return (Queue<T>) elements;
        } else if (!elements.iterator().hasNext()) {
            return empty();
        } else if (elements instanceof io.vavr.collection.List) {
            return new Queue<>((io.vavr.collection.List<T>) elements, io.vavr.collection.List.empty());
        } else {
            return new Queue<>(io.vavr.collection.List.ofAll(elements), io.vavr.collection.List.empty());
        }
    }

    /**
     * Creates a Queue 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 Queue containing the given elements in the same order.
     */
    public static <T> Queue<T> ofAll(java.util.stream.Stream<? extends T> javaStream) {
        Objects.requireNonNull(javaStream, "javaStream is null");
        return new Queue<>(io.vavr.collection.List.ofAll(javaStream), io.vavr.collection.List.empty());
    }

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

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

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

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

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

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

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

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

    /**
     * Returns a Queue 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 Queue
     * @param n   The number of elements in the Queue
     * @param f   The Function computing element values
     * @return A Queue consisting of elements {@code f(0),f(1), ..., f(n - 1)}
     * @throws NullPointerException if {@code f} is null
     */
    public static <T> Queue<T> tabulate(int n, Function<? super Integer, ? extends T> f) {
        Objects.requireNonNull(f, "f is null");
        return io.vavr.collection.Collections.tabulate(n, f, empty(), Queue::of);
    }

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

    public static Queue<Character> range(char from, char toExclusive) {
        return ofAll(Iterator.range(from, toExclusive));
    }

    public static Queue<Character> rangeBy(char from, char toExclusive, int step) {
        return ofAll(Iterator.rangeBy(from, toExclusive, step));
    }

    @GwtIncompatible
    public static Queue<Double> rangeBy(double from, double toExclusive, double step) {
        return ofAll(Iterator.rangeBy(from, toExclusive, step));
    }

    /**
     * Creates a Queue of int numbers starting from {@code from}, extending to {@code toExclusive - 1}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.range(0, 0)  // = Queue()
     * Queue.range(2, 0)  // = Queue()
     * Queue.range(-2, 2) // = Queue(-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 {@code Nil} if {@code from >= toExclusive}
     */
    public static Queue<Integer> range(int from, int toExclusive) {
        return ofAll(Iterator.range(from, toExclusive));
    }

    /**
     * Creates a Queue of int numbers starting from {@code from}, extending to {@code toExclusive - 1},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.rangeBy(1, 3, 1)  // = Queue(1, 2)
     * Queue.rangeBy(1, 4, 2)  // = Queue(1, 3)
     * Queue.rangeBy(4, 1, -2) // = Queue(4, 2)
     * Queue.rangeBy(4, 1, 2)  // = Queue()
     * </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 {@code Nil} 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 Queue<Integer> rangeBy(int from, int toExclusive, int step) {
        return ofAll(Iterator.rangeBy(from, toExclusive, step));
    }

    /**
     * Creates a Queue of long numbers starting from {@code from}, extending to {@code toExclusive - 1}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.range(0L, 0L)  // = Queue()
     * Queue.range(2L, 0L)  // = Queue()
     * Queue.range(-2L, 2L) // = Queue(-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 {@code Nil} if {@code from >= toExclusive}
     */
    public static Queue<Long> range(long from, long toExclusive) {
        return ofAll(Iterator.range(from, toExclusive));
    }

    /**
     * Creates a Queue of long numbers starting from {@code from}, extending to {@code toExclusive - 1},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.rangeBy(1L, 3L, 1L)  // = Queue(1L, 2L)
     * Queue.rangeBy(1L, 4L, 2L)  // = Queue(1L, 3L)
     * Queue.rangeBy(4L, 1L, -2L) // = Queue(4L, 2L)
     * Queue.rangeBy(4L, 1L, 2L)  // = Queue()
     * </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 {@code Nil} 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 Queue<Long> rangeBy(long from, long toExclusive, long step) {
        return ofAll(Iterator.rangeBy(from, toExclusive, step));
    }

    public static Queue<Character> rangeClosed(char from, char toInclusive) {
        return ofAll(Iterator.rangeClosed(from, toInclusive));
    }

    public static Queue<Character> rangeClosedBy(char from, char toInclusive, int step) {
        return ofAll(Iterator.rangeClosedBy(from, toInclusive, step));
    }

    @GwtIncompatible
    public static Queue<Double> rangeClosedBy(double from, double toInclusive, double step) {
        return ofAll(Iterator.rangeClosedBy(from, toInclusive, step));
    }

    /**
     * Creates a Queue of int numbers starting from {@code from}, extending to {@code toInclusive}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.rangeClosed(0, 0)  // = Queue(0)
     * Queue.rangeClosed(2, 0)  // = Queue()
     * Queue.rangeClosed(-2, 2) // = Queue(-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 {@code Nil} if {@code from > toInclusive}
     */
    public static Queue<Integer> rangeClosed(int from, int toInclusive) {
        return ofAll(Iterator.rangeClosed(from, toInclusive));
    }

    /**
     * Creates a Queue of int numbers starting from {@code from}, extending to {@code toInclusive},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.rangeClosedBy(1, 3, 1)  // = Queue(1, 2, 3)
     * Queue.rangeClosedBy(1, 4, 2)  // = Queue(1, 3)
     * Queue.rangeClosedBy(4, 1, -2) // = Queue(4, 2)
     * Queue.rangeClosedBy(4, 1, 2)  // = Queue()
     * </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 {@code Nil} 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 Queue<Integer> rangeClosedBy(int from, int toInclusive, int step) {
        return ofAll(Iterator.rangeClosedBy(from, toInclusive, step));
    }

    /**
     * Creates a Queue of long numbers starting from {@code from}, extending to {@code toInclusive}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.rangeClosed(0L, 0L)  // = Queue(0L)
     * Queue.rangeClosed(2L, 0L)  // = Queue()
     * Queue.rangeClosed(-2L, 2L) // = Queue(-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 {@code Nil} if {@code from > toInclusive}
     */
    public static Queue<Long> rangeClosed(long from, long toInclusive) {
        return ofAll(Iterator.rangeClosed(from, toInclusive));
    }

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

    /**
     * Creates a Queue of long numbers starting from {@code from}, extending to {@code toInclusive},
     * with {@code step}.
     * <p>
     * Examples:
     * <pre>
     * <code>
     * Queue.rangeClosedBy(1L, 3L, 1L)  // = Queue(1L, 2L, 3L)
     * Queue.rangeClosedBy(1L, 4L, 2L)  // = Queue(1L, 3L)
     * Queue.rangeClosedBy(4L, 1L, -2L) // = Queue(4L, 2L)
     * Queue.rangeClosedBy(4L, 1L, 2L)  // = Queue()
     * </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 {@code Nil} 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 Queue<Long> rangeClosedBy(long from, long toInclusive, long step) {
        return ofAll(Iterator.rangeClosedBy(from, toInclusive, step));
    }

    /**
     * Creates a Queue 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 Queue, otherwise {@code Some} {@code Tuple}
     * of the element for the next call and the value to add to the
     * resulting Queue.
     * <p>
     * Example:
     * <pre>
     * <code>
     * Queue.unfoldRight(10, x -&gt; x == 0
     *             ? Option.none()
     *             : Option.of(new Tuple2&lt;&gt;(x, x-1)));
     * // Queue(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 Queue with the values built up by the iteration
     * @throws NullPointerException if {@code f} is null
     */
    public static <T, U> Queue<U> unfoldRight(T seed, Function<? super T, Option<Tuple2<? extends U, ? extends T>>> f) {
        return Iterator.unfoldRight(seed, f).toQueue();
    }

    /**
     * Creates a Queue 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 Queue, otherwise {@code Some} {@code Tuple}
     * of the value to add to the resulting Queue and
     * the element for the next call.
     * <p>
     * Example:
     * <pre>
     * <code>
     * Queue.unfoldLeft(10, x -&gt; x == 0
     *             ? Option.none()
     *             : Option.of(new Tuple2&lt;&gt;(x-1, x)));
     * // Queue(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 Queue with the values built up by the iteration
     * @throws NullPointerException if {@code f} is null
     */
    public static <T, U> Queue<U> unfoldLeft(T seed, Function<? super T, Option<Tuple2<? extends T, ? extends U>>> f) {
        return Iterator.unfoldLeft(seed, f).toQueue();
    }

    /**
     * Creates a Queue 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 Queue, otherwise {@code Some} {@code Tuple}
     * of the value to add to the resulting Queue and
     * the element for the next call.
     * <p>
     * Example:
     * <pre>
     * <code>
     * Queue.unfold(10, x -&gt; x == 0
     *             ? Option.none()
     *             : Option.of(new Tuple2&lt;&gt;(x-1, x)));
     * // Queue(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 Queue with the values built up by the iteration
     * @throws NullPointerException if {@code f} is null
     */
    public static <T> Queue<T> unfold(T seed, Function<? super T, Option<Tuple2<? extends T, ? extends T>>> f) {
        return Iterator.unfold(seed, f).toQueue();
    }

    /**
     * Enqueues a new element.
     *
     * @param element The new element
     * @return a new {@code Queue} instance, containing the new element
     */
    @Override
    public Queue<T> enqueue(T element) {
        return new Queue<>(front, rear.prepend(element));
    }

    /**
     * Enqueues the given elements. A queue has FIFO order, i.e. the first of the given elements is
     * the first which will be retrieved.
     *
     * @param elements An Iterable of elements, may be empty
     * @return a new {@code Queue} instance, containing the new elements
     * @throws NullPointerException if elements is null
     */
    @SuppressWarnings("unchecked")
    public Queue<T> enqueueAll(Iterable<? extends T> elements) {
        Objects.requireNonNull(elements, "elements is null");
        if (isEmpty() && elements instanceof Queue) {
            return (Queue<T>) elements;
        } else {
            return io.vavr.collection.List.ofAll(elements).foldLeft(this, Queue::enqueue);
        }
    }

    // -- Adjusted return types of Seq methods

    @Override
    public Queue<T> append(T element) {
        return enqueue(element);
    }

    @Override
    public Queue<T> appendAll(Iterable<? extends T> elements) {
        return enqueueAll(elements);
    }

    @Override
    public java.util.List<T> asJava() {
        return JavaConverters.asJava(this, IMMUTABLE);
    }

    @Override
    public Queue<T> asJava(Consumer<? super java.util.List<T>> action) {
        return Collections.asJava(this, action, IMMUTABLE);
    }

    @Override
    public java.util.List<T> asJavaMutable() {
        return JavaConverters.asJava(this, MUTABLE);
    }

    @Override
    public Queue<T> asJavaMutable(Consumer<? super java.util.List<T>> action) {
        return Collections.asJava(this, action, MUTABLE);
    }

    @Override
    public <R> Queue<R> collect(PartialFunction<? super T, ? extends R> partialFunction) {
        return ofAll(iterator().<R> collect(partialFunction));
    }

    @Override
    public Queue<Queue<T>> combinations() {
        return ofAll(toList().combinations().map(Queue::ofAll));
    }

    @Override
    public Queue<Queue<T>> combinations(int k) {
        return ofAll(toList().combinations(k).map(Queue::ofAll));
    }

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

    @Override
    public Queue<T> distinct() {
        return ofAll(toList().distinct());
    }

    @Override
    public Queue<T> distinctBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator, "comparator is null");
        return ofAll(toList().distinctBy(comparator));
    }

    @Override
    public <U> Queue<T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
        return ofAll(toList().distinctBy(keyExtractor));
    }

    @Override
    public Queue<T> drop(int n) {
        if (n <= 0) {
            return this;
        }
        if (n >= length()) {
            return empty();
        }
        return new Queue<>(front.drop(n), rear.dropRight(n - front.length()));
    }

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

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

    @Override
    public Queue<T> dropRight(int n) {
        if (n <= 0) {
            return this;
        }
        if (n >= length()) {
            return empty();
        }
        return new Queue<>(front.dropRight(n - rear.length()), rear.drop(n));
    }

    @Override
    public Queue<T> dropRightUntil(Predicate<? super T> predicate) {
        return io.vavr.collection.Collections.dropUntil(reverse(), predicate).reverse();
    }

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

    @Override
    public Queue<T> filter(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        final io.vavr.collection.List<T> filtered = toList().filter(predicate);

        if (filtered.isEmpty()) {
            return empty();
        } else if (filtered.length() == length()) {
            return this;
        } else {
            return ofAll(filtered);
        }
    }

    @Override
    public <U> Queue<U> flatMap(Function<? super T, ? extends Iterable<? extends U>> mapper) {
        Objects.requireNonNull(mapper, "mapper is null");
        if (isEmpty()) {
            return empty();
        } else {
            return new Queue<>(front.flatMap(mapper), rear.flatMap(mapper));
        }
    }

    @Override
    public T get(int index) {
        if (isEmpty()) {
            throw new IndexOutOfBoundsException("get(" + index + ") on empty Queue");
        }
        if (index < 0) {
            throw new IndexOutOfBoundsException("get(" + index + ")");
        }
        final int length = front.length();
        if (index < length) {
            return front.get(index);
        } else {
            final int rearIndex = index - length;
            final int rearLength = rear.length();
            if (rearIndex < rearLength) {
                final int reverseRearIndex = rearLength - rearIndex - 1;
                return rear.get(reverseRearIndex);
            } else {
                throw new IndexOutOfBoundsException("get(" + index + ") on Queue of length " + length());
            }
        }
    }

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

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

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

    @Override
    public T head() {
        if (isEmpty()) {
            throw new NoSuchElementException("head of empty " + stringPrefix());
        } else {
            return front.head();
        }
    }

    @Override
    public int indexOf(T element, int from) {
        final int frontIndex = front.indexOf(element, from);
        if (frontIndex != -1) {
            return frontIndex;
        } else {
            // we need to reverse because we search the first occurrence
            final int rearIndex = rear.reverse().indexOf(element, from - front.length());
            return (rearIndex == -1) ? -1 : rearIndex + front.length();
        }
    }

    @Override
    public Queue<T> init() {
        if (isEmpty()) {
            throw new UnsupportedOperationException("init of empty " + stringPrefix());
        } else if (rear.isEmpty()) {
            return new Queue<>(front.init(), rear);
        } else {
            return new Queue<>(front, rear.tail());
        }
    }

    @Override
    public Queue<T> insert(int index, T element) {
        if (index < 0) {
            throw new IndexOutOfBoundsException("insert(" + index + ", element)");
        }
        final int length = front.length();
        if (index <= length) {
            return new Queue<>(front.insert(index, element), rear);
        } else {
            final int rearIndex = index - length;
            final int rearLength = rear.length();
            if (rearIndex <= rearLength) {
                final int reverseRearIndex = rearLength - rearIndex;
                return new Queue<>(front, rear.insert(reverseRearIndex, element));
            } else {
                throw new IndexOutOfBoundsException("insert(" + index + ", element) on Queue of length " + length());
            }
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public Queue<T> insertAll(int index, Iterable<? extends T> elements) {
        Objects.requireNonNull(elements, "elements is null");
        if (index < 0) {
            throw new IndexOutOfBoundsException("insertAll(" + index + ", elements)");
        }
        final int length = front.length();
        if (index <= length) {
            if (isEmpty() && elements instanceof Queue) {
                return (Queue<T>) elements;
            } else {
                final io.vavr.collection.List<T> newFront = front.insertAll(index, elements);
                return (newFront == front) ? this : new Queue<>(newFront, rear);
            }
        } else {
            final int rearIndex = index - length;
            final int rearLength = rear.length();
            if (rearIndex <= rearLength) {
                final int reverseRearIndex = rearLength - rearIndex;
                final io.vavr.collection.List<T> newRear = rear.insertAll(reverseRearIndex, io.vavr.collection.List.ofAll(elements).reverse());
                return (newRear == rear) ? this : new Queue<>(front, newRear);
            } else {
                throw new IndexOutOfBoundsException("insertAll(" + index + ", elements) on Queue of length " + length());
            }
        }
    }

    @Override
    public Queue<T> intersperse(T element) {
        if (isEmpty()) {
            return this;
        } else if (rear.isEmpty()) {
            return new Queue<>(front.intersperse(element), rear);
        } else {
            return new Queue<>(front.intersperse(element), rear.intersperse(element).append(element));
        }
    }

    /**
     * A {@code Queue} is computed synchronously.
     *
     * @return false
     */
    @Override
    public boolean isAsync() {
        return false;
    }

    @Override
    public boolean isEmpty() {
        return front.isEmpty();
    }

    /**
     * A {@code Queue} is computed eagerly.
     *
     * @return false
     */
    @Override
    public boolean isLazy() {
        return false;
    }

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

    @Override
    public int lastIndexOf(T element, int end) {
        return toList().lastIndexOf(element, end);
    }

    @Override
    public int length() {
        return front.length() + rear.length();
    }

    @Override
    public <U> Queue<U> map(Function<? super T, ? extends U> mapper) {
        Objects.requireNonNull(mapper, "mapper is null");
        return new Queue<>(front.map(mapper), rear.map(mapper));
    }

    @Override
    public Queue<T> orElse(Iterable<? extends T> other) {
        return isEmpty() ? ofAll(other) : this;
    }

    @Override
    public Queue<T> orElse(Supplier<? extends Iterable<? extends T>> supplier) {
        return isEmpty() ? ofAll(supplier.get()) : this;
    }

    @Override
    public Queue<T> padTo(int length, T element) {
        final int actualLength = length();
        if (length <= actualLength) {
            return this;
        } else {
            return ofAll(toList().padTo(length, element));
        }
    }

    @Override
    public Queue<T> leftPadTo(int length, T element) {
        final int actualLength = length();
        if (length <= actualLength) {
            return this;
        } else {
            return ofAll(toList().leftPadTo(length, element));
        }
    }

    @Override
    public Queue<T> patch(int from, Iterable<? extends T> that, int replaced) {
        from = from < 0 ? 0 : from;
        replaced = replaced < 0 ? 0 : replaced;
        Queue<T> result = take(from).appendAll(that);
        from += replaced;
        result = result.appendAll(drop(from));
        return result;
    }

    @Override
    public Tuple2<Queue<T>, Queue<T>> partition(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate, "predicate is null");
        return toList().partition(predicate).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
    }

    @Override
    public Queue<Queue<T>> permutations() {
         return ofAll(toList().permutations().map(io.vavr.collection.List::toQueue));
     }

     @Override
     public Queue<T> prepend(T element) {
         return new Queue<>(front.prepend(element), rear);
     }

     @SuppressWarnings("unchecked")
     @Override
     public Queue<T> prependAll(Iterable<? extends T> elements) {
         Objects.requireNonNull(elements, "elements is null");
         if (isEmpty() && elements instanceof Queue) {
             return (Queue<T>) elements;
         } else {
             final io.vavr.collection.List<T> newFront = front.prependAll(elements);
             return (newFront == front) ? this : new Queue<>(newFront, rear);
         }
     }

     @Override
     public Queue<T> remove(T element) {
         final io.vavr.collection.List<T> removed = toList().remove(element);
         return ofAll(removed.length() == length() ? this : removed);
     }

     @Override
     public Queue<T> removeFirst(Predicate<T> predicate) {
         final io.vavr.collection.List<T> removed = toList().removeFirst(predicate);
         return ofAll(removed.length() == length() ? this : removed);
     }

     @Override
     public Queue<T> removeLast(Predicate<T> predicate) {
         final io.vavr.collection.List<T> removed = toList().removeLast(predicate);
         return ofAll(removed.length() == length() ? this : removed);
     }

     @Override
     public Queue<T> removeAt(int index) {
         return ofAll(toList().removeAt(index));
     }

     @Override
     public Queue<T> removeAll(T element) {
         return io.vavr.collection.Collections.removeAll(this, element);
     }

     @Override
     public Queue<T> replace(T currentElement, T newElement) {
         final io.vavr.collection.List<T> newFront = front.replace(currentElement, newElement);
         final io.vavr.collection.List<T> newRear = rear.replace(currentElement, newElement);
         return newFront.size() + newRear.size() == 0 ? empty()
                                                      : newFront == front && newRear == rear ? this
                                                                                             : new Queue<>(newFront, newRear);
     }

     @Override
     public Queue<T> replaceAll(T currentElement, T newElement) {
         final io.vavr.collection.List<T> newFront = front.replaceAll(currentElement, newElement);
         final io.vavr.collection.List<T> newRear = rear.replaceAll(currentElement, newElement);
         return newFront.size() + newRear.size() == 0 ? empty()
                                                      : newFront == front && newRear == rear ? this
                                                                                             : new Queue<>(newFront, newRear);
     }

     @Override
     public Queue<T> reverse() {
         return isEmpty() ? this : ofAll(toList().reverse());
     }

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

     @Override
     public <U> Queue<U> scanLeft(U zero, BiFunction<? super U, ? super T, ? extends U> operation) {
         return io.vavr.collection.Collections.scanLeft(this, zero, operation, Iterator::toQueue);
     }

     @Override
     public <U> Queue<U> scanRight(U zero, BiFunction<? super T, ? super U, ? extends U> operation) {
         return io.vavr.collection.Collections.scanRight(this, zero, operation, Iterator::toQueue);
     }

     @Override
     public Queue<T> shuffle() {
         return io.vavr.collection.Collections.shuffle(this, Queue::ofAll);
     }

     @Override
     public Queue<T> slice(int beginIndex, int endIndex) {
         return ofAll(toList().slice(beginIndex, endIndex));
     }

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

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

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

     @Override
     public Queue<T> sorted() {
         return ofAll(toList().sorted());
     }

     @Override
     public Queue<T> sorted(Comparator<? super T> comparator) {
         Objects.requireNonNull(comparator, "comparator is null");
         return ofAll(toList().sorted(comparator));
     }

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

     @Override
     public <U> Queue<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<Queue<T>, Queue<T>> span(Predicate<? super T> predicate) {
         Objects.requireNonNull(predicate, "predicate is null");
         return toList().span(predicate).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
     }

     @Override
     public Tuple2<Queue<T>, Queue<T>> splitAt(int n) {
         return toList().splitAt(n).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
     }

     @Override
     public Tuple2<Queue<T>, Queue<T>> splitAt(Predicate<? super T> predicate) {
         return toList().splitAt(predicate).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
     }

     @Override
     public Tuple2<Queue<T>, Queue<T>> splitAtInclusive(Predicate<? super T> predicate) {
         return toList().splitAtInclusive(predicate).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
     }

     @Override
     public boolean startsWith(Iterable<? extends T> that, int offset) {
         return toList().startsWith(that, offset);
     }

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

     @Override
     public Queue<T> subSequence(int beginIndex, int endIndex) {
         Collections.subSequenceRangeCheck(beginIndex, endIndex, length());
         if (beginIndex == endIndex) {
             return empty();
         } else if (beginIndex == 0 && endIndex == length()) {
             return this;
         } else {
             return ofAll(toList().subSequence(beginIndex, endIndex));
         }
     }

     @Override
     public Queue<T> tail() {
         if (isEmpty()) {
             throw new UnsupportedOperationException("tail of empty " + stringPrefix());
         } else {
             return new Queue<>(front.tail(), rear);
         }
     }

     @Override
     public Queue<T> take(int n) {
         if (n <= 0) {
             return empty();
         }
         if (n >= length()) {
             return this;
         }
         final int frontLength = front.length();
         if (n < frontLength) {
             return new Queue<>(front.take(n), io.vavr.collection.List.empty());
         } else if (n == frontLength) {
             return new Queue<>(front, io.vavr.collection.List.empty());
         } else {
             return new Queue<>(front, rear.takeRight(n - frontLength));
         }
     }

     @Override
     public Queue<T> takeRight(int n) {
         if (n <= 0) {
             return empty();
         }
         if (n >= length()) {
             return this;
         }
         final int rearLength = rear.length();
         if (n < rearLength) {
             return new Queue<>(rear.take(n).reverse(), io.vavr.collection.List.empty());
         } else if (n == rearLength) {
             return new Queue<>(rear.reverse(), io.vavr.collection.List.empty());
         } else {
             return new Queue<>(front.takeRight(n - rearLength), rear);
         }
     }

     @Override
     public Queue<T> takeUntil(Predicate<? super T> predicate) {
         Objects.requireNonNull(predicate, "predicate is null");
         final io.vavr.collection.List<T> taken = toList().takeUntil(predicate);
         return taken.length() == length() ? this : ofAll(taken);
     }

     /**
      * Transforms this {@code Queue}.
      *
      * @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 Queue<T>, ? extends U> f) {
         Objects.requireNonNull(f, "f is null");
         return f.apply(this);
     }

     @Override
     public <T1, T2> Tuple2<Queue<T1>, Queue<T2>> unzip(
             Function<? super T, Tuple2<? extends T1, ? extends T2>> unzipper) {
         Objects.requireNonNull(unzipper, "unzipper is null");
         return toList().unzip(unzipper).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
     }

     @Override
     public <T1, T2, T3> Tuple3<Queue<T1>, Queue<T2>, Queue<T3>> unzip3(Function<? super T, Tuple3<? extends T1, ? extends T2, ? extends T3>> unzipper) {
         Objects.requireNonNull(unzipper, "unzipper is null");
         return toList().unzip3(unzipper).map(io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue, io.vavr.collection.List::toQueue);
     }

     @Override
     public Queue<T> update(int index, T element) {
         return ofAll(toList().update(index, element));
     }

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

     @SuppressWarnings("unchecked")
     @Override
     public <U, R> Queue<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(toList().zipWith(that, mapper));
     }

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

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

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

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

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

     @Override
     public boolean equals(Object o) {
         return io.vavr.collection.Collections.equals(this, o);
     }

     @Override
     public int hashCode() {
         return io.vavr.collection.Collections.hashOrdered(this);
     }
 }