/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Josh Bloch of Google Inc. and released to the public domain,

 * as explained at http://creativecommons.org/publicdomain/zero/1.0/.

 */

package java.util;

import java.io.*;

/**

 * Resizable-array implementation of the {@link Deque} interface.  Array

 * deques have no capacity restrictions; they grow as necessary to support

 * usage.  They are not thread-safe; in the absence of external

 * synchronization, they do not support concurrent access by multiple threads.

 * Null elements are prohibited.  This class is likely to be faster than

 * {@link Stack} when used as a stack, and faster than {@link LinkedList}

 * when used as a queue.

 *

 * <p>Most <tt>ArrayDeque</tt> operations run in amortized constant time.

 * Exceptions include {@link #remove(Object) remove}, {@link

 * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence

 * removeLastOccurrence}, {@link #contains contains}, {@link #iterator

 * iterator.remove()}, and the bulk operations, all of which run in linear

 * time.

 *

 * <p>The iterators returned by this class's <tt>iterator</tt> method are

 * <i>fail-fast</i>: If the deque is modified at any time after the iterator

 * is created, in any way except through the iterator's own <tt>remove</tt>

 * method, the iterator will generally throw a {@link

 * ConcurrentModificationException}.  Thus, in the face of concurrent

 * modification, the iterator fails quickly and cleanly, rather than risking

 * arbitrary, non-deterministic behavior at an undetermined time in the

 * future.

 *

 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

 * as it is, generally speaking, impossible to make any hard guarantees in the

 * presence of unsynchronized concurrent modification.  Fail-fast iterators

 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.

 * Therefore, it would be wrong to write a program that depended on this

 * exception for its correctness: <i>the fail-fast behavior of iterators

 * should be used only to detect bugs.</i>

 *

 * <p>This class and its iterator implement all of the

 * <em>optional</em> methods of the {@link Collection} and {@link

 * Iterator} interfaces.

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @author  Josh Bloch and Doug Lea

 * @since   1.6

 * @param <E> the type of elements held in this collection

 */

public class ArrayDeque<E> extends AbstractCollection<E>

                           implements Deque<E>, Cloneable, Serializable

{

    /**

     * The array in which the elements of the deque are stored.

     * The capacity of the deque is the length of this array, which is

     * always a power of two. The array is never allowed to become

     * full, except transiently within an addX method where it is

     * resized (see doubleCapacity) immediately upon becoming full,

     * thus avoiding head and tail wrapping around to equal each

     * other.  We also guarantee that all array cells not holding

     * deque elements are always null.

     */

    private transient E[] elements;

    /**

     * The index of the element at the head of the deque (which is the

     * element that would be removed by remove() or pop()); or an

     * arbitrary number equal to tail if the deque is empty.

     */

    private transient int head;

    /**

     * The index at which the next element would be added to the tail

     * of the deque (via addLast(E), add(E), or push(E)).

     */

    private transient int tail;

    /**

     * The minimum capacity that we'll use for a newly created deque.

     * Must be a power of 2.

     */

    private static final int MIN_INITIAL_CAPACITY = 8;

    // ******  Array allocation and resizing utilities ******

    /**

     * Allocate empty array to hold the given number of elements.

     *

     * @param numElements  the number of elements to hold

     */

    private void allocateElements(int numElements) {

        int initialCapacity = MIN_INITIAL_CAPACITY;

        // Find the best power of two to hold elements.

        // Tests "<=" because arrays aren't kept full.

        if (numElements >= initialCapacity) {

            initialCapacity = numElements;

            initialCapacity |= (initialCapacity >>>  1);

            initialCapacity |= (initialCapacity >>>  2);

            initialCapacity |= (initialCapacity >>>  4);

            initialCapacity |= (initialCapacity >>>  8);

            initialCapacity |= (initialCapacity >>> 16);

            initialCapacity++;

            if (initialCapacity < 0)   // Too many elements, must back off

                initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements

        }

        elements = (E[]) new Object[initialCapacity];

    }

    /**

     * Double the capacity of this deque.  Call only when full, i.e.,

     * when head and tail have wrapped around to become equal.

     */

    private void doubleCapacity() {

        assert head == tail;

        int p = head;

        int n = elements.length;

        int r = n - p; // number of elements to the right of p

        int newCapacity = n << 1;

        if (newCapacity < 0)

            throw new IllegalStateException("Sorry, deque too big");

        Object[] a = new Object[newCapacity];

        System.arraycopy(elements, p, a, 0, r);

        System.arraycopy(elements, 0, a, r, p);

        elements = (E[])a;

        head = 0;

        tail = n;

    }

    /**

     * Copies the elements from our element array into the specified array,

     * in order (from first to last element in the deque).  It is assumed

     * that the array is large enough to hold all elements in the deque.

     *

     * @return its argument

     */

    private <T> T[] copyElements(T[] a) {

        if (head < tail) {

            System.arraycopy(elements, head, a, 0, size());

        } else if (head > tail) {

            int headPortionLen = elements.length - head;

            System.arraycopy(elements, head, a, 0, headPortionLen);

            System.arraycopy(elements, 0, a, headPortionLen, tail);

        }

        return a;

    }

    /**

     * Constructs an empty array deque with an initial capacity

     * sufficient to hold 16 elements.

     */

    public ArrayDeque() {

        elements = (E[]) new Object[16];

    }

    /**

     * Constructs an empty array deque with an initial capacity

     * sufficient to hold the specified number of elements.

     *

     * @param numElements  lower bound on initial capacity of the deque

     */

    public ArrayDeque(int numElements) {

        allocateElements(numElements);

    }

    /**

     * Constructs a deque containing the elements of the specified

     * collection, in the order they are returned by the collection's

     * iterator.  (The first element returned by the collection's

     * iterator becomes the first element, or <i>front</i> of the

     * deque.)

     *

     * @param c the collection whose elements are to be placed into the deque

     * @throws NullPointerException if the specified collection is null

     */

    public ArrayDeque(Collection<? extends E> c) {

        allocateElements(c.size());

        addAll(c);

    }

    // The main insertion and extraction methods are addFirst,

    // addLast, pollFirst, pollLast. The other methods are defined in

    // terms of these.

    /**

     * Inserts the specified element at the front of this deque.

     *

     * @param e the element to add

     * @throws NullPointerException if the specified element is null

     */

    public void addFirst(E e) {

        if (e == null)

            throw new NullPointerException();

        elements[head = (head - 1) & (elements.length - 1)] = e;

        if (head == tail)

            doubleCapacity();

    }

    /**

     * Inserts the specified element at the end of this deque.

     *

     * <p>This method is equivalent to {@link #add}.

     *

     * @param e the element to add

     * @throws NullPointerException if the specified element is null

     */

    public void addLast(E e) {

        if (e == null)

            throw new NullPointerException();

        elements[tail] = e;

        if ( (tail = (tail + 1) & (elements.length - 1)) == head)

            doubleCapacity();

    }

    /**

     * Inserts the specified element at the front of this deque.

     *

     * @param e the element to add

     * @return <tt>true</tt> (as specified by {@link Deque#offerFirst})

     * @throws NullPointerException if the specified element is null

     */

    public boolean offerFirst(E e) {

        addFirst(e);

        return true;

    }

    /**

     * Inserts the specified element at the end of this deque.

     *

     * @param e the element to add

     * @return <tt>true</tt> (as specified by {@link Deque#offerLast})

     * @throws NullPointerException if the specified element is null

     */

    public boolean offerLast(E e) {

        addLast(e);

        return true;

    }

    /**

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E removeFirst() {

        E x = pollFirst();

        if (x == null)

            throw new NoSuchElementException();

        return x;

    }

    /**

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E removeLast() {

        E x = pollLast();

        if (x == null)

            throw new NoSuchElementException();

        return x;

    }

    public E pollFirst() {

        int h = head;

        E result = elements[h]; // Element is null if deque empty

        if (result == null)

            return null;

        elements[h] = null;     // Must null out slot

        head = (h + 1) & (elements.length - 1);

        return result;

    }

    public E pollLast() {

        int t = (tail - 1) & (elements.length - 1);

        E result = elements[t];

        if (result == null)

            return null;

        elements[t] = null;

        tail = t;

        return result;

    }

    /**

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E getFirst() {

        E x = elements[head];

        if (x == null)

            throw new NoSuchElementException();

        return x;

    }

    /**

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E getLast() {

        E x = elements[(tail - 1) & (elements.length - 1)];

        if (x == null)

            throw new NoSuchElementException();

        return x;

    }

    public E peekFirst() {

        return elements[head]; // elements[head] is null if deque empty

    }

    public E peekLast() {

        return elements[(tail - 1) & (elements.length - 1)];

    }

    /**

     * Removes the first occurrence of the specified element in this

     * deque (when traversing the deque from head to tail).

     * If the deque does not contain the element, it is unchanged.

     * More formally, removes the first element <tt>e</tt> such that

     * <tt>o.equals(e)</tt> (if such an element exists).

     * Returns <tt>true</tt> if this deque contained the specified element

     * (or equivalently, if this deque changed as a result of the call).

     *

     * @param o element to be removed from this deque, if present

     * @return <tt>true</tt> if the deque contained the specified element

     */

    public boolean removeFirstOccurrence(Object o) {

        if (o == null)

            return false;

        int mask = elements.length - 1;

        int i = head;

        E x;

        while ( (x = elements[i]) != null) {

            if (o.equals(x)) {

                delete(i);

                return true;

            }

            i = (i + 1) & mask;

        }

        return false;

    }

    /**

     * Removes the last occurrence of the specified element in this

     * deque (when traversing the deque from head to tail).

     * If the deque does not contain the element, it is unchanged.

     * More formally, removes the last element <tt>e</tt> such that

     * <tt>o.equals(e)</tt> (if such an element exists).

     * Returns <tt>true</tt> if this deque contained the specified element

     * (or equivalently, if this deque changed as a result of the call).

     *

     * @param o element to be removed from this deque, if present

     * @return <tt>true</tt> if the deque contained the specified element

     */

    public boolean removeLastOccurrence(Object o) {

        if (o == null)

            return false;

        int mask = elements.length - 1;

        int i = (tail - 1) & mask;

        E x;

        while ( (x = elements[i]) != null) {

            if (o.equals(x)) {

                delete(i);

                return true;

            }

            i = (i - 1) & mask;

        }

        return false;

    }

    // *** Queue methods ***

    /**

     * Inserts the specified element at the end of this deque.

     *

     * <p>This method is equivalent to {@link #addLast}.

     *

     * @param e the element to add

     * @return <tt>true</tt> (as specified by {@link Collection#add})

     * @throws NullPointerException if the specified element is null

     */

    public boolean add(E e) {

        addLast(e);

        return true;

    }

    /**

     * Inserts the specified element at the end of this deque.

     *

     * <p>This method is equivalent to {@link #offerLast}.

     *

     * @param e the element to add

     * @return <tt>true</tt> (as specified by {@link Queue#offer})

     * @throws NullPointerException if the specified element is null

     */

    public boolean offer(E e) {

        return offerLast(e);

    }

    /**

     * Retrieves and removes the head of the queue represented by this deque.

     *

     * This method differs from {@link #poll poll} only in that it throws an

     * exception if this deque is empty.

     *

     * <p>This method is equivalent to {@link #removeFirst}.

     *

     * @return the head of the queue represented by this deque

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E remove() {

        return removeFirst();

    }

    /**

     * Retrieves and removes the head of the queue represented by this deque

     * (in other words, the first element of this deque), or returns

     * <tt>null</tt> if this deque is empty.

     *

     * <p>This method is equivalent to {@link #pollFirst}.

     *

     * @return the head of the queue represented by this deque, or

     *         <tt>null</tt> if this deque is empty

     */

    public E poll() {

        return pollFirst();

    }

    /**

     * Retrieves, but does not remove, the head of the queue represented by

     * this deque.  This method differs from {@link #peek peek} only in

     * that it throws an exception if this deque is empty.

     *

     * <p>This method is equivalent to {@link #getFirst}.

     *

     * @return the head of the queue represented by this deque

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E element() {

        return getFirst();

    }

    /**

     * Retrieves, but does not remove, the head of the queue represented by

     * this deque, or returns <tt>null</tt> if this deque is empty.

     *

     * <p>This method is equivalent to {@link #peekFirst}.

     *

     * @return the head of the queue represented by this deque, or

     *         <tt>null</tt> if this deque is empty

     */

    public E peek() {

        return peekFirst();

    }

    // *** Stack methods ***

    /**

     * Pushes an element onto the stack represented by this deque.  In other

     * words, inserts the element at the front of this deque.

     *

     * <p>This method is equivalent to {@link #addFirst}.

     *

     * @param e the element to push

     * @throws NullPointerException if the specified element is null

     */

    public void push(E e) {

        addFirst(e);

    }

    /**

     * Pops an element from the stack represented by this deque.  In other

     * words, removes and returns the first element of this deque.

     *

     * <p>This method is equivalent to {@link #removeFirst()}.

     *

     * @return the element at the front of this deque (which is the top

     *         of the stack represented by this deque)

     * @throws NoSuchElementException {@inheritDoc}

     */

    public E pop() {

        return removeFirst();

    }

    private void checkInvariants() {

        assert elements[tail] == null;

        assert head == tail ? elements[head] == null :

            (elements[head] != null &&

             elements[(tail - 1) & (elements.length - 1)] != null);

        assert elements[(head - 1) & (elements.length - 1)] == null;

    }

    /**

     * Removes the element at the specified position in the elements array,

     * adjusting head and tail as necessary.  This can result in motion of

     * elements backwards or forwards in the array.

     *

     * <p>This method is called delete rather than remove to emphasize

     * that its semantics differ from those of {@link List#remove(int)}.

     *

     * @return true if elements moved backwards

     */

    private boolean delete(int i) {

        checkInvariants();

        final E[] elements = this.elements;

        final int mask = elements.length - 1;

        final int h = head;

        final int t = tail;

        final int front = (i - h) & mask;

        final int back  = (t - i) & mask;

        // Invariant: head <= i < tail mod circularity

        if (front >= ((t - h) & mask))

            throw new ConcurrentModificationException();

        // Optimize for least element motion

        if (front < back) {

            if (h <= i) {

                System.arraycopy(elements, h, elements, h + 1, front);

            } else { // Wrap around

                System.arraycopy(elements, 0, elements, 1, i);

                elements[0] = elements[mask];

                System.arraycopy(elements, h, elements, h + 1, mask - h);

            }

            elements[h] = null;

            head = (h + 1) & mask;

            return false;

        } else {

            if (i < t) { // Copy the null tail as well

                System.arraycopy(elements, i + 1, elements, i, back);

                tail = t - 1;

            } else { // Wrap around

                System.arraycopy(elements, i + 1, elements, i, mask - i);

                elements[mask] = elements[0];

                System.arraycopy(elements, 1, elements, 0, t);

                tail = (t - 1) & mask;

            }

            return true;

        }

    }

    // *** Collection Methods ***

    /**

     * Returns the number of elements in this deque.

     *

     * @return the number of elements in this deque

     */

    public int size() {

        return (tail - head) & (elements.length - 1);

    }

    /**

     * Returns <tt>true</tt> if this deque contains no elements.

     *

     * @return <tt>true</tt> if this deque contains no elements

     */

    public boolean isEmpty() {

        return head == tail;

    }

    /**

     * Returns an iterator over the elements in this deque.  The elements

     * will be ordered from first (head) to last (tail).  This is the same

     * order that elements would be dequeued (via successive calls to

     * {@link #remove} or popped (via successive calls to {@link #pop}).

     *

     * @return an iterator over the elements in this deque

     */

    public Iterator<E> iterator() {

        return new DeqIterator();

    }

    public Iterator<E> descendingIterator() {

        return new DescendingIterator();

    }

    private class DeqIterator implements Iterator<E> {

        /**

         * Index of element to be returned by subsequent call to next.

         */

        private int cursor = head;

        /**

         * Tail recorded at construction (also in remove), to stop

         * iterator and also to check for comodification.

         */

        private int fence = tail;

        /**

         * Index of element returned by most recent call to next.

         * Reset to -1 if element is deleted by a call to remove.

         */

        private int lastRet = -1;

        public boolean hasNext() {

            return cursor != fence;

        }

        public E next() {

            if (cursor == fence)

                throw new NoSuchElementException();

            E result = elements[cursor];

            // This check doesn't catch all possible comodifications,

            // but does catch the ones that corrupt traversal

            if (tail != fence || result == null)

                throw new ConcurrentModificationException();

            lastRet = cursor;

            cursor = (cursor + 1) & (elements.length - 1);

            return result;

        }

        public void remove() {

            if (lastRet < 0)

                throw new IllegalStateException();

            if (delete(lastRet)) { // if left-shifted, undo increment in next()

                cursor = (cursor - 1) & (elements.length - 1);

                fence = tail;

            }

            lastRet = -1;

        }

    }

    private class DescendingIterator implements Iterator<E> {

        /*

         * This class is nearly a mirror-image of DeqIterator, using

         * tail instead of head for initial cursor, and head instead of

         * tail for fence.

         */

        private int cursor = tail;

        private int fence = head;

        private int lastRet = -1;

        public boolean hasNext() {

            return cursor != fence;

        }

        public E next() {

            if (cursor == fence)

                throw new NoSuchElementException();

            cursor = (cursor - 1) & (elements.length - 1);

            E result = elements[cursor];

            if (head != fence || result == null)

                throw new ConcurrentModificationException();

            lastRet = cursor;

            return result;

        }

        public void remove() {

            if (lastRet < 0)

                throw new IllegalStateException();

            if (!delete(lastRet)) {

                cursor = (cursor + 1) & (elements.length - 1);

                fence = head;

            }

            lastRet = -1;

        }

    }

    /**

     * Returns <tt>true</tt> if this deque contains the specified element.

     * More formally, returns <tt>true</tt> if and only if this deque contains

     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.

     *

     * @param o object to be checked for containment in this deque

     * @return <tt>true</tt> if this deque contains the specified element

     */

    public boolean contains(Object o) {

        if (o == null)

            return false;

        int mask = elements.length - 1;

        int i = head;

        E x;

        while ( (x = elements[i]) != null) {

            if (o.equals(x))

                return true;

            i = (i + 1) & mask;

        }

        return false;

    }

    /**

     * Removes a single instance of the specified element from this deque.

     * If the deque does not contain the element, it is unchanged.

     * More formally, removes the first element <tt>e</tt> such that

     * <tt>o.equals(e)</tt> (if such an element exists).

     * Returns <tt>true</tt> if this deque contained the specified element

     * (or equivalently, if this deque changed as a result of the call).

     *

     * <p>This method is equivalent to {@link #removeFirstOccurrence}.

     *

     * @param o element to be removed from this deque, if present

     * @return <tt>true</tt> if this deque contained the specified element

     */

    public boolean remove(Object o) {

        return removeFirstOccurrence(o);

    }

    /**

     * Removes all of the elements from this deque.

     * The deque will be empty after this call returns.

     */

    public void clear() {

        int h = head;

        int t = tail;

        if (h != t) { // clear all cells

            head = tail = 0;

            int i = h;

            int mask = elements.length - 1;

            do {

                elements[i] = null;

                i = (i + 1) & mask;

            } while (i != t);

        }

    }

    /**

     * Returns an array containing all of the elements in this deque

     * in proper sequence (from first to last element).

     *

     * <p>The returned array will be "safe" in that no references to it are

     * maintained by this deque.  (In other words, this method must allocate

     * a new array).  The caller is thus free to modify the returned array.

     *

     * <p>This method acts as bridge between array-based and collection-based

     * APIs.

     *

     * @return an array containing all of the elements in this deque

     */

    public Object[] toArray() {

        return copyElements(new Object[size()]);

    }

    /**

     * Returns an array containing all of the elements in this deque in

     * proper sequence (from first to last element); the runtime type of the

     * returned array is that of the specified array.  If the deque fits in

     * the specified array, it is returned therein.  Otherwise, a new array

     * is allocated with the runtime type of the specified array and the

     * size of this deque.

     *

     * <p>If this deque fits in the specified array with room to spare

     * (i.e., the array has more elements than this deque), the element in

     * the array immediately following the end of the deque is set to

     * <tt>null</tt>.

     *

     * <p>Like the {@link #toArray()} method, this method acts as bridge between

     * array-based and collection-based APIs.  Further, this method allows

     * precise control over the runtime type of the output array, and may,

     * under certain circumstances, be used to save allocation costs.

     *

     * <p>Suppose <tt>x</tt> is a deque known to contain only strings.

     * The following code can be used to dump the deque into a newly

     * allocated array of <tt>String</tt>:

     *

     * <pre>

     *     String[] y = x.toArray(new String[0]);</pre>

     *

     * Note that <tt>toArray(new Object[0])</tt> is identical in function to

     * <tt>toArray()</tt>.

     *

     * @param a the array into which the elements of the deque are to

     *          be stored, if it is big enough; otherwise, a new array of the

     *          same runtime type is allocated for this purpose

     * @return an array containing all of the elements in this deque

     * @throws ArrayStoreException if the runtime type of the specified array

     *         is not a supertype of the runtime type of every element in

     *         this deque

     * @throws NullPointerException if the specified array is null

     */

    public <T> T[] toArray(T[] a) {

        int size = size();

        if (a.length < size)

            a = (T[])java.lang.reflect.Array.newInstance(

                    a.getClass().getComponentType(), size);

        copyElements(a);

        if (a.length > size)

            a[size] = null;

        return a;

    }

    // *** Object methods ***

    /**

     * Returns a copy of this deque.

     *

     * @return a copy of this deque

     */

    public ArrayDeque<E> clone() {

        try {

            ArrayDeque<E> result = (ArrayDeque<E>) super.clone();

            result.elements = Arrays.copyOf(elements, elements.length);

            return result;

        } catch (CloneNotSupportedException e) {

            throw new AssertionError();

        }

    }

    /**

     * Appease the serialization gods.

     */

    private static final long serialVersionUID = 2340985798034038923L;

}