/*
 * @(#)ArrayDeque.java  1.6 06/04/21
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package org.xerial.util;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.AbstractCollection;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;

/**
 * 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 = copyOf(elements, elements.length);
            return result;
        }
        catch (CloneNotSupportedException e)
        {
            throw new AssertionError();
        }
    }

    private static <T> T[] copyOf(T[] original, int newLength)
    {
        return (T[]) copyOf(original, newLength, original.getClass());
    }

    private static <T, U> T[] copyOf(U[] original, int newLength, Class< ? extends T[]> newType)
    {
        T[] copy = ((Object) newType == (Object) Object[].class) ? (T[]) new Object[newLength] : (T[]) Array
                .newInstance(newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength));
        return copy;
    }

    /**
     * Appease the serialization gods.
     */
    private static final long serialVersionUID = 2340985798034038923L;

    /**
     * Serialize this deque.
     * 
     * @serialData The current size (<tt>int</tt>) of the deque, followed by all
     *             of its elements (each an object reference) in first-to-last
     *             order.
     */
    private void writeObject(ObjectOutputStream s) throws IOException
    {
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size());

        // Write out elements in order.
        int mask = elements.length - 1;
        for (int i = head; i != tail; i = (i + 1) & mask)
            s.writeObject(elements[i]);
    }

    /**
     * Deserialize this deque.
     */
    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException
    {
        s.defaultReadObject();

        // Read in size and allocate array
        int size = s.readInt();
        allocateElements(size);
        head = 0;
        tail = size;

        // Read in all elements in the proper order.
        for (int i = 0; i < size; i++)
            elements[i] = (E) s.readObject();
    }
}
