/*

  * 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;

 }