/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * 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.Serializable;
import java.util.function.Consumer;
import sun.misc.SharedSecrets;

/**
 * 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 {@code ArrayDeque} 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 {@code iterator} 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 {@code remove}
 * 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 {@code ConcurrentModificationException} 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
 */
@SuppressWarnings("all")
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.
 */
 transient Object[] elements; // non-private to simplify nested class access

 /**
 * 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.
 */
 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)).
 */
 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 ******

 private static int calculateSize(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
 }
 return initialCapacity;
 }

 /**
 * Allocates empty array to hold the given number of elements.
 *
 * @param numElements the number of elements to hold
 */
 private void allocateElements(int numElements) {
 elements = new Object[calculateSize(numElements)];
 }

 /**
 * Doubles 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 = 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 = 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 {@code true} (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 {@code true} (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;
 @SuppressWarnings("unchecked")
 E result = (E) 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);
 @SuppressWarnings("unchecked")
 E result = (E) elements[t];
 if (result == null)
 return null;
 elements[t] = null;
 tail = t;
 return result;
 }

 /**
 * @throws NoSuchElementException {@inheritDoc}
 */
 public E getFirst() {
 @SuppressWarnings("unchecked")
 E result = (E) elements[head];
 if (result == null)
 throw new NoSuchElementException();
 return result;
 }

 /**
 * @throws NoSuchElementException {@inheritDoc}
 */
 public E getLast() {
 @SuppressWarnings("unchecked")
 E result = (E) elements[(tail - 1) & (elements.length - 1)];
 if (result == null)
 throw new NoSuchElementException();
 return result;
 }

 @SuppressWarnings("unchecked")
 public E peekFirst() {
 // elements[head] is null if deque empty
 return (E) elements[head];
 }

 @SuppressWarnings("unchecked")
 public E peekLast() {
 return (E) 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 {@code e} such that
 * {@code o.equals(e)} (if such an element exists).
 * Returns {@code true} 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 {@code true} 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;
 Object 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 {@code e} such that
 * {@code o.equals(e)} (if such an element exists).
 * Returns {@code true} 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 {@code true} 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;
 Object 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 {@code true} (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 {@code true} (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
 * {@code null} if this deque is empty.
 *
 * <p>This method is equivalent to {@link #pollFirst}.
 *
 * @return the head of the queue represented by this deque, or
 * {@code null} 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 {@code null} if this deque is empty.
 *
 * <p>This method is equivalent to {@link #peekFirst}.
 *
 * @return the head of the queue represented by this deque, or
 * {@code null} 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 Object[] 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 {@code true} if this deque contains no elements.
 *
 * @return {@code true} 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();
 @SuppressWarnings("unchecked")
 E result = (E) 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;
 }

 public void forEachRemaining(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 Object[] a = elements;
 int m = a.length - 1, f = fence, i = cursor;
 cursor = f;
 while (i != f) {
 @SuppressWarnings("unchecked") E e = (E)a[i];
 i = (i + 1) & m;
 if (e == null)
 throw new ConcurrentModificationException();
 action.accept(e);
 }
 }
 }

 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);
 @SuppressWarnings("unchecked")
 E result = (E) 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 {@code true} if this deque contains the specified element.
 * More formally, returns {@code true} if and only if this deque contains
 * at least one element {@code e} such that {@code o.equals(e)}.
 *
 * @param o object to be checked for containment in this deque
 * @return {@code true} 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;
 Object 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 {@code e} such that
 * {@code o.equals(e)} (if such an element exists).
 * Returns {@code true} 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(Object)}.
 *
 * @param o element to be removed from this deque, if present
 * @return {@code true} 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
 * {@code null}.
 *
 * <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 {@code x} is a deque known to contain only strings.
 * The following code can be used to dump the deque into a newly
 * allocated array of {@code String}:
 *
 * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
 *
 * Note that {@code toArray(new Object[0])} is identical in function to
 * {@code toArray()}.
 *
 * @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
 */
 @SuppressWarnings("unchecked")
 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 {
 @SuppressWarnings("unchecked")
 ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
 result.elements = Arrays.copyOf(elements, elements.length);
 return result;
 } catch (CloneNotSupportedException e) {
 throw new AssertionError();
 }
 }

 private static final long serialVersionUID = 2340985798034038923L;

 /**
 * Saves this deque to a stream (that is, serializes it).
 *
 * @serialData The current size ({@code int}) of the deque,
 * followed by all of its elements (each an object reference) in
 * first-to-last order.
 */
 private void writeObject(java.io.ObjectOutputStream s)
 throws java.io.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]);
 }

 /**
 * Reconstitutes this deque from a stream (that is, deserializes it).
 */
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
 s.defaultReadObject();

 // Read in size and allocate array
 int size = s.readInt();
 int capacity = calculateSize(size);
 SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
 allocateElements(size);
 head = 0;
 tail = size;

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

 /**
 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
 * and <em>fail-fast</em> {@link Spliterator} over the elements in this
 * deque.
 *
 * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
 * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
 * {@link Spliterator#NONNULL}. Overriding implementations should document
 * the reporting of additional characteristic values.
 *
 * @return a {@code Spliterator} over the elements in this deque
 * @since 1.8
 */
 public Spliterator<E> spliterator() {
 return new DeqSpliterator<E>(this, -1, -1);
 }

 static final class DeqSpliterator<E> implements Spliterator<E> {
 private final ArrayDeque<E> deq;
 private int fence; // -1 until first use
 private int index; // current index, modified on traverse/split

 /** Creates new spliterator covering the given array and range */
 DeqSpliterator(ArrayDeque<E> deq, int origin, int fence) {
 this.deq = deq;
 this.index = origin;
 this.fence = fence;
 }

 private int getFence() { // force initialization
 int t;
 if ((t = fence) < 0) {
 t = fence = deq.tail;
 index = deq.head;
 }
 return t;
 }

 public DeqSpliterator<E> trySplit() {
 int t = getFence(), h = index, n = deq.elements.length;
 if (h != t && ((h + 1) & (n - 1)) != t) {
 if (h > t)
 t += n;
 int m = ((h + t) >>> 1) & (n - 1);
 return new DeqSpliterator<>(deq, h, index = m);
 }
 return null;
 }

 public void forEachRemaining(Consumer<? super E> consumer) {
 if (consumer == null)
 throw new NullPointerException();
 Object[] a = deq.elements;
 int m = a.length - 1, f = getFence(), i = index;
 index = f;
 while (i != f) {
 @SuppressWarnings("unchecked") E e = (E)a[i];
 i = (i + 1) & m;
 if (e == null)
 throw new ConcurrentModificationException();
 consumer.accept(e);
 }
 }

 public boolean tryAdvance(Consumer<? super E> consumer) {
 if (consumer == null)
 throw new NullPointerException();
 Object[] a = deq.elements;
 int m = a.length - 1, f = getFence(), i = index;
 if (i != fence) {
 @SuppressWarnings("unchecked") E e = (E)a[i];
 index = (i + 1) & m;
 if (e == null)
 throw new ConcurrentModificationException();
 consumer.accept(e);
 return true;
 }
 return false;
 }

 public long estimateSize() {
 int n = getFence() - index;
 if (n < 0)
 n += deq.elements.length;
 return (long) n;
 }

 @Override
 public int characteristics() {
 return Spliterator.ORDERED | Spliterator.SIZED |
 Spliterator.NONNULL | Spliterator.SUBSIZED;
 }
 }

}