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	/*
	* 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
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	/*
	* 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 Doug Lea with assistance from members of JCP JSR-166
	* Expert Group and released to the public domain, as explained at
	* http://creativecommons.org/publicdomain/zero/1.0/
	*/

	package java.util.concurrent;

	import java.util.AbstractQueue;
	import java.util.Collection;
	import java.util.Iterator;
	import java.util.NoSuchElementException;
	import java.util.Objects;
	import java.util.Spliterator;
	import java.util.Spliterators;
	import java.util.concurrent.locks.Condition;
	import java.util.concurrent.locks.ReentrantLock;
	import java.util.function.Consumer;
	import java.util.function.Predicate;

	/**
	* An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
	* linked nodes.
	*
	* <p>The optional capacity bound constructor argument serves as a
	* way to prevent excessive expansion. The capacity, if unspecified,
	* is equal to {@link Integer#MAX_VALUE}. Linked nodes are
	* dynamically created upon each insertion unless this would bring the
	* deque above capacity.
	*
	* <p>Most operations run in constant time (ignoring time spent
	* blocking). 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>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}/java.base/java/util/package-summary.html#CollectionsFramework">
	* Java Collections Framework</a>.
	*
	* @since 1.6
	* @author Doug Lea
	* @param <E> the type of elements held in this deque
	*/
	public class LinkedBlockingDeque<E>
	extends AbstractQueue<E>
	implements BlockingDeque<E>, java.io.Serializable {

	/*
	* Implemented as a simple doubly-linked list protected by a
	* single lock and using conditions to manage blocking.
	*
	* To implement weakly consistent iterators, it appears we need to
	* keep all Nodes GC-reachable from a predecessor dequeued Node.
	* That would cause two problems:
	* - allow a rogue Iterator to cause unbounded memory retention
	* - cause cross-generational linking of old Nodes to new Nodes if
	* a Node was tenured while live, which generational GCs have a
	* hard time dealing with, causing repeated major collections.
	* However, only non-deleted Nodes need to be reachable from
	* dequeued Nodes, and reachability does not necessarily have to
	* be of the kind understood by the GC. We use the trick of
	* linking a Node that has just been dequeued to itself. Such a
	* self-link implicitly means to jump to "first" (for next links)
	* or "last" (for prev links).
	*/

	/*
	* We have "diamond" multiple interface/abstract class inheritance
	* here, and that introduces ambiguities. Often we want the
	* BlockingDeque javadoc combined with the AbstractQueue
	* implementation, so a lot of method specs are duplicated here.
	*/

	private static final long serialVersionUID = -387911632671998426L;

	/** Doubly-linked list node class */
	static final class Node<E> {
	/**
	* The item, or null if this node has been removed.
	*/
	E item;

	/**
	* One of:
	* - the real predecessor Node
	* - this Node, meaning the predecessor is tail
	* - null, meaning there is no predecessor
	*/
	Node<E> prev;

	/**
	* One of:
	* - the real successor Node
	* - this Node, meaning the successor is head
	* - null, meaning there is no successor
	*/
	Node<E> next;

	Node(E x) {
	item = x;
	}
	}

	/**
	* Pointer to first node.
	* Invariant: (first == null && last == null) \|\|
	* (first.prev == null && first.item != null)
	*/
	transient Node<E> first;

	/**
	* Pointer to last node.
	* Invariant: (first == null && last == null) \|\|
	* (last.next == null && last.item != null)
	*/
	transient Node<E> last;

	/** Number of items in the deque */
	private transient int count;

	/** Maximum number of items in the deque */
	private final int capacity;

	/** Main lock guarding all access */
	final ReentrantLock lock = new ReentrantLock();

	/** Condition for waiting takes */
	private final Condition notEmpty = lock.newCondition();

	/** Condition for waiting puts */
	private final Condition notFull = lock.newCondition();

	/**
	* Creates a {@code LinkedBlockingDeque} with a capacity of
	* {@link Integer#MAX_VALUE}.
	*/
	public LinkedBlockingDeque() {
	this(Integer.MAX_VALUE);
	}

	/**
	* Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
	*
	* @param capacity the capacity of this deque
	* @throws IllegalArgumentException if {@code capacity} is less than 1
	*/
	public LinkedBlockingDeque(int capacity) {
	if (capacity <= 0) throw new IllegalArgumentException();
	this.capacity = capacity;
	}

	/**
	* Creates a {@code LinkedBlockingDeque} with a capacity of
	* {@link Integer#MAX_VALUE}, initially containing the elements of
	* the given collection, added in traversal order of the
	* collection's iterator.
	*
	* @param c the collection of elements to initially contain
	* @throws NullPointerException if the specified collection or any
	* of its elements are null
	*/
	public LinkedBlockingDeque(Collection<? extends E> c) {
	this(Integer.MAX_VALUE);
	addAll(c);
	}


	// Basic linking and unlinking operations, called only while holding lock

	/**
	* Links node as first element, or returns false if full.
	*/
	private boolean linkFirst(Node<E> node) {
	// assert lock.isHeldByCurrentThread();
	if (count >= capacity)
	return false;
	Node<E> f = first;
	node.next = f;
	first = node;
	if (last == null)
	last = node;
	else
	f.prev = node;
	++count;
	notEmpty.signal();
	return true;
	}

	/**
	* Links node as last element, or returns false if full.
	*/
	private boolean linkLast(Node<E> node) {
	// assert lock.isHeldByCurrentThread();
	if (count >= capacity)
	return false;
	Node<E> l = last;
	node.prev = l;
	last = node;
	if (first == null)
	first = node;
	else
	l.next = node;
	++count;
	notEmpty.signal();
	return true;
	}

	/**
	* Removes and returns first element, or null if empty.
	*/
	private E unlinkFirst() {
	// assert lock.isHeldByCurrentThread();
	Node<E> f = first;
	if (f == null)
	return null;
	Node<E> n = f.next;
	E item = f.item;
	f.item = null;
	f.next = f; // help GC
	first = n;
	if (n == null)
	last = null;
	else
	n.prev = null;
	--count;
	notFull.signal();
	return item;
	}

	/**
	* Removes and returns last element, or null if empty.
	*/
	private E unlinkLast() {
	// assert lock.isHeldByCurrentThread();
	Node<E> l = last;
	if (l == null)
	return null;
	Node<E> p = l.prev;
	E item = l.item;
	l.item = null;
	l.prev = l; // help GC
	last = p;
	if (p == null)
	first = null;
	else
	p.next = null;
	--count;
	notFull.signal();
	return item;
	}

	/**
	* Unlinks x.
	*/
	void unlink(Node<E> x) {
	// assert lock.isHeldByCurrentThread();
	// assert x.item != null;
	Node<E> p = x.prev;
	Node<E> n = x.next;
	if (p == null) {
	unlinkFirst();
	} else if (n == null) {
	unlinkLast();
	} else {
	p.next = n;
	n.prev = p;
	x.item = null;
	// Don't mess with x's links. They may still be in use by
	// an iterator.
	--count;
	notFull.signal();
	}
	}

	// BlockingDeque methods

	/**
	* @throws IllegalStateException if this deque is full
	* @throws NullPointerException {@inheritDoc}
	*/
	public void addFirst(E e) {
	if (!offerFirst(e))
	throw new IllegalStateException("Deque full");
	}

	/**
	* @throws IllegalStateException if this deque is full
	* @throws NullPointerException {@inheritDoc}
	*/
	public void addLast(E e) {
	if (!offerLast(e))
	throw new IllegalStateException("Deque full");
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean offerFirst(E e) {
	if (e == null) throw new NullPointerException();
	Node<E> node = new Node<E>(e);
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return linkFirst(node);
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean offerLast(E e) {
	if (e == null) throw new NullPointerException();
	Node<E> node = new Node<E>(e);
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return linkLast(node);
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws InterruptedException {@inheritDoc}
	*/
	public void putFirst(E e) throws InterruptedException {
	if (e == null) throw new NullPointerException();
	Node<E> node = new Node<E>(e);
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	while (!linkFirst(node))
	notFull.await();
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws InterruptedException {@inheritDoc}
	*/
	public void putLast(E e) throws InterruptedException {
	if (e == null) throw new NullPointerException();
	Node<E> node = new Node<E>(e);
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	while (!linkLast(node))
	notFull.await();
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws InterruptedException {@inheritDoc}
	*/
	public boolean offerFirst(E e, long timeout, TimeUnit unit)
	throws InterruptedException {
	if (e == null) throw new NullPointerException();
	Node<E> node = new Node<E>(e);
	long nanos = unit.toNanos(timeout);
	final ReentrantLock lock = this.lock;
	lock.lockInterruptibly();
	try {
	while (!linkFirst(node)) {
	if (nanos <= 0L)
	return false;
	nanos = notFull.awaitNanos(nanos);
	}
	return true;
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws InterruptedException {@inheritDoc}
	*/
	public boolean offerLast(E e, long timeout, TimeUnit unit)
	throws InterruptedException {
	if (e == null) throw new NullPointerException();
	Node<E> node = new Node<E>(e);
	long nanos = unit.toNanos(timeout);
	final ReentrantLock lock = this.lock;
	lock.lockInterruptibly();
	try {
	while (!linkLast(node)) {
	if (nanos <= 0L)
	return false;
	nanos = notFull.awaitNanos(nanos);
	}
	return true;
	} finally {
	lock.unlock();
	}
	}

	/**
	* @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() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return unlinkFirst();
	} finally {
	lock.unlock();
	}
	}

	public E pollLast() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return unlinkLast();
	} finally {
	lock.unlock();
	}
	}

	public E takeFirst() throws InterruptedException {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	E x;
	while ( (x = unlinkFirst()) == null)
	notEmpty.await();
	return x;
	} finally {
	lock.unlock();
	}
	}

	public E takeLast() throws InterruptedException {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	E x;
	while ( (x = unlinkLast()) == null)
	notEmpty.await();
	return x;
	} finally {
	lock.unlock();
	}
	}

	public E pollFirst(long timeout, TimeUnit unit)
	throws InterruptedException {
	long nanos = unit.toNanos(timeout);
	final ReentrantLock lock = this.lock;
	lock.lockInterruptibly();
	try {
	E x;
	while ( (x = unlinkFirst()) == null) {
	if (nanos <= 0L)
	return null;
	nanos = notEmpty.awaitNanos(nanos);
	}
	return x;
	} finally {
	lock.unlock();
	}
	}

	public E pollLast(long timeout, TimeUnit unit)
	throws InterruptedException {
	long nanos = unit.toNanos(timeout);
	final ReentrantLock lock = this.lock;
	lock.lockInterruptibly();
	try {
	E x;
	while ( (x = unlinkLast()) == null) {
	if (nanos <= 0L)
	return null;
	nanos = notEmpty.awaitNanos(nanos);
	}
	return x;
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws NoSuchElementException {@inheritDoc}
	*/
	public E getFirst() {
	E x = peekFirst();
	if (x == null) throw new NoSuchElementException();
	return x;
	}

	/**
	* @throws NoSuchElementException {@inheritDoc}
	*/
	public E getLast() {
	E x = peekLast();
	if (x == null) throw new NoSuchElementException();
	return x;
	}

	public E peekFirst() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return (first == null) ? null : first.item;
	} finally {
	lock.unlock();
	}
	}

	public E peekLast() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return (last == null) ? null : last.item;
	} finally {
	lock.unlock();
	}
	}

	public boolean removeFirstOccurrence(Object o) {
	if (o == null) return false;
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	for (Node<E> p = first; p != null; p = p.next) {
	if (o.equals(p.item)) {
	unlink(p);
	return true;
	}
	}
	return false;
	} finally {
	lock.unlock();
	}
	}

	public boolean removeLastOccurrence(Object o) {
	if (o == null) return false;
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	for (Node<E> p = last; p != null; p = p.prev) {
	if (o.equals(p.item)) {
	unlink(p);
	return true;
	}
	}
	return false;
	} finally {
	lock.unlock();
	}
	}

	// BlockingQueue methods

	/**
	* Inserts the specified element at the end of this deque unless it would
	* violate capacity restrictions. When using a capacity-restricted deque,
	* it is generally preferable to use method {@link #offer(Object) offer}.
	*
	* <p>This method is equivalent to {@link #addLast}.
	*
	* @throws IllegalStateException if this deque is full
	* @throws NullPointerException if the specified element is null
	*/
	public boolean add(E e) {
	addLast(e);
	return true;
	}

	/**
	* @throws NullPointerException if the specified element is null
	*/
	public boolean offer(E e) {
	return offerLast(e);
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws InterruptedException {@inheritDoc}
	*/
	public void put(E e) throws InterruptedException {
	putLast(e);
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws InterruptedException {@inheritDoc}
	*/
	public boolean offer(E e, long timeout, TimeUnit unit)
	throws InterruptedException {
	return offerLast(e, timeout, unit);
	}

	/**
	* 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() removeFirst}.
	*
	* @return the head of the queue represented by this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	public E remove() {
	return removeFirst();
	}

	public E poll() {
	return pollFirst();
	}

	public E take() throws InterruptedException {
	return takeFirst();
	}

	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
	return pollFirst(timeout, unit);
	}

	/**
	* 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() getFirst}.
	*
	* @return the head of the queue represented by this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	public E element() {
	return getFirst();
	}

	public E peek() {
	return peekFirst();
	}

	/**
	* Returns the number of additional elements that this deque can ideally
	* (in the absence of memory or resource constraints) accept without
	* blocking. This is always equal to the initial capacity of this deque
	* less the current {@code size} of this deque.
	*
	* <p>Note that you <em>cannot</em> always tell if an attempt to insert
	* an element will succeed by inspecting {@code remainingCapacity}
	* because it may be the case that another thread is about to
	* insert or remove an element.
	*/
	public int remainingCapacity() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return capacity - count;
	} finally {
	lock.unlock();
	}
	}

	/**
	* @throws UnsupportedOperationException {@inheritDoc}
	* @throws ClassCastException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c) {
	return drainTo(c, Integer.MAX_VALUE);
	}

	/**
	* @throws UnsupportedOperationException {@inheritDoc}
	* @throws ClassCastException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c, int maxElements) {
	Objects.requireNonNull(c);
	if (c == this)
	throw new IllegalArgumentException();
	if (maxElements <= 0)
	return 0;
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	int n = Math.min(maxElements, count);
	for (int i = 0; i < n; i++) {
	c.add(first.item); // In this order, in case add() throws.
	unlinkFirst();
	}
	return n;
	} finally {
	lock.unlock();
	}
	}

	// Stack methods

	/**
	* @throws IllegalStateException if this deque is full
	* @throws NullPointerException {@inheritDoc}
	*/
	public void push(E e) {
	addFirst(e);
	}

	/**
	* @throws NoSuchElementException {@inheritDoc}
	*/
	public E pop() {
	return removeFirst();
	}

	// Collection methods

	/**
	* Removes the first occurrence 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) removeFirstOccurrence}.
	*
	* @param o element to be removed from this deque, if present
	* @return {@code true} if this deque changed as a result of the call
	*/
	public boolean remove(Object o) {
	return removeFirstOccurrence(o);
	}

	/**
	* Returns the number of elements in this deque.
	*
	* @return the number of elements in this deque
	*/
	public int size() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return count;
	} finally {
	lock.unlock();
	}
	}

	/**
	* 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;
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	for (Node<E> p = first; p != null; p = p.next)
	if (o.equals(p.item))
	return true;
	return false;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Appends all of the elements in the specified collection to the end of
	* this deque, in the order that they are returned by the specified
	* collection's iterator. Attempts to {@code addAll} of a deque to
	* itself result in {@code IllegalArgumentException}.
	*
	* @param c the elements to be inserted into this deque
	* @return {@code true} if this deque changed as a result of the call
	* @throws NullPointerException if the specified collection or any
	* of its elements are null
	* @throws IllegalArgumentException if the collection is this deque
	* @throws IllegalStateException if this deque is full
	* @see #add(Object)
	*/
	public boolean addAll(Collection<? extends E> c) {
	if (c == this)
	// As historically specified in AbstractQueue#addAll
	throw new IllegalArgumentException();

	// Copy c into a private chain of Nodes
	Node<E> beg = null, end = null;
	int n = 0;
	for (E e : c) {
	Objects.requireNonNull(e);
	n++;
	Node<E> newNode = new Node<E>(e);
	if (beg == null)
	beg = end = newNode;
	else {
	end.next = newNode;
	newNode.prev = end;
	end = newNode;
	}
	}
	if (beg == null)
	return false;

	// Atomically append the chain at the end
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	if (count + n <= capacity) {
	beg.prev = last;
	if (first == null)
	first = beg;
	else
	last.next = beg;
	last = end;
	count += n;
	notEmpty.signalAll();
	return true;
	}
	} finally {
	lock.unlock();
	}
	// Fall back to historic non-atomic implementation, failing
	// with IllegalStateException when the capacity is exceeded.
	return super.addAll(c);
	}

	/**
	* 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
	*/
	@SuppressWarnings("unchecked")
	public Object[] toArray() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	Object[] a = new Object[count];
	int k = 0;
	for (Node<E> p = first; p != null; p = p.next)
	a[k++] = p.item;
	return a;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns an array containing all of the elements in this deque, in
	* proper sequence; 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) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	if (a.length < count)
	a = (T[])java.lang.reflect.Array.newInstance
	(a.getClass().getComponentType(), count);

	int k = 0;
	for (Node<E> p = first; p != null; p = p.next)
	a[k++] = (T)p.item;
	if (a.length > k)
	a[k] = null;
	return a;
	} finally {
	lock.unlock();
	}
	}

	public String toString() {
	return Helpers.collectionToString(this);
	}

	/**
	* Atomically removes all of the elements from this deque.
	* The deque will be empty after this call returns.
	*/
	public void clear() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	for (Node<E> f = first; f != null; ) {
	f.item = null;
	Node<E> n = f.next;
	f.prev = null;
	f.next = null;
	f = n;
	}
	first = last = null;
	count = 0;
	notFull.signalAll();
	} finally {
	lock.unlock();
	}
	}

	/**
	* Used for any element traversal that is not entirely under lock.
	* Such traversals must handle both:
	* - dequeued nodes (p.next == p)
	* - (possibly multiple) interior removed nodes (p.item == null)
	*/
	Node<E> succ(Node<E> p) {
	if (p == (p = p.next))
	p = first;
	return p;
	}

	/**
	* Returns an iterator over the elements in this deque in proper sequence.
	* The elements will be returned in order from first (head) to last (tail).
	*
	* <p>The returned iterator is
	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* @return an iterator over the elements in this deque in proper sequence
	*/
	public Iterator<E> iterator() {
	return new Itr();
	}

	/**
	* Returns an iterator over the elements in this deque in reverse
	* sequential order. The elements will be returned in order from
	* last (tail) to first (head).
	*
	* <p>The returned iterator is
	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* @return an iterator over the elements in this deque in reverse order
	*/
	public Iterator<E> descendingIterator() {
	return new DescendingItr();
	}

	/**
	* Base class for LinkedBlockingDeque iterators.
	*/
	private abstract class AbstractItr implements Iterator<E> {
	/**
	* The next node to return in next().
	*/
	Node<E> next;

	/**
	* nextItem holds on to item fields because once we claim that
	* an element exists in hasNext(), we must return item read
	* under lock even if it was in the process of being removed
	* when hasNext() was called.
	*/
	E nextItem;

	/**
	* Node returned by most recent call to next. Needed by remove.
	* Reset to null if this element is deleted by a call to remove.
	*/
	private Node<E> lastRet;

	abstract Node<E> firstNode();
	abstract Node<E> nextNode(Node<E> n);

	private Node<E> succ(Node<E> p) {
	if (p == (p = nextNode(p)))
	p = firstNode();
	return p;
	}

	AbstractItr() {
	// set to initial position
	final ReentrantLock lock = LinkedBlockingDeque.this.lock;
	lock.lock();
	try {
	if ((next = firstNode()) != null)
	nextItem = next.item;
	} finally {
	lock.unlock();
	}
	}

	public boolean hasNext() {
	return next != null;
	}

	public E next() {
	Node<E> p;
	if ((p = next) == null)
	throw new NoSuchElementException();
	lastRet = p;
	E x = nextItem;
	final ReentrantLock lock = LinkedBlockingDeque.this.lock;
	lock.lock();
	try {
	E e = null;
	for (p = nextNode(p); p != null && (e = p.item) == null; )
	p = succ(p);
	next = p;
	nextItem = e;
	} finally {
	lock.unlock();
	}
	return x;
	}

	public void forEachRemaining(Consumer<? super E> action) {
	// A variant of forEachFrom
	Objects.requireNonNull(action);
	Node<E> p;
	if ((p = next) == null) return;
	lastRet = p;
	next = null;
	final ReentrantLock lock = LinkedBlockingDeque.this.lock;
	final int batchSize = 64;
	Object[] es = null;
	int n, len = 1;
	do {
	lock.lock();
	try {
	if (es == null) {
	p = nextNode(p);
	for (Node<E> q = p; q != null; q = succ(q))
	if (q.item != null && ++len == batchSize)
	break;
	es = new Object[len];
	es[0] = nextItem;
	nextItem = null;
	n = 1;
	} else
	n = 0;
	for (; p != null && n < len; p = succ(p))
	if ((es[n] = p.item) != null) {
	lastRet = p;
	n++;
	}
	} finally {
	lock.unlock();
	}
	for (int i = 0; i < n; i++) {
	@SuppressWarnings("unchecked") E e = (E) es[i];
	action.accept(e);
	}
	} while (n > 0 && p != null);
	}

	public void remove() {
	Node<E> n = lastRet;
	if (n == null)
	throw new IllegalStateException();
	lastRet = null;
	final ReentrantLock lock = LinkedBlockingDeque.this.lock;
	lock.lock();
	try {
	if (n.item != null)
	unlink(n);
	} finally {
	lock.unlock();
	}
	}
	}

	/** Forward iterator */
	private class Itr extends AbstractItr {
	Itr() {} // prevent access constructor creation
	Node<E> firstNode() { return first; }
	Node<E> nextNode(Node<E> n) { return n.next; }
	}

	/** Descending iterator */
	private class DescendingItr extends AbstractItr {
	DescendingItr() {} // prevent access constructor creation
	Node<E> firstNode() { return last; }
	Node<E> nextNode(Node<E> n) { return n.prev; }
	}

	/**
	* A customized variant of Spliterators.IteratorSpliterator.
	* Keep this class in sync with (very similar) LBQSpliterator.
	*/
	private final class LBDSpliterator implements Spliterator<E> {
	static final int MAX_BATCH = 1 << 25; // max batch array size;
	Node<E> current; // current node; null until initialized
	int batch; // batch size for splits
	boolean exhausted; // true when no more nodes
	long est = size(); // size estimate

	LBDSpliterator() {}

	public long estimateSize() { return est; }

	public Spliterator<E> trySplit() {
	Node<E> h;
	if (!exhausted &&
	((h = current) != null \|\| (h = first) != null)
	&& h.next != null) {
	int n = batch = Math.min(batch + 1, MAX_BATCH);
	Object[] a = new Object[n];
	final ReentrantLock lock = LinkedBlockingDeque.this.lock;
	int i = 0;
	Node<E> p = current;
	lock.lock();
	try {
	if (p != null \|\| (p = first) != null)
	for (; p != null && i < n; p = succ(p))
	if ((a[i] = p.item) != null)
	i++;
	} finally {
	lock.unlock();
	}
	if ((current = p) == null) {
	est = 0L;
	exhausted = true;
	}
	else if ((est -= i) < 0L)
	est = 0L;
	if (i > 0)
	return Spliterators.spliterator
	(a, 0, i, (Spliterator.ORDERED \|
	Spliterator.NONNULL \|
	Spliterator.CONCURRENT));
	}
	return null;
	}

	public boolean tryAdvance(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	if (!exhausted) {
	E e = null;
	final ReentrantLock lock = LinkedBlockingDeque.this.lock;
	lock.lock();
	try {
	Node<E> p;
	if ((p = current) != null \|\| (p = first) != null)
	do {
	e = p.item;
	p = succ(p);
	} while (e == null && p != null);
	if ((current = p) == null)
	exhausted = true;
	} finally {
	lock.unlock();
	}
	if (e != null) {
	action.accept(e);
	return true;
	}
	}
	return false;
	}

	public void forEachRemaining(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	if (!exhausted) {
	exhausted = true;
	Node<E> p = current;
	current = null;
	forEachFrom(action, p);
	}
	}

	public int characteristics() {
	return (Spliterator.ORDERED \|
	Spliterator.NONNULL \|
	Spliterator.CONCURRENT);
	}
	}

	/**
	* Returns a {@link Spliterator} over the elements in this deque.
	*
	* <p>The returned spliterator is
	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
	* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
	*
	* @implNote
	* The {@code Spliterator} implements {@code trySplit} to permit limited
	* parallelism.
	*
	* @return a {@code Spliterator} over the elements in this deque
	* @since 1.8
	*/
	public Spliterator<E> spliterator() {
	return new LBDSpliterator();
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public void forEach(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	forEachFrom(action, null);
	}

	/**
	* Runs action on each element found during a traversal starting at p.
	* If p is null, traversal starts at head.
	*/
	void forEachFrom(Consumer<? super E> action, Node<E> p) {
	// Extract batches of elements while holding the lock; then
	// run the action on the elements while not
	final ReentrantLock lock = this.lock;
	final int batchSize = 64; // max number of elements per batch
	Object[] es = null; // container for batch of elements
	int n, len = 0;
	do {
	lock.lock();
	try {
	if (es == null) {
	if (p == null) p = first;
	for (Node<E> q = p; q != null; q = succ(q))
	if (q.item != null && ++len == batchSize)
	break;
	es = new Object[len];
	}
	for (n = 0; p != null && n < len; p = succ(p))
	if ((es[n] = p.item) != null)
	n++;
	} finally {
	lock.unlock();
	}
	for (int i = 0; i < n; i++) {
	@SuppressWarnings("unchecked") E e = (E) es[i];
	action.accept(e);
	}
	} while (n > 0 && p != null);
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean removeIf(Predicate<? super E> filter) {
	Objects.requireNonNull(filter);
	return bulkRemove(filter);
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean removeAll(Collection<?> c) {
	Objects.requireNonNull(c);
	return bulkRemove(e -> c.contains(e));
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean retainAll(Collection<?> c) {
	Objects.requireNonNull(c);
	return bulkRemove(e -> !c.contains(e));
	}

	/** Implementation of bulk remove methods. */
	@SuppressWarnings("unchecked")
	private boolean bulkRemove(Predicate<? super E> filter) {
	boolean removed = false;
	final ReentrantLock lock = this.lock;
	Node<E> p = null;
	Node<E>[] nodes = null;
	int n, len = 0;
	do {
	// 1. Extract batch of up to 64 elements while holding the lock.
	lock.lock();
	try {
	if (nodes == null) { // first batch; initialize
	p = first;
	for (Node<E> q = p; q != null; q = succ(q))
	if (q.item != null && ++len == 64)
	break;
	nodes = (Node<E>[]) new Node<?>[len];
	}
	for (n = 0; p != null && n < len; p = succ(p))
	nodes[n++] = p;
	} finally {
	lock.unlock();
	}

	// 2. Run the filter on the elements while lock is free.
	long deathRow = 0L; // "bitset" of size 64
	for (int i = 0; i < n; i++) {
	final E e;
	if ((e = nodes[i].item) != null && filter.test(e))
	deathRow \|= 1L << i;
	}

	// 3. Remove any filtered elements while holding the lock.
	if (deathRow != 0) {
	lock.lock();
	try {
	for (int i = 0; i < n; i++) {
	final Node<E> q;
	if ((deathRow & (1L << i)) != 0L
	&& (q = nodes[i]).item != null) {
	unlink(q);
	removed = true;
	}
	nodes[i] = null; // help GC
	}
	} finally {
	lock.unlock();
	}
	}
	} while (n > 0 && p != null);
	return removed;
	}

	/**
	* Saves this deque to a stream (that is, serializes it).
	*
	* @param s the stream
	* @throws java.io.IOException if an I/O error occurs
	* @serialData The capacity (int), followed by elements (each an
	* {@code Object}) in the proper order, followed by a null
	*/
	private void writeObject(java.io.ObjectOutputStream s)
	throws java.io.IOException {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	// Write out capacity and any hidden stuff
	s.defaultWriteObject();
	// Write out all elements in the proper order.
	for (Node<E> p = first; p != null; p = p.next)
	s.writeObject(p.item);
	// Use trailing null as sentinel
	s.writeObject(null);
	} finally {
	lock.unlock();
	}
	}

	/**
	* Reconstitutes this deque from a stream (that is, deserializes it).
	* @param s the stream
	* @throws ClassNotFoundException if the class of a serialized object
	* could not be found
	* @throws java.io.IOException if an I/O error occurs
	*/
	private void readObject(java.io.ObjectInputStream s)
	throws java.io.IOException, ClassNotFoundException {
	s.defaultReadObject();
	count = 0;
	first = null;
	last = null;
	// Read in all elements and place in queue
	for (;;) {
	@SuppressWarnings("unchecked") E item = (E)s.readObject();
	if (item == null)
	break;
	add(item);
	}
	}

	void checkInvariants() {
	// assert lock.isHeldByCurrentThread();
	// Nodes may get self-linked or lose their item, but only
	// after being unlinked and becoming unreachable from first.
	for (Node<E> p = first; p != null; p = p.next) {
	// assert p.next != p;
	// assert p.item != null;
	}
	}

	}

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