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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.atomic.AtomicInteger;
	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 BlockingQueue blocking queue} based on
	* linked nodes.
	* This queue orders elements FIFO (first-in-first-out).
	* The <em>head</em> of the queue is that element that has been on the
	* queue the longest time.
	* The <em>tail</em> of the queue is that element that has been on the
	* queue the shortest time. New elements
	* are inserted at the tail of the queue, and the queue retrieval
	* operations obtain elements at the head of the queue.
	* Linked queues typically have higher throughput than array-based queues but
	* less predictable performance in most concurrent applications.
	*
	* <p>The optional capacity bound constructor argument serves as a
	* way to prevent excessive queue 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
	* queue above capacity.
	*
	* <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 1.5
	* @author Doug Lea
	* @param <E> the type of elements held in this queue
	*/
	public class LinkedBlockingQueue<E> extends AbstractQueue<E>
	implements BlockingQueue<E>, java.io.Serializable {
	private static final long serialVersionUID = -6903933977591709194L;

	/*
	* A variant of the "two lock queue" algorithm. The putLock gates
	* entry to put (and offer), and has an associated condition for
	* waiting puts. Similarly for the takeLock. The "count" field
	* that they both rely on is maintained as an atomic to avoid
	* needing to get both locks in most cases. Also, to minimize need
	* for puts to get takeLock and vice-versa, cascading notifies are
	* used. When a put notices that it has enabled at least one take,
	* it signals taker. That taker in turn signals others if more
	* items have been entered since the signal. And symmetrically for
	* takes signalling puts. Operations such as remove(Object) and
	* iterators acquire both locks.
	*
	* Visibility between writers and readers is provided as follows:
	*
	* Whenever an element is enqueued, the putLock is acquired and
	* count updated. A subsequent reader guarantees visibility to the
	* enqueued Node by either acquiring the putLock (via fullyLock)
	* or by acquiring the takeLock, and then reading n = count.get();
	* this gives visibility to the first n items.
	*
	* 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 advance to head.next.
	*/

	/**
	* Linked list node class.
	*/
	static class Node<E> {
	E item;

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

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

	/** The capacity bound, or Integer.MAX_VALUE if none */
	private final int capacity;

	/** Current number of elements */
	private final AtomicInteger count = new AtomicInteger();

	/**
	* Head of linked list.
	* Invariant: head.item == null
	*/
	transient Node<E> head;

	/**
	* Tail of linked list.
	* Invariant: last.next == null
	*/
	private transient Node<E> last;

	/** Lock held by take, poll, etc */
	private final ReentrantLock takeLock = new ReentrantLock();

	/** Wait queue for waiting takes */
	private final Condition notEmpty = takeLock.newCondition();

	/** Lock held by put, offer, etc */
	private final ReentrantLock putLock = new ReentrantLock();

	/** Wait queue for waiting puts */
	private final Condition notFull = putLock.newCondition();

	/**
	* Signals a waiting take. Called only from put/offer (which do not
	* otherwise ordinarily lock takeLock.)
	*/
	private void signalNotEmpty() {
	final ReentrantLock takeLock = this.takeLock;
	takeLock.lock();
	try {
	notEmpty.signal();
	} finally {
	takeLock.unlock();
	}
	}

	/**
	* Signals a waiting put. Called only from take/poll.
	*/
	private void signalNotFull() {
	final ReentrantLock putLock = this.putLock;
	putLock.lock();
	try {
	notFull.signal();
	} finally {
	putLock.unlock();
	}
	}

	/**
	* Links node at end of queue.
	*
	* @param node the node
	*/
	private void enqueue(Node<E> node) {
	// assert putLock.isHeldByCurrentThread();
	// assert last.next == null;
	last = last.next = node;
	}

	/**
	* Removes a node from head of queue.
	*
	* @return the node
	*/
	private E dequeue() {
	// assert takeLock.isHeldByCurrentThread();
	// assert head.item == null;
	Node<E> h = head;
	Node<E> first = h.next;
	h.next = h; // help GC
	head = first;
	E x = first.item;
	first.item = null;
	return x;
	}

	/**
	* Locks to prevent both puts and takes.
	*/
	void fullyLock() {
	putLock.lock();
	takeLock.lock();
	}

	/**
	* Unlocks to allow both puts and takes.
	*/
	void fullyUnlock() {
	takeLock.unlock();
	putLock.unlock();
	}

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

	/**
	* Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
	*
	* @param capacity the capacity of this queue
	* @throws IllegalArgumentException if {@code capacity} is not greater
	* than zero
	*/
	public LinkedBlockingQueue(int capacity) {
	if (capacity <= 0) throw new IllegalArgumentException();
	this.capacity = capacity;
	last = head = new Node<E>(null);
	}

	/**
	* Creates a {@code LinkedBlockingQueue} 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 LinkedBlockingQueue(Collection<? extends E> c) {
	this(Integer.MAX_VALUE);
	final ReentrantLock putLock = this.putLock;
	putLock.lock(); // Never contended, but necessary for visibility
	try {
	int n = 0;
	for (E e : c) {
	if (e == null)
	throw new NullPointerException();
	if (n == capacity)
	throw new IllegalStateException("Queue full");
	enqueue(new Node<E>(e));
	++n;
	}
	count.set(n);
	} finally {
	putLock.unlock();
	}
	}

	// this doc comment is overridden to remove the reference to collections
	// greater in size than Integer.MAX_VALUE
	/**
	* Returns the number of elements in this queue.
	*
	* @return the number of elements in this queue
	*/
	public int size() {
	return count.get();
	}

	// this doc comment is a modified copy of the inherited doc comment,
	// without the reference to unlimited queues.
	/**
	* Returns the number of additional elements that this queue can ideally
	* (in the absence of memory or resource constraints) accept without
	* blocking. This is always equal to the initial capacity of this queue
	* less the current {@code size} of this queue.
	*
	* <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() {
	return capacity - count.get();
	}

	/**
	* Inserts the specified element at the tail of this queue, waiting if
	* necessary for space to become available.
	*
	* @throws InterruptedException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	*/
	public void put(E e) throws InterruptedException {
	if (e == null) throw new NullPointerException();
	final int c;
	final Node<E> node = new Node<E>(e);
	final ReentrantLock putLock = this.putLock;
	final AtomicInteger count = this.count;
	putLock.lockInterruptibly();
	try {
	/*
	* Note that count is used in wait guard even though it is
	* not protected by lock. This works because count can
	* only decrease at this point (all other puts are shut
	* out by lock), and we (or some other waiting put) are
	* signalled if it ever changes from capacity. Similarly
	* for all other uses of count in other wait guards.
	*/
	while (count.get() == capacity) {
	notFull.await();
	}
	enqueue(node);
	c = count.getAndIncrement();
	if (c + 1 < capacity)
	notFull.signal();
	} finally {
	putLock.unlock();
	}
	if (c == 0)
	signalNotEmpty();
	}

	/**
	* Inserts the specified element at the tail of this queue, waiting if
	* necessary up to the specified wait time for space to become available.
	*
	* @return {@code true} if successful, or {@code false} if
	* the specified waiting time elapses before space is available
	* @throws InterruptedException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	*/
	public boolean offer(E e, long timeout, TimeUnit unit)
	throws InterruptedException {

	if (e == null) throw new NullPointerException();
	long nanos = unit.toNanos(timeout);
	final int c;
	final ReentrantLock putLock = this.putLock;
	final AtomicInteger count = this.count;
	putLock.lockInterruptibly();
	try {
	while (count.get() == capacity) {
	if (nanos <= 0L)
	return false;
	nanos = notFull.awaitNanos(nanos);
	}
	enqueue(new Node<E>(e));
	c = count.getAndIncrement();
	if (c + 1 < capacity)
	notFull.signal();
	} finally {
	putLock.unlock();
	}
	if (c == 0)
	signalNotEmpty();
	return true;
	}

	/**
	* Inserts the specified element at the tail of this queue if it is
	* possible to do so immediately without exceeding the queue's capacity,
	* returning {@code true} upon success and {@code false} if this queue
	* is full.
	* When using a capacity-restricted queue, this method is generally
	* preferable to method {@link BlockingQueue#add add}, which can fail to
	* insert an element only by throwing an exception.
	*
	* @throws NullPointerException if the specified element is null
	*/
	public boolean offer(E e) {
	if (e == null) throw new NullPointerException();
	final AtomicInteger count = this.count;
	if (count.get() == capacity)
	return false;
	final int c;
	final Node<E> node = new Node<E>(e);
	final ReentrantLock putLock = this.putLock;
	putLock.lock();
	try {
	if (count.get() == capacity)
	return false;
	enqueue(node);
	c = count.getAndIncrement();
	if (c + 1 < capacity)
	notFull.signal();
	} finally {
	putLock.unlock();
	}
	if (c == 0)
	signalNotEmpty();
	return true;
	}

	public E take() throws InterruptedException {
	final E x;
	final int c;
	final AtomicInteger count = this.count;
	final ReentrantLock takeLock = this.takeLock;
	takeLock.lockInterruptibly();
	try {
	while (count.get() == 0) {
	notEmpty.await();
	}
	x = dequeue();
	c = count.getAndDecrement();
	if (c > 1)
	notEmpty.signal();
	} finally {
	takeLock.unlock();
	}
	if (c == capacity)
	signalNotFull();
	return x;
	}

	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
	final E x;
	final int c;
	long nanos = unit.toNanos(timeout);
	final AtomicInteger count = this.count;
	final ReentrantLock takeLock = this.takeLock;
	takeLock.lockInterruptibly();
	try {
	while (count.get() == 0) {
	if (nanos <= 0L)
	return null;
	nanos = notEmpty.awaitNanos(nanos);
	}
	x = dequeue();
	c = count.getAndDecrement();
	if (c > 1)
	notEmpty.signal();
	} finally {
	takeLock.unlock();
	}
	if (c == capacity)
	signalNotFull();
	return x;
	}

	public E poll() {
	final AtomicInteger count = this.count;
	if (count.get() == 0)
	return null;
	final E x;
	final int c;
	final ReentrantLock takeLock = this.takeLock;
	takeLock.lock();
	try {
	if (count.get() == 0)
	return null;
	x = dequeue();
	c = count.getAndDecrement();
	if (c > 1)
	notEmpty.signal();
	} finally {
	takeLock.unlock();
	}
	if (c == capacity)
	signalNotFull();
	return x;
	}

	public E peek() {
	final AtomicInteger count = this.count;
	if (count.get() == 0)
	return null;
	final ReentrantLock takeLock = this.takeLock;
	takeLock.lock();
	try {
	return (count.get() > 0) ? head.next.item : null;
	} finally {
	takeLock.unlock();
	}
	}

	/**
	* Unlinks interior Node p with predecessor pred.
	*/
	void unlink(Node<E> p, Node<E> pred) {
	// assert putLock.isHeldByCurrentThread();
	// assert takeLock.isHeldByCurrentThread();
	// p.next is not changed, to allow iterators that are
	// traversing p to maintain their weak-consistency guarantee.
	p.item = null;
	pred.next = p.next;
	if (last == p)
	last = pred;
	if (count.getAndDecrement() == capacity)
	notFull.signal();
	}

	/**
	* Removes a single instance of the specified element from this queue,
	* if it is present. More formally, removes an element {@code e} such
	* that {@code o.equals(e)}, if this queue contains one or more such
	* elements.
	* Returns {@code true} if this queue contained the specified element
	* (or equivalently, if this queue changed as a result of the call).
	*
	* @param o element to be removed from this queue, if present
	* @return {@code true} if this queue changed as a result of the call
	*/
	public boolean remove(Object o) {
	if (o == null) return false;
	fullyLock();
	try {
	for (Node<E> pred = head, p = pred.next;
	p != null;
	pred = p, p = p.next) {
	if (o.equals(p.item)) {
	unlink(p, pred);
	return true;
	}
	}
	return false;
	} finally {
	fullyUnlock();
	}
	}

	/**
	* Returns {@code true} if this queue contains the specified element.
	* More formally, returns {@code true} if and only if this queue contains
	* at least one element {@code e} such that {@code o.equals(e)}.
	*
	* @param o object to be checked for containment in this queue
	* @return {@code true} if this queue contains the specified element
	*/
	public boolean contains(Object o) {
	if (o == null) return false;
	fullyLock();
	try {
	for (Node<E> p = head.next; p != null; p = p.next)
	if (o.equals(p.item))
	return true;
	return false;
	} finally {
	fullyUnlock();
	}
	}

	/**
	* Returns an array containing all of the elements in this queue, in
	* proper sequence.
	*
	* <p>The returned array will be "safe" in that no references to it are
	* maintained by this queue. (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 queue
	*/
	public Object[] toArray() {
	fullyLock();
	try {
	int size = count.get();
	Object[] a = new Object[size];
	int k = 0;
	for (Node<E> p = head.next; p != null; p = p.next)
	a[k++] = p.item;
	return a;
	} finally {
	fullyUnlock();
	}
	}

	/**
	* Returns an array containing all of the elements in this queue, in
	* proper sequence; the runtime type of the returned array is that of
	* the specified array. If the queue 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 queue.
	*
	* <p>If this queue fits in the specified array with room to spare
	* (i.e., the array has more elements than this queue), the element in
	* the array immediately following the end of the queue 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 queue known to contain only strings.
	* The following code can be used to dump the queue 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 queue 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 queue
	* @throws ArrayStoreException if the runtime type of the specified array
	* is not a supertype of the runtime type of every element in
	* this queue
	* @throws NullPointerException if the specified array is null
	*/
	@SuppressWarnings("unchecked")
	public <T> T[] toArray(T[] a) {
	fullyLock();
	try {
	int size = count.get();
	if (a.length < size)
	a = (T[])java.lang.reflect.Array.newInstance
	(a.getClass().getComponentType(), size);

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

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

	/**
	* Atomically removes all of the elements from this queue.
	* The queue will be empty after this call returns.
	*/
	public void clear() {
	fullyLock();
	try {
	for (Node<E> p, h = head; (p = h.next) != null; h = p) {
	h.next = h;
	p.item = null;
	}
	head = last;
	// assert head.item == null && head.next == null;
	if (count.getAndSet(0) == capacity)
	notFull.signal();
	} finally {
	fullyUnlock();
	}
	}

	/**
	* @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;
	boolean signalNotFull = false;
	final ReentrantLock takeLock = this.takeLock;
	takeLock.lock();
	try {
	int n = Math.min(maxElements, count.get());
	// count.get provides visibility to first n Nodes
	Node<E> h = head;
	int i = 0;
	try {
	while (i < n) {
	Node<E> p = h.next;
	c.add(p.item);
	p.item = null;
	h.next = h;
	h = p;
	++i;
	}
	return n;
	} finally {
	// Restore invariants even if c.add() threw
	if (i > 0) {
	// assert h.item == null;
	head = h;
	signalNotFull = (count.getAndAdd(-i) == capacity);
	}
	}
	} finally {
	takeLock.unlock();
	if (signalNotFull)
	signalNotFull();
	}
	}

	/**
	* 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 = head.next;
	return p;
	}

	/**
	* Returns an iterator over the elements in this queue 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 queue in proper sequence
	*/
	public Iterator<E> iterator() {
	return new Itr();
	}

	/**
	* Weakly-consistent iterator.
	*
	* Lazily updated ancestor field provides expected O(1) remove(),
	* but still O(n) in the worst case, whenever the saved ancestor
	* is concurrently deleted.
	*/
	private class Itr implements Iterator<E> {
	private Node<E> next; // Node holding nextItem
	private E nextItem; // next item to hand out
	private Node<E> lastRet;
	private Node<E> ancestor; // Helps unlink lastRet on remove()

	Itr() {
	fullyLock();
	try {
	if ((next = head.next) != null)
	nextItem = next.item;
	} finally {
	fullyUnlock();
	}
	}

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

	public E next() {
	Node<E> p;
	if ((p = next) == null)
	throw new NoSuchElementException();
	lastRet = p;
	E x = nextItem;
	fullyLock();
	try {
	E e = null;
	for (p = p.next; p != null && (e = p.item) == null; )
	p = succ(p);
	next = p;
	nextItem = e;
	} finally {
	fullyUnlock();
	}
	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 int batchSize = 64;
	Object[] es = null;
	int n, len = 1;
	do {
	fullyLock();
	try {
	if (es == null) {
	p = p.next;
	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 {
	fullyUnlock();
	}
	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> p = lastRet;
	if (p == null)
	throw new IllegalStateException();
	lastRet = null;
	fullyLock();
	try {
	if (p.item != null) {
	if (ancestor == null)
	ancestor = head;
	ancestor = findPred(p, ancestor);
	unlink(p, ancestor);
	}
	} finally {
	fullyUnlock();
	}
	}
	}

	/**
	* A customized variant of Spliterators.IteratorSpliterator.
	* Keep this class in sync with (very similar) LBDSpliterator.
	*/
	private final class LBQSpliterator 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

	LBQSpliterator() {}

	public long estimateSize() { return est; }

	public Spliterator<E> trySplit() {
	Node<E> h;
	if (!exhausted &&
	((h = current) != null \|\| (h = head.next) != null)
	&& h.next != null) {
	int n = batch = Math.min(batch + 1, MAX_BATCH);
	Object[] a = new Object[n];
	int i = 0;
	Node<E> p = current;
	fullyLock();
	try {
	if (p != null \|\| (p = head.next) != null)
	for (; p != null && i < n; p = succ(p))
	if ((a[i] = p.item) != null)
	i++;
	} finally {
	fullyUnlock();
	}
	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;
	fullyLock();
	try {
	Node<E> p;
	if ((p = current) != null \|\| (p = head.next) != null)
	do {
	e = p.item;
	p = succ(p);
	} while (e == null && p != null);
	if ((current = p) == null)
	exhausted = true;
	} finally {
	fullyUnlock();
	}
	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 queue.
	*
	* <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 queue
	* @since 1.8
	*/
	public Spliterator<E> spliterator() {
	return new LBQSpliterator();
	}

	/**
	* @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 int batchSize = 64; // max number of elements per batch
	Object[] es = null; // container for batch of elements
	int n, len = 0;
	do {
	fullyLock();
	try {
	if (es == null) {
	if (p == null) p = head.next;
	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 {
	fullyUnlock();
	}
	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));
	}

	/**
	* Returns the predecessor of live node p, given a node that was
	* once a live ancestor of p (or head); allows unlinking of p.
	*/
	Node<E> findPred(Node<E> p, Node<E> ancestor) {
	// assert p.item != null;
	if (ancestor.item == null)
	ancestor = head;
	// Fails with NPE if precondition not satisfied
	for (Node<E> q; (q = ancestor.next) != p; )
	ancestor = q;
	return ancestor;
	}

	/** Implementation of bulk remove methods. */
	@SuppressWarnings("unchecked")
	private boolean bulkRemove(Predicate<? super E> filter) {
	boolean removed = false;
	Node<E> p = null, ancestor = head;
	Node<E>[] nodes = null;
	int n, len = 0;
	do {
	// 1. Extract batch of up to 64 elements while holding the lock.
	fullyLock();
	try {
	if (nodes == null) { // first batch; initialize
	p = head.next;
	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 {
	fullyUnlock();
	}

	// 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) {
	fullyLock();
	try {
	for (int i = 0; i < n; i++) {
	final Node<E> q;
	if ((deathRow & (1L << i)) != 0L
	&& (q = nodes[i]).item != null) {
	ancestor = findPred(q, ancestor);
	unlink(q, ancestor);
	removed = true;
	}
	nodes[i] = null; // help GC
	}
	} finally {
	fullyUnlock();
	}
	}
	} while (n > 0 && p != null);
	return removed;
	}

	/**
	* Saves this queue to a stream (that is, serializes it).
	*
	* @param s the stream
	* @throws java.io.IOException if an I/O error occurs
	* @serialData The capacity is emitted (int), followed by all of
	* its elements (each an {@code Object}) in the proper order,
	* followed by a null
	*/
	private void writeObject(java.io.ObjectOutputStream s)
	throws java.io.IOException {

	fullyLock();
	try {
	// Write out any hidden stuff, plus capacity
	s.defaultWriteObject();

	// Write out all elements in the proper order.
	for (Node<E> p = head.next; p != null; p = p.next)
	s.writeObject(p.item);

	// Use trailing null as sentinel
	s.writeObject(null);
	} finally {
	fullyUnlock();
	}
	}

	/**
	* Reconstitutes this queue 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 {
	// Read in capacity, and any hidden stuff
	s.defaultReadObject();

	count.set(0);
	last = head = new Node<E>(null);

	// Read in all elements and place in queue
	for (;;) {
	@SuppressWarnings("unchecked")
	E item = (E)s.readObject();
	if (item == null)
	break;
	add(item);
	}
	}
	}

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