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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.Queue;
	import java.util.concurrent.TimeUnit;
	import java.util.concurrent.locks.LockSupport;
	import java.util.Spliterator;
	import java.util.Spliterators;
	import java.util.function.Consumer;

	/**
	* An unbounded {@link TransferQueue} based on linked nodes.
	* This queue orders elements FIFO (first-in-first-out) with respect
	* to any given producer. The <em>head</em> of the queue is that
	* element that has been on the queue the longest time for some
	* producer. The <em>tail</em> of the queue is that element that has
	* been on the queue the shortest time for some producer.
	*
	* <p>Beware that, unlike in most collections, the {@code size} method
	* is <em>NOT</em> a constant-time operation. Because of the
	* asynchronous nature of these queues, determining the current number
	* of elements requires a traversal of the elements, and so may report
	* inaccurate results if this collection is modified during traversal.
	* Additionally, the bulk operations {@code addAll},
	* {@code removeAll}, {@code retainAll}, {@code containsAll},
	* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
	* to be performed atomically. For example, an iterator operating
	* concurrently with an {@code addAll} operation might view only some
	* of the added elements.
	*
	* <p>This class and its iterator implement all of the
	* <em>optional</em> methods of the {@link Collection} and {@link
	* Iterator} interfaces.
	*
	* <p>Memory consistency effects: As with other concurrent
	* collections, actions in a thread prior to placing an object into a
	* {@code LinkedTransferQueue}
	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	* actions subsequent to the access or removal of that element from
	* the {@code LinkedTransferQueue} in another thread.
	*
	* <p>This class is a member of the
	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
	* Java Collections Framework</a>.
	*
	* @since 1.7
	* @author Doug Lea
	* @param <E> the type of elements held in this collection
	*/
	public class LinkedTransferQueue<E> extends AbstractQueue<E>
	implements TransferQueue<E>, java.io.Serializable {
	private static final long serialVersionUID = -3223113410248163686L;

	/*
	* * Overview of Dual Queues with Slack *
	*
	* Dual Queues, introduced by Scherer and Scott
	* (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
	* (linked) queues in which nodes may represent either data or
	* requests. When a thread tries to enqueue a data node, but
	* encounters a request node, it instead "matches" and removes it;
	* and vice versa for enqueuing requests. Blocking Dual Queues
	* arrange that threads enqueuing unmatched requests block until
	* other threads provide the match. Dual Synchronous Queues (see
	* Scherer, Lea, & Scott
	* http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
	* additionally arrange that threads enqueuing unmatched data also
	* block. Dual Transfer Queues support all of these modes, as
	* dictated by callers.
	*
	* A FIFO dual queue may be implemented using a variation of the
	* Michael & Scott (M&S) lock-free queue algorithm
	* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
	* It maintains two pointer fields, "head", pointing to a
	* (matched) node that in turn points to the first actual
	* (unmatched) queue node (or null if empty); and "tail" that
	* points to the last node on the queue (or again null if
	* empty). For example, here is a possible queue with four data
	* elements:
	*
	* head tail
	* \| \|
	* v v
	* M -> U -> U -> U -> U
	*
	* The M&S queue algorithm is known to be prone to scalability and
	* overhead limitations when maintaining (via CAS) these head and
	* tail pointers. This has led to the development of
	* contention-reducing variants such as elimination arrays (see
	* Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
	* optimistic back pointers (see Ladan-Mozes & Shavit
	* http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
	* However, the nature of dual queues enables a simpler tactic for
	* improving M&S-style implementations when dual-ness is needed.
	*
	* In a dual queue, each node must atomically maintain its match
	* status. While there are other possible variants, we implement
	* this here as: for a data-mode node, matching entails CASing an
	* "item" field from a non-null data value to null upon match, and
	* vice-versa for request nodes, CASing from null to a data
	* value. (Note that the linearization properties of this style of
	* queue are easy to verify -- elements are made available by
	* linking, and unavailable by matching.) Compared to plain M&S
	* queues, this property of dual queues requires one additional
	* successful atomic operation per enq/deq pair. But it also
	* enables lower cost variants of queue maintenance mechanics. (A
	* variation of this idea applies even for non-dual queues that
	* support deletion of interior elements, such as
	* j.u.c.ConcurrentLinkedQueue.)
	*
	* Once a node is matched, its match status can never again
	* change. We may thus arrange that the linked list of them
	* contain a prefix of zero or more matched nodes, followed by a
	* suffix of zero or more unmatched nodes. (Note that we allow
	* both the prefix and suffix to be zero length, which in turn
	* means that we do not use a dummy header.) If we were not
	* concerned with either time or space efficiency, we could
	* correctly perform enqueue and dequeue operations by traversing
	* from a pointer to the initial node; CASing the item of the
	* first unmatched node on match and CASing the next field of the
	* trailing node on appends. (Plus some special-casing when
	* initially empty). While this would be a terrible idea in
	* itself, it does have the benefit of not requiring ANY atomic
	* updates on head/tail fields.
	*
	* We introduce here an approach that lies between the extremes of
	* never versus always updating queue (head and tail) pointers.
	* This offers a tradeoff between sometimes requiring extra
	* traversal steps to locate the first and/or last unmatched
	* nodes, versus the reduced overhead and contention of fewer
	* updates to queue pointers. For example, a possible snapshot of
	* a queue is:
	*
	* head tail
	* \| \|
	* v v
	* M -> M -> U -> U -> U -> U
	*
	* The best value for this "slack" (the targeted maximum distance
	* between the value of "head" and the first unmatched node, and
	* similarly for "tail") is an empirical matter. We have found
	* that using very small constants in the range of 1-3 work best
	* over a range of platforms. Larger values introduce increasing
	* costs of cache misses and risks of long traversal chains, while
	* smaller values increase CAS contention and overhead.
	*
	* Dual queues with slack differ from plain M&S dual queues by
	* virtue of only sometimes updating head or tail pointers when
	* matching, appending, or even traversing nodes; in order to
	* maintain a targeted slack. The idea of "sometimes" may be
	* operationalized in several ways. The simplest is to use a
	* per-operation counter incremented on each traversal step, and
	* to try (via CAS) to update the associated queue pointer
	* whenever the count exceeds a threshold. Another, that requires
	* more overhead, is to use random number generators to update
	* with a given probability per traversal step.
	*
	* In any strategy along these lines, because CASes updating
	* fields may fail, the actual slack may exceed targeted
	* slack. However, they may be retried at any time to maintain
	* targets. Even when using very small slack values, this
	* approach works well for dual queues because it allows all
	* operations up to the point of matching or appending an item
	* (hence potentially allowing progress by another thread) to be
	* read-only, thus not introducing any further contention. As
	* described below, we implement this by performing slack
	* maintenance retries only after these points.
	*
	* As an accompaniment to such techniques, traversal overhead can
	* be further reduced without increasing contention of head
	* pointer updates: Threads may sometimes shortcut the "next" link
	* path from the current "head" node to be closer to the currently
	* known first unmatched node, and similarly for tail. Again, this
	* may be triggered with using thresholds or randomization.
	*
	* These ideas must be further extended to avoid unbounded amounts
	* of costly-to-reclaim garbage caused by the sequential "next"
	* links of nodes starting at old forgotten head nodes: As first
	* described in detail by Boehm
	* (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
	* delays noticing that any arbitrarily old node has become
	* garbage, all newer dead nodes will also be unreclaimed.
	* (Similar issues arise in non-GC environments.) To cope with
	* this in our implementation, upon CASing to advance the head
	* pointer, we set the "next" link of the previous head to point
	* only to itself; thus limiting the length of connected dead lists.
	* (We also take similar care to wipe out possibly garbage
	* retaining values held in other Node fields.) However, doing so
	* adds some further complexity to traversal: If any "next"
	* pointer links to itself, it indicates that the current thread
	* has lagged behind a head-update, and so the traversal must
	* continue from the "head". Traversals trying to find the
	* current tail starting from "tail" may also encounter
	* self-links, in which case they also continue at "head".
	*
	* It is tempting in slack-based scheme to not even use CAS for
	* updates (similarly to Ladan-Mozes & Shavit). However, this
	* cannot be done for head updates under the above link-forgetting
	* mechanics because an update may leave head at a detached node.
	* And while direct writes are possible for tail updates, they
	* increase the risk of long retraversals, and hence long garbage
	* chains, which can be much more costly than is worthwhile
	* considering that the cost difference of performing a CAS vs
	* write is smaller when they are not triggered on each operation
	* (especially considering that writes and CASes equally require
	* additional GC bookkeeping ("write barriers") that are sometimes
	* more costly than the writes themselves because of contention).
	*
	* * Overview of implementation *
	*
	* We use a threshold-based approach to updates, with a slack
	* threshold of two -- that is, we update head/tail when the
	* current pointer appears to be two or more steps away from the
	* first/last node. The slack value is hard-wired: a path greater
	* than one is naturally implemented by checking equality of
	* traversal pointers except when the list has only one element,
	* in which case we keep slack threshold at one. Avoiding tracking
	* explicit counts across method calls slightly simplifies an
	* already-messy implementation. Using randomization would
	* probably work better if there were a low-quality dirt-cheap
	* per-thread one available, but even ThreadLocalRandom is too
	* heavy for these purposes.
	*
	* With such a small slack threshold value, it is not worthwhile
	* to augment this with path short-circuiting (i.e., unsplicing
	* interior nodes) except in the case of cancellation/removal (see
	* below).
	*
	* We allow both the head and tail fields to be null before any
	* nodes are enqueued; initializing upon first append. This
	* simplifies some other logic, as well as providing more
	* efficient explicit control paths instead of letting JVMs insert
	* implicit NullPointerExceptions when they are null. While not
	* currently fully implemented, we also leave open the possibility
	* of re-nulling these fields when empty (which is complicated to
	* arrange, for little benefit.)
	*
	* All enqueue/dequeue operations are handled by the single method
	* "xfer" with parameters indicating whether to act as some form
	* of offer, put, poll, take, or transfer (each possibly with
	* timeout). The relative complexity of using one monolithic
	* method outweighs the code bulk and maintenance problems of
	* using separate methods for each case.
	*
	* Operation consists of up to three phases. The first is
	* implemented within method xfer, the second in tryAppend, and
	* the third in method awaitMatch.
	*
	* 1. Try to match an existing node
	*
	* Starting at head, skip already-matched nodes until finding
	* an unmatched node of opposite mode, if one exists, in which
	* case matching it and returning, also if necessary updating
	* head to one past the matched node (or the node itself if the
	* list has no other unmatched nodes). If the CAS misses, then
	* a loop retries advancing head by two steps until either
	* success or the slack is at most two. By requiring that each
	* attempt advances head by two (if applicable), we ensure that
	* the slack does not grow without bound. Traversals also check
	* if the initial head is now off-list, in which case they
	* start at the new head.
	*
	* If no candidates are found and the call was untimed
	* poll/offer, (argument "how" is NOW) return.
	*
	* 2. Try to append a new node (method tryAppend)
	*
	* Starting at current tail pointer, find the actual last node
	* and try to append a new node (or if head was null, establish
	* the first node). Nodes can be appended only if their
	* predecessors are either already matched or are of the same
	* mode. If we detect otherwise, then a new node with opposite
	* mode must have been appended during traversal, so we must
	* restart at phase 1. The traversal and update steps are
	* otherwise similar to phase 1: Retrying upon CAS misses and
	* checking for staleness. In particular, if a self-link is
	* encountered, then we can safely jump to a node on the list
	* by continuing the traversal at current head.
	*
	* On successful append, if the call was ASYNC, return.
	*
	* 3. Await match or cancellation (method awaitMatch)
	*
	* Wait for another thread to match node; instead cancelling if
	* the current thread was interrupted or the wait timed out. On
	* multiprocessors, we use front-of-queue spinning: If a node
	* appears to be the first unmatched node in the queue, it
	* spins a bit before blocking. In either case, before blocking
	* it tries to unsplice any nodes between the current "head"
	* and the first unmatched node.
	*
	* Front-of-queue spinning vastly improves performance of
	* heavily contended queues. And so long as it is relatively
	* brief and "quiet", spinning does not much impact performance
	* of less-contended queues. During spins threads check their
	* interrupt status and generate a thread-local random number
	* to decide to occasionally perform a Thread.yield. While
	* yield has underdefined specs, we assume that it might help,
	* and will not hurt, in limiting impact of spinning on busy
	* systems. We also use smaller (1/2) spins for nodes that are
	* not known to be front but whose predecessors have not
	* blocked -- these "chained" spins avoid artifacts of
	* front-of-queue rules which otherwise lead to alternating
	* nodes spinning vs blocking. Further, front threads that
	* represent phase changes (from data to request node or vice
	* versa) compared to their predecessors receive additional
	* chained spins, reflecting longer paths typically required to
	* unblock threads during phase changes.
	*
	*
	* Unlinking removed interior nodes
	*
	* In addition to minimizing garbage retention via self-linking
	* described above, we also unlink removed interior nodes. These
	* may arise due to timed out or interrupted waits, or calls to
	* remove(x) or Iterator.remove. Normally, given a node that was
	* at one time known to be the predecessor of some node s that is
	* to be removed, we can unsplice s by CASing the next field of
	* its predecessor if it still points to s (otherwise s must
	* already have been removed or is now offlist). But there are two
	* situations in which we cannot guarantee to make node s
	* unreachable in this way: (1) If s is the trailing node of list
	* (i.e., with null next), then it is pinned as the target node
	* for appends, so can only be removed later after other nodes are
	* appended. (2) We cannot necessarily unlink s given a
	* predecessor node that is matched (including the case of being
	* cancelled): the predecessor may already be unspliced, in which
	* case some previous reachable node may still point to s.
	* (For further explanation see Herlihy & Shavit "The Art of
	* Multiprocessor Programming" chapter 9). Although, in both
	* cases, we can rule out the need for further action if either s
	* or its predecessor are (or can be made to be) at, or fall off
	* from, the head of list.
	*
	* Without taking these into account, it would be possible for an
	* unbounded number of supposedly removed nodes to remain
	* reachable. Situations leading to such buildup are uncommon but
	* can occur in practice; for example when a series of short timed
	* calls to poll repeatedly time out but never otherwise fall off
	* the list because of an untimed call to take at the front of the
	* queue.
	*
	* When these cases arise, rather than always retraversing the
	* entire list to find an actual predecessor to unlink (which
	* won't help for case (1) anyway), we record a conservative
	* estimate of possible unsplice failures (in "sweepVotes").
	* We trigger a full sweep when the estimate exceeds a threshold
	* ("SWEEP_THRESHOLD") indicating the maximum number of estimated
	* removal failures to tolerate before sweeping through, unlinking
	* cancelled nodes that were not unlinked upon initial removal.
	* We perform sweeps by the thread hitting threshold (rather than
	* background threads or by spreading work to other threads)
	* because in the main contexts in which removal occurs, the
	* caller is already timed-out, cancelled, or performing a
	* potentially O(n) operation (e.g. remove(x)), none of which are
	* time-critical enough to warrant the overhead that alternatives
	* would impose on other threads.
	*
	* Because the sweepVotes estimate is conservative, and because
	* nodes become unlinked "naturally" as they fall off the head of
	* the queue, and because we allow votes to accumulate even while
	* sweeps are in progress, there are typically significantly fewer
	* such nodes than estimated. Choice of a threshold value
	* balances the likelihood of wasted effort and contention, versus
	* providing a worst-case bound on retention of interior nodes in
	* quiescent queues. The value defined below was chosen
	* empirically to balance these under various timeout scenarios.
	*
	* Note that we cannot self-link unlinked interior nodes during
	* sweeps. However, the associated garbage chains terminate when
	* some successor ultimately falls off the head of the list and is
	* self-linked.
	*/

	/** True if on multiprocessor */
	private static final boolean MP =
	Runtime.getRuntime().availableProcessors() > 1;

	/**
	* The number of times to spin (with randomly interspersed calls
	* to Thread.yield) on multiprocessor before blocking when a node
	* is apparently the first waiter in the queue. See above for
	* explanation. Must be a power of two. The value is empirically
	* derived -- it works pretty well across a variety of processors,
	* numbers of CPUs, and OSes.
	*/
	private static final int FRONT_SPINS = 1 << 7;

	/**
	* The number of times to spin before blocking when a node is
	* preceded by another node that is apparently spinning. Also
	* serves as an increment to FRONT_SPINS on phase changes, and as
	* base average frequency for yielding during spins. Must be a
	* power of two.
	*/
	private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;

	/**
	* The maximum number of estimated removal failures (sweepVotes)
	* to tolerate before sweeping through the queue unlinking
	* cancelled nodes that were not unlinked upon initial
	* removal. See above for explanation. The value must be at least
	* two to avoid useless sweeps when removing trailing nodes.
	*/
	static final int SWEEP_THRESHOLD = 32;

	/**
	* Queue nodes. Uses Object, not E, for items to allow forgetting
	* them after use. Relies heavily on Unsafe mechanics to minimize
	* unnecessary ordering constraints: Writes that are intrinsically
	* ordered wrt other accesses or CASes use simple relaxed forms.
	*/
	static final class Node {
	final boolean isData; // false if this is a request node
	volatile Object item; // initially non-null if isData; CASed to match
	volatile Node next;
	volatile Thread waiter; // null until waiting

	// CAS methods for fields
	final boolean casNext(Node cmp, Node val) {
	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
	}

	final boolean casItem(Object cmp, Object val) {
	// assert cmp == null \|\| cmp.getClass() != Node.class;
	return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
	}

	/**
	* Constructs a new node. Uses relaxed write because item can
	* only be seen after publication via casNext.
	*/
	Node(Object item, boolean isData) {
	UNSAFE.putObject(this, itemOffset, item); // relaxed write
	this.isData = isData;
	}

	/**
	* Links node to itself to avoid garbage retention. Called
	* only after CASing head field, so uses relaxed write.
	*/
	final void forgetNext() {
	UNSAFE.putObject(this, nextOffset, this);
	}

	/**
	* Sets item to self and waiter to null, to avoid garbage
	* retention after matching or cancelling. Uses relaxed writes
	* because order is already constrained in the only calling
	* contexts: item is forgotten only after volatile/atomic
	* mechanics that extract items. Similarly, clearing waiter
	* follows either CAS or return from park (if ever parked;
	* else we don't care).
	*/
	final void forgetContents() {
	UNSAFE.putObject(this, itemOffset, this);
	UNSAFE.putObject(this, waiterOffset, null);
	}

	/**
	* Returns true if this node has been matched, including the
	* case of artificial matches due to cancellation.
	*/
	final boolean isMatched() {
	Object x = item;
	return (x == this) \|\| ((x == null) == isData);
	}

	/**
	* Returns true if this is an unmatched request node.
	*/
	final boolean isUnmatchedRequest() {
	return !isData && item == null;
	}

	/**
	* Returns true if a node with the given mode cannot be
	* appended to this node because this node is unmatched and
	* has opposite data mode.
	*/
	final boolean cannotPrecede(boolean haveData) {
	boolean d = isData;
	Object x;
	return d != haveData && (x = item) != this && (x != null) == d;
	}

	/**
	* Tries to artificially match a data node -- used by remove.
	*/
	final boolean tryMatchData() {
	// assert isData;
	Object x = item;
	if (x != null && x != this && casItem(x, null)) {
	LockSupport.unpark(waiter);
	return true;
	}
	return false;
	}

	private static final long serialVersionUID = -3375979862319811754L;

	// Unsafe mechanics
	private static final sun.misc.Unsafe UNSAFE;
	private static final long itemOffset;
	private static final long nextOffset;
	private static final long waiterOffset;
	static {
	try {
	UNSAFE = sun.misc.Unsafe.getUnsafe();
	Class<?> k = Node.class;
	itemOffset = UNSAFE.objectFieldOffset
	(k.getDeclaredField("item"));
	nextOffset = UNSAFE.objectFieldOffset
	(k.getDeclaredField("next"));
	waiterOffset = UNSAFE.objectFieldOffset
	(k.getDeclaredField("waiter"));
	} catch (Exception e) {
	throw new Error(e);
	}
	}
	}

	/** head of the queue; null until first enqueue */
	transient volatile Node head;

	/** tail of the queue; null until first append */
	private transient volatile Node tail;

	/** The number of apparent failures to unsplice removed nodes */
	private transient volatile int sweepVotes;

	// CAS methods for fields
	private boolean casTail(Node cmp, Node val) {
	return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
	}

	private boolean casHead(Node cmp, Node val) {
	return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
	}

	private boolean casSweepVotes(int cmp, int val) {
	return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
	}

	/*
	* Possible values for "how" argument in xfer method.
	*/
	private static final int NOW = 0; // for untimed poll, tryTransfer
	private static final int ASYNC = 1; // for offer, put, add
	private static final int SYNC = 2; // for transfer, take
	private static final int TIMED = 3; // for timed poll, tryTransfer

	@SuppressWarnings("unchecked")
	static <E> E cast(Object item) {
	// assert item == null \|\| item.getClass() != Node.class;
	return (E) item;
	}

	/**
	* Implements all queuing methods. See above for explanation.
	*
	* @param e the item or null for take
	* @param haveData true if this is a put, else a take
	* @param how NOW, ASYNC, SYNC, or TIMED
	* @param nanos timeout in nanosecs, used only if mode is TIMED
	* @return an item if matched, else e
	* @throws NullPointerException if haveData mode but e is null
	*/
	private E xfer(E e, boolean haveData, int how, long nanos) {
	if (haveData && (e == null))
	throw new NullPointerException();
	Node s = null; // the node to append, if needed

	retry:
	for (;;) { // restart on append race

	for (Node h = head, p = h; p != null;) { // find & match first node
	boolean isData = p.isData;
	Object item = p.item;
	if (item != p && (item != null) == isData) { // unmatched
	if (isData == haveData) // can't match
	break;
	if (p.casItem(item, e)) { // match
	for (Node q = p; q != h;) {
	Node n = q.next; // update by 2 unless singleton
	if (head == h && casHead(h, n == null ? q : n)) {
	h.forgetNext();
	break;
	} // advance and retry
	if ((h = head) == null \|\|
	(q = h.next) == null \|\| !q.isMatched())
	break; // unless slack < 2
	}
	LockSupport.unpark(p.waiter);
	return LinkedTransferQueue.<E>cast(item);
	}
	}
	Node n = p.next;
	p = (p != n) ? n : (h = head); // Use head if p offlist
	}

	if (how != NOW) { // No matches available
	if (s == null)
	s = new Node(e, haveData);
	Node pred = tryAppend(s, haveData);
	if (pred == null)
	continue retry; // lost race vs opposite mode
	if (how != ASYNC)
	return awaitMatch(s, pred, e, (how == TIMED), nanos);
	}
	return e; // not waiting
	}
	}

	/**
	* Tries to append node s as tail.
	*
	* @param s the node to append
	* @param haveData true if appending in data mode
	* @return null on failure due to losing race with append in
	* different mode, else s's predecessor, or s itself if no
	* predecessor
	*/
	private Node tryAppend(Node s, boolean haveData) {
	for (Node t = tail, p = t;;) { // move p to last node and append
	Node n, u; // temps for reads of next & tail
	if (p == null && (p = head) == null) {
	if (casHead(null, s))
	return s; // initialize
	}
	else if (p.cannotPrecede(haveData))
	return null; // lost race vs opposite mode
	else if ((n = p.next) != null) // not last; keep traversing
	p = p != t && t != (u = tail) ? (t = u) : // stale tail
	(p != n) ? n : null; // restart if off list
	else if (!p.casNext(null, s))
	p = p.next; // re-read on CAS failure
	else {
	if (p != t) { // update if slack now >= 2
	while ((tail != t \|\| !casTail(t, s)) &&
	(t = tail) != null &&
	(s = t.next) != null && // advance and retry
	(s = s.next) != null && s != t);
	}
	return p;
	}
	}
	}

	/**
	* Spins/yields/blocks until node s is matched or caller gives up.
	*
	* @param s the waiting node
	* @param pred the predecessor of s, or s itself if it has no
	* predecessor, or null if unknown (the null case does not occur
	* in any current calls but may in possible future extensions)
	* @param e the comparison value for checking match
	* @param timed if true, wait only until timeout elapses
	* @param nanos timeout in nanosecs, used only if timed is true
	* @return matched item, or e if unmatched on interrupt or timeout
	*/
	private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
	final long deadline = timed ? System.nanoTime() + nanos : 0L;
	Thread w = Thread.currentThread();
	int spins = -1; // initialized after first item and cancel checks
	ThreadLocalRandom randomYields = null; // bound if needed

	for (;;) {
	Object item = s.item;
	if (item != e) { // matched
	// assert item != s;
	s.forgetContents(); // avoid garbage
	return LinkedTransferQueue.<E>cast(item);
	}
	if ((w.isInterrupted() \|\| (timed && nanos <= 0)) &&
	s.casItem(e, s)) { // cancel
	unsplice(pred, s);
	return e;
	}

	if (spins < 0) { // establish spins at/near front
	if ((spins = spinsFor(pred, s.isData)) > 0)
	randomYields = ThreadLocalRandom.current();
	}
	else if (spins > 0) { // spin
	--spins;
	if (randomYields.nextInt(CHAINED_SPINS) == 0)
	Thread.yield(); // occasionally yield
	}
	else if (s.waiter == null) {
	s.waiter = w; // request unpark then recheck
	}
	else if (timed) {
	nanos = deadline - System.nanoTime();
	if (nanos > 0L)
	LockSupport.parkNanos(this, nanos);
	}
	else {
	LockSupport.park(this);
	}
	}
	}

	/**
	* Returns spin/yield value for a node with given predecessor and
	* data mode. See above for explanation.
	*/
	private static int spinsFor(Node pred, boolean haveData) {
	if (MP && pred != null) {
	if (pred.isData != haveData) // phase change
	return FRONT_SPINS + CHAINED_SPINS;
	if (pred.isMatched()) // probably at front
	return FRONT_SPINS;
	if (pred.waiter == null) // pred apparently spinning
	return CHAINED_SPINS;
	}
	return 0;
	}

	/* -------------- Traversal methods -------------- */

	/**
	* Returns the successor of p, or the head node if p.next has been
	* linked to self, which will only be true if traversing with a
	* stale pointer that is now off the list.
	*/
	final Node succ(Node p) {
	Node next = p.next;
	return (p == next) ? head : next;
	}

	/**
	* Returns the first unmatched node of the given mode, or null if
	* none. Used by methods isEmpty, hasWaitingConsumer.
	*/
	private Node firstOfMode(boolean isData) {
	for (Node p = head; p != null; p = succ(p)) {
	if (!p.isMatched())
	return (p.isData == isData) ? p : null;
	}
	return null;
	}

	/**
	* Version of firstOfMode used by Spliterator. Callers must
	* recheck if the returned node's item field is null or
	* self-linked before using.
	*/
	final Node firstDataNode() {
	for (Node p = head; p != null;) {
	Object item = p.item;
	if (p.isData) {
	if (item != null && item != p)
	return p;
	}
	else if (item == null)
	break;
	if (p == (p = p.next))
	p = head;
	}
	return null;
	}

	/**
	* Returns the item in the first unmatched node with isData; or
	* null if none. Used by peek.
	*/
	private E firstDataItem() {
	for (Node p = head; p != null; p = succ(p)) {
	Object item = p.item;
	if (p.isData) {
	if (item != null && item != p)
	return LinkedTransferQueue.<E>cast(item);
	}
	else if (item == null)
	return null;
	}
	return null;
	}

	/**
	* Traverses and counts unmatched nodes of the given mode.
	* Used by methods size and getWaitingConsumerCount.
	*/
	private int countOfMode(boolean data) {
	int count = 0;
	for (Node p = head; p != null; ) {
	if (!p.isMatched()) {
	if (p.isData != data)
	return 0;
	if (++count == Integer.MAX_VALUE) // saturated
	break;
	}
	Node n = p.next;
	if (n != p)
	p = n;
	else {
	count = 0;
	p = head;
	}
	}
	return count;
	}

	final class Itr implements Iterator<E> {
	private Node nextNode; // next node to return item for
	private E nextItem; // the corresponding item
	private Node lastRet; // last returned node, to support remove
	private Node lastPred; // predecessor to unlink lastRet

	/**
	* Moves to next node after prev, or first node if prev null.
	*/
	private void advance(Node prev) {
	/*
	* To track and avoid buildup of deleted nodes in the face
	* of calls to both Queue.remove and Itr.remove, we must
	* include variants of unsplice and sweep upon each
	* advance: Upon Itr.remove, we may need to catch up links
	* from lastPred, and upon other removes, we might need to
	* skip ahead from stale nodes and unsplice deleted ones
	* found while advancing.
	*/

	Node r, b; // reset lastPred upon possible deletion of lastRet
	if ((r = lastRet) != null && !r.isMatched())
	lastPred = r; // next lastPred is old lastRet
	else if ((b = lastPred) == null \|\| b.isMatched())
	lastPred = null; // at start of list
	else {
	Node s, n; // help with removal of lastPred.next
	while ((s = b.next) != null &&
	s != b && s.isMatched() &&
	(n = s.next) != null && n != s)
	b.casNext(s, n);
	}

	this.lastRet = prev;

	for (Node p = prev, s, n;;) {
	s = (p == null) ? head : p.next;
	if (s == null)
	break;
	else if (s == p) {
	p = null;
	continue;
	}
	Object item = s.item;
	if (s.isData) {
	if (item != null && item != s) {
	nextItem = LinkedTransferQueue.<E>cast(item);
	nextNode = s;
	return;
	}
	}
	else if (item == null)
	break;
	// assert s.isMatched();
	if (p == null)
	p = s;
	else if ((n = s.next) == null)
	break;
	else if (s == n)
	p = null;
	else
	p.casNext(s, n);
	}
	nextNode = null;
	nextItem = null;
	}

	Itr() {
	advance(null);
	}

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

	public final E next() {
	Node p = nextNode;
	if (p == null) throw new NoSuchElementException();
	E e = nextItem;
	advance(p);
	return e;
	}

	public final void remove() {
	final Node lastRet = this.lastRet;
	if (lastRet == null)
	throw new IllegalStateException();
	this.lastRet = null;
	if (lastRet.tryMatchData())
	unsplice(lastPred, lastRet);
	}
	}

	/** A customized variant of Spliterators.IteratorSpliterator */
	static final class LTQSpliterator<E> implements Spliterator<E> {
	static final int MAX_BATCH = 1 << 25; // max batch array size;
	final LinkedTransferQueue<E> queue;
	Node current; // current node; null until initialized
	int batch; // batch size for splits
	boolean exhausted; // true when no more nodes
	LTQSpliterator(LinkedTransferQueue<E> queue) {
	this.queue = queue;
	}

	public Spliterator<E> trySplit() {
	Node p;
	final LinkedTransferQueue<E> q = this.queue;
	int b = batch;
	int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
	if (!exhausted &&
	((p = current) != null \|\| (p = q.firstDataNode()) != null) &&
	p.next != null) {
	Object[] a = new Object[n];
	int i = 0;
	do {
	Object e = p.item;
	if (e != p && (a[i] = e) != null)
	++i;
	if (p == (p = p.next))
	p = q.firstDataNode();
	} while (p != null && i < n && p.isData);
	if ((current = p) == null)
	exhausted = true;
	if (i > 0) {
	batch = i;
	return Spliterators.spliterator
	(a, 0, i, Spliterator.ORDERED \| Spliterator.NONNULL \|
	Spliterator.CONCURRENT);
	}
	}
	return null;
	}

	@SuppressWarnings("unchecked")
	public void forEachRemaining(Consumer<? super E> action) {
	Node p;
	if (action == null) throw new NullPointerException();
	final LinkedTransferQueue<E> q = this.queue;
	if (!exhausted &&
	((p = current) != null \|\| (p = q.firstDataNode()) != null)) {
	exhausted = true;
	do {
	Object e = p.item;
	if (e != null && e != p)
	action.accept((E)e);
	if (p == (p = p.next))
	p = q.firstDataNode();
	} while (p != null && p.isData);
	}
	}

	@SuppressWarnings("unchecked")
	public boolean tryAdvance(Consumer<? super E> action) {
	Node p;
	if (action == null) throw new NullPointerException();
	final LinkedTransferQueue<E> q = this.queue;
	if (!exhausted &&
	((p = current) != null \|\| (p = q.firstDataNode()) != null)) {
	Object e;
	do {
	if ((e = p.item) == p)
	e = null;
	if (p == (p = p.next))
	p = q.firstDataNode();
	} while (e == null && p != null && p.isData);
	if ((current = p) == null)
	exhausted = true;
	if (e != null) {
	action.accept((E)e);
	return true;
	}
	}
	return false;
	}

	public long estimateSize() { return Long.MAX_VALUE; }

	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 LTQSpliterator<E>(this);
	}

	/* -------------- Removal methods -------------- */

	/**
	* Unsplices (now or later) the given deleted/cancelled node with
	* the given predecessor.
	*
	* @param pred a node that was at one time known to be the
	* predecessor of s, or null or s itself if s is/was at head
	* @param s the node to be unspliced
	*/
	final void unsplice(Node pred, Node s) {
	s.forgetContents(); // forget unneeded fields
	/*
	* See above for rationale. Briefly: if pred still points to
	* s, try to unlink s. If s cannot be unlinked, because it is
	* trailing node or pred might be unlinked, and neither pred
	* nor s are head or offlist, add to sweepVotes, and if enough
	* votes have accumulated, sweep.
	*/
	if (pred != null && pred != s && pred.next == s) {
	Node n = s.next;
	if (n == null \|\|
	(n != s && pred.casNext(s, n) && pred.isMatched())) {
	for (;;) { // check if at, or could be, head
	Node h = head;
	if (h == pred \|\| h == s \|\| h == null)
	return; // at head or list empty
	if (!h.isMatched())
	break;
	Node hn = h.next;
	if (hn == null)
	return; // now empty
	if (hn != h && casHead(h, hn))
	h.forgetNext(); // advance head
	}
	if (pred.next != pred && s.next != s) { // recheck if offlist
	for (;;) { // sweep now if enough votes
	int v = sweepVotes;
	if (v < SWEEP_THRESHOLD) {
	if (casSweepVotes(v, v + 1))
	break;
	}
	else if (casSweepVotes(v, 0)) {
	sweep();
	break;
	}
	}
	}
	}
	}
	}

	/**
	* Unlinks matched (typically cancelled) nodes encountered in a
	* traversal from head.
	*/
	private void sweep() {
	for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
	if (!s.isMatched())
	// Unmatched nodes are never self-linked
	p = s;
	else if ((n = s.next) == null) // trailing node is pinned
	break;
	else if (s == n) // stale
	// No need to also check for p == s, since that implies s == n
	p = head;
	else
	p.casNext(s, n);
	}
	}

	/**
	* Main implementation of remove(Object)
	*/
	private boolean findAndRemove(Object e) {
	if (e != null) {
	for (Node pred = null, p = head; p != null; ) {
	Object item = p.item;
	if (p.isData) {
	if (item != null && item != p && e.equals(item) &&
	p.tryMatchData()) {
	unsplice(pred, p);
	return true;
	}
	}
	else if (item == null)
	break;
	pred = p;
	if ((p = p.next) == pred) { // stale
	pred = null;
	p = head;
	}
	}
	}
	return false;
	}

	/**
	* Creates an initially empty {@code LinkedTransferQueue}.
	*/
	public LinkedTransferQueue() {
	}

	/**
	* Creates a {@code LinkedTransferQueue}
	* 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 LinkedTransferQueue(Collection<? extends E> c) {
	this();
	addAll(c);
	}

	/**
	* Inserts the specified element at the tail of this queue.
	* As the queue is unbounded, this method will never block.
	*
	* @throws NullPointerException if the specified element is null
	*/
	public void put(E e) {
	xfer(e, true, ASYNC, 0);
	}

	/**
	* Inserts the specified element at the tail of this queue.
	* As the queue is unbounded, this method will never block or
	* return {@code false}.
	*
	* @return {@code true} (as specified by
	* {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit)
	* BlockingQueue.offer})
	* @throws NullPointerException if the specified element is null
	*/
	public boolean offer(E e, long timeout, TimeUnit unit) {
	xfer(e, true, ASYNC, 0);
	return true;
	}

	/**
	* Inserts the specified element at the tail of this queue.
	* As the queue is unbounded, this method will never return {@code false}.
	*
	* @return {@code true} (as specified by {@link Queue#offer})
	* @throws NullPointerException if the specified element is null
	*/
	public boolean offer(E e) {
	xfer(e, true, ASYNC, 0);
	return true;
	}

	/**
	* Inserts the specified element at the tail of this queue.
	* As the queue is unbounded, this method will never throw
	* {@link IllegalStateException} or return {@code false}.
	*
	* @return {@code true} (as specified by {@link Collection#add})
	* @throws NullPointerException if the specified element is null
	*/
	public boolean add(E e) {
	xfer(e, true, ASYNC, 0);
	return true;
	}

	/**
	* Transfers the element to a waiting consumer immediately, if possible.
	*
	* <p>More precisely, transfers the specified element immediately
	* if there exists a consumer already waiting to receive it (in
	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
	* otherwise returning {@code false} without enqueuing the element.
	*
	* @throws NullPointerException if the specified element is null
	*/
	public boolean tryTransfer(E e) {
	return xfer(e, true, NOW, 0) == null;
	}

	/**
	* Transfers the element to a consumer, waiting if necessary to do so.
	*
	* <p>More precisely, transfers the specified element immediately
	* if there exists a consumer already waiting to receive it (in
	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
	* else inserts the specified element at the tail of this queue
	* and waits until the element is received by a consumer.
	*
	* @throws NullPointerException if the specified element is null
	*/
	public void transfer(E e) throws InterruptedException {
	if (xfer(e, true, SYNC, 0) != null) {
	Thread.interrupted(); // failure possible only due to interrupt
	throw new InterruptedException();
	}
	}

	/**
	* Transfers the element to a consumer if it is possible to do so
	* before the timeout elapses.
	*
	* <p>More precisely, transfers the specified element immediately
	* if there exists a consumer already waiting to receive it (in
	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
	* else inserts the specified element at the tail of this queue
	* and waits until the element is received by a consumer,
	* returning {@code false} if the specified wait time elapses
	* before the element can be transferred.
	*
	* @throws NullPointerException if the specified element is null
	*/
	public boolean tryTransfer(E e, long timeout, TimeUnit unit)
	throws InterruptedException {
	if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
	return true;
	if (!Thread.interrupted())
	return false;
	throw new InterruptedException();
	}

	public E take() throws InterruptedException {
	E e = xfer(null, false, SYNC, 0);
	if (e != null)
	return e;
	Thread.interrupted();
	throw new InterruptedException();
	}

	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
	E e = xfer(null, false, TIMED, unit.toNanos(timeout));
	if (e != null \|\| !Thread.interrupted())
	return e;
	throw new InterruptedException();
	}

	public E poll() {
	return xfer(null, false, NOW, 0);
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c) {
	if (c == null)
	throw new NullPointerException();
	if (c == this)
	throw new IllegalArgumentException();
	int n = 0;
	for (E e; (e = poll()) != null;) {
	c.add(e);
	++n;
	}
	return n;
	}

	/**
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c, int maxElements) {
	if (c == null)
	throw new NullPointerException();
	if (c == this)
	throw new IllegalArgumentException();
	int n = 0;
	for (E e; n < maxElements && (e = poll()) != null;) {
	c.add(e);
	++n;
	}
	return n;
	}

	/**
	* 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();
	}

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

	/**
	* Returns {@code true} if this queue contains no elements.
	*
	* @return {@code true} if this queue contains no elements
	*/
	public boolean isEmpty() {
	for (Node p = head; p != null; p = succ(p)) {
	if (!p.isMatched())
	return !p.isData;
	}
	return true;
	}

	public boolean hasWaitingConsumer() {
	return firstOfMode(false) != null;
	}

	/**
	* Returns the number of elements in this queue. If this queue
	* contains more than {@code Integer.MAX_VALUE} elements, returns
	* {@code Integer.MAX_VALUE}.
	*
	* <p>Beware that, unlike in most collections, this method is
	* <em>NOT</em> a constant-time operation. Because of the
	* asynchronous nature of these queues, determining the current
	* number of elements requires an O(n) traversal.
	*
	* @return the number of elements in this queue
	*/
	public int size() {
	return countOfMode(true);
	}

	public int getWaitingConsumerCount() {
	return countOfMode(false);
	}

	/**
	* 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) {
	return findAndRemove(o);
	}

	/**
	* 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;
	for (Node p = head; p != null; p = succ(p)) {
	Object item = p.item;
	if (p.isData) {
	if (item != null && item != p && o.equals(item))
	return true;
	}
	else if (item == null)
	break;
	}
	return false;
	}

	/**
	* Always returns {@code Integer.MAX_VALUE} because a
	* {@code LinkedTransferQueue} is not capacity constrained.
	*
	* @return {@code Integer.MAX_VALUE} (as specified by
	* {@link java.util.concurrent.BlockingQueue#remainingCapacity()
	* BlockingQueue.remainingCapacity})
	*/
	public int remainingCapacity() {
	return Integer.MAX_VALUE;
	}

	/**
	* 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 All of the elements (each an {@code E}) in
	* the proper order, followed by a null
	*/
	private void writeObject(java.io.ObjectOutputStream s)
	throws java.io.IOException {
	s.defaultWriteObject();
	for (E e : this)
	s.writeObject(e);
	// Use trailing null as sentinel
	s.writeObject(null);
	}

	/**
	* 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 {
	s.defaultReadObject();
	for (;;) {
	@SuppressWarnings("unchecked")
	E item = (E) s.readObject();
	if (item == null)
	break;
	else
	offer(item);
	}
	}

	// Unsafe mechanics

	private static final sun.misc.Unsafe UNSAFE;
	private static final long headOffset;
	private static final long tailOffset;
	private static final long sweepVotesOffset;
	static {
	try {
	UNSAFE = sun.misc.Unsafe.getUnsafe();
	Class<?> k = LinkedTransferQueue.class;
	headOffset = UNSAFE.objectFieldOffset
	(k.getDeclaredField("head"));
	tailOffset = UNSAFE.objectFieldOffset
	(k.getDeclaredField("tail"));
	sweepVotesOffset = UNSAFE.objectFieldOffset
	(k.getDeclaredField("sweepVotes"));
	} catch (Exception e) {
	throw new Error(e);
	}
	}
	}

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