/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * 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.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.AbstractQueue;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Queue;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.locks.LockSupport;
import java.util.function.Consumer;
import java.util.function.Predicate;

/**
 * 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.
 *
 * <p>Bulk operations that add, remove, or examine multiple elements,
 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
 * are <em>not</em> guaranteed to be performed atomically.
 * For example, a {@code forEach} traversal concurrent with an {@code
 * addAll} operation might observe 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @since 1.7
 * @author Doug Lea
 * @param <E> the type of elements held in this queue
 */
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.rochester.edu/~scott/papers/2004_DISC_dual_DS.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/~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. 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 chains of dead nodes.
 * (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).
 *
 * 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 two phases. The first is implemented
 * in method xfer, the second in method awaitMatch.
 *
 * 1. Traverse until matching or appending (method xfer)
 *
 * Conceptually, we simply traverse all nodes starting from head.
 * If we encounter an unmatched node of opposite mode, we match
 * it and return, also updating head (by at least 2 hops) to
 * one past the matched node (or the node itself if it's the
 * pinned trailing node). Traversals also check for the
 * possibility of falling off-list, in which case they restart.
 *
 * If the trailing node of the list is reached, a match is not
 * possible. If this call was untimed poll or tryTransfer
 * (argument "how" is NOW), return empty-handed immediately.
 * Else a new node is CAS-appended. On successful append, if
 * this call was ASYNC (e.g. offer), an element was
 * successfully added to the end of the queue and we return.
 *
 * Of course, this naive traversal is O(n) when no match is
 * possible. We optimize the traversal by maintaining a tail
 * pointer, which is expected to be "near" the end of the list.
 * It is only safe to fast-forward to tail (in the presence of
 * arbitrary concurrent changes) if it is pointing to a node of
 * the same mode, even if it is dead (in this case no preceding
 * node could still be matchable by this traversal). If we
 * need to restart due to falling off-list, we can again
 * fast-forward to tail, but only if it has changed since the
 * last traversal (else we might loop forever). If tail cannot
 * be used, traversal starts at head (but in this case we
 * expect to be able to match near head). As with head, we
 * CAS-advance the tail pointer by at least two hops.
 *
 * 2. 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 at the trailing node but otherwise
 * never 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 timed-out or cancelled, which are not 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.
 *
 * Because traversal operations on the linked list of nodes are a
 * natural opportunity to sweep dead nodes, we generally do so,
 * including all the operations that might remove elements as they
 * traverse, such as removeIf and Iterator.remove. This largely
 * eliminates long chains of dead interior nodes, except from
 * cancelled or timed out blocking operations.
 *
 * 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. 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 when not waiting for a match

 /**
 * Constructs a data node holding item if item is non-null,
 * else a request node. Uses relaxed write because item can
 * only be seen after piggy-backing publication via CAS.
 */
 Node(Object item) {
 ITEM.set(this, item);
 isData = (item != null);
 }

 /** Constructs a (matched data) dummy node. */
 Node() {
 isData = true;
 }

 final boolean casNext(Node cmp, Node val) {
 // assert val != null;
 return NEXT.compareAndSet(this, cmp, val);
 }

 final boolean casItem(Object cmp, Object val) {
 // assert isData == (cmp != null);
 // assert isData == (val == null);
 // assert !(cmp instanceof Node);
 return ITEM.compareAndSet(this, cmp, val);
 }

 /**
 * Links node to itself to avoid garbage retention. Called
 * only after CASing head field, so uses relaxed write.
 */
 final void selfLink() {
 // assert isMatched();
 NEXT.setRelease(this, this);
 }

 final void appendRelaxed(Node next) {
 // assert next != null;
 // assert this.next == null;
 NEXT.set(this, next);
 }

 /**
 * Sets item (of a request node) 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, and visitors
 * of request nodes only ever check whether item is null.
 * Similarly, clearing waiter follows either CAS or return
 * from park (if ever parked; else we don't care).
 */
 final void forgetContents() {
 // assert isMatched();
 if (!isData)
 ITEM.set(this, this);
 WAITER.set(this, null);
 }

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

 /** Tries to CAS-match this node; if successful, wakes waiter. */
 final boolean tryMatch(Object cmp, Object val) {
 if (casItem(cmp, val)) {
 LockSupport.unpark(waiter);
 return true;
 }
 return false;
 }

 /**
 * 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;
 return d != haveData && d != (item == null);
 }

 private static final long serialVersionUID = -3375979862319811754L;
 }

 /**
 * A node from which the first live (non-matched) node (if any)
 * can be reached in O(1) time.
 * Invariants:
 * - all live nodes are reachable from head via .next
 * - head != null
 * - (tmp = head).next != tmp || tmp != head
 * Non-invariants:
 * - head may or may not be live
 * - it is permitted for tail to lag behind head, that is, for tail
 * to not be reachable from head!
 */
 transient volatile Node head;

 /**
 * A node from which the last node on list (that is, the unique
 * node with node.next == null) can be reached in O(1) time.
 * Invariants:
 * - the last node is always reachable from tail via .next
 * - tail != null
 * Non-invariants:
 * - tail may or may not be live
 * - it is permitted for tail to lag behind head, that is, for tail
 * to not be reachable from head!
 * - tail.next may or may not be self-linked.
 */
 private transient volatile Node tail;

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

 private boolean casTail(Node cmp, Node val) {
 // assert cmp != null;
 // assert val != null;
 return TAIL.compareAndSet(this, cmp, val);
 }

 private boolean casHead(Node cmp, Node val) {
 return HEAD.compareAndSet(this, cmp, val);
 }

 /** Atomic version of ++sweepVotes. */
 private int incSweepVotes() {
 return (int) SWEEPVOTES.getAndAdd(this, 1) + 1;
 }

 /**
 * Tries to CAS pred.next (or head, if pred is null) from c to p.
 * Caller must ensure that we're not unlinking the trailing node.
 */
 private boolean tryCasSuccessor(Node pred, Node c, Node p) {
 // assert p != null;
 // assert c.isData != (c.item != null);
 // assert c != p;
 if (pred != null)
 return pred.casNext(c, p);
 if (casHead(c, p)) {
 c.selfLink();
 return true;
 }
 return false;
 }

 /**
 * Collapses dead (matched) nodes between pred and q.
 * @param pred the last known live node, or null if none
 * @param c the first dead node
 * @param p the last dead node
 * @param q p.next: the next live node, or null if at end
 * @return pred if pred still alive and CAS succeeded; else p
 */
 private Node skipDeadNodes(Node pred, Node c, Node p, Node q) {
 // assert pred != c;
 // assert p != q;
 // assert c.isMatched();
 // assert p.isMatched();
 if (q == null) {
 // Never unlink trailing node.
 if (c == p) return pred;
 q = p;
 }
 return (tryCasSuccessor(pred, c, q)
 && (pred == null || !pred.isMatched()))
 ? pred : p;
 }

 /**
 * Collapses dead (matched) nodes from h (which was once head) to p.
 * Caller ensures all nodes from h up to and including p are dead.
 */
 private void skipDeadNodesNearHead(Node h, Node p) {
 // assert h != null;
 // assert h != p;
 // assert p.isMatched();
 for (;;) {
 final Node q;
 if ((q = p.next) == null) break;
 else if (!q.isMatched()) { p = q; break; }
 else if (p == (p = q)) return;
 }
 if (casHead(h, p))
 h.selfLink();
 }

 /* 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

 /**
 * 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
 */
 @SuppressWarnings("unchecked")
 private E xfer(E e, boolean haveData, int how, long nanos) {
 if (haveData && (e == null))
 throw new NullPointerException();

 restart: for (Node s = null, t = null, h = null;;) {
 for (Node p = (t != (t = tail) && t.isData == haveData) ? t
 : (h = head);; ) {
 final Node q; final Object item;
 if (p.isData != haveData
 && haveData == ((item = p.item) == null)) {
 if (h == null) h = head;
 if (p.tryMatch(item, e)) {
 if (h != p) skipDeadNodesNearHead(h, p);
 return (E) item;
 }
 }
 if ((q = p.next) == null) {
 if (how == NOW) return e;
 if (s == null) s = new Node(e);
 if (!p.casNext(null, s)) continue;
 if (p != t) casTail(t, s);
 if (how == ASYNC) return e;
 return awaitMatch(s, p, e, (how == TIMED), nanos);
 }
 if (p == (p = q)) continue restart;
 }
 }
 }

 /**
 * Spins/yields/blocks until node s is matched or caller gives up.
 *
 * @param s the waiting node
 * @param pred the predecessor of s, 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 (;;) {
 final Object item;
 if ((item = s.item) != e) { // matched
 // assert item != s;
 s.forgetContents(); // avoid garbage
 @SuppressWarnings("unchecked") E itemE = (E) item;
 return itemE;
 }
 else if (w.isInterrupted() || (timed && nanos <= 0L)) {
 // try to cancel and unlink
 if (s.casItem(e, s.isData ? null : s)) {
 unsplice(pred, s);
 return e;
 }
 // return normally if lost CAS
 }
 else 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 first unmatched data node, or null if none.
 * Callers must recheck if the returned node is unmatched
 * before using.
 */
 final Node firstDataNode() {
 Node first = null;
 restartFromHead: for (;;) {
 Node h = head, p = h;
 while (p != null) {
 if (p.item != null) {
 if (p.isData) {
 first = p;
 break;
 }
 }
 else if (!p.isData)
 break;
 final Node q;
 if ((q = p.next) == null)
 break;
 if (p == (p = q))
 continue restartFromHead;
 }
 if (p != h && casHead(h, p))
 h.selfLink();
 return first;
 }
 }

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

 public String toString() {
 String[] a = null;
 restartFromHead: for (;;) {
 int charLength = 0;
 int size = 0;
 for (Node p = head; p != null;) {
 Object item = p.item;
 if (p.isData) {
 if (item != null) {
 if (a == null)
 a = new String[4];
 else if (size == a.length)
 a = Arrays.copyOf(a, 2 * size);
 String s = item.toString();
 a[size++] = s;
 charLength += s.length();
 }
 } else if (item == null)
 break;
 if (p == (p = p.next))
 continue restartFromHead;
 }

 if (size == 0)
 return "[]";

 return Helpers.toString(a, size, charLength);
 }
 }

 private Object[] toArrayInternal(Object[] a) {
 Object[] x = a;
 restartFromHead: for (;;) {
 int size = 0;
 for (Node p = head; p != null;) {
 Object item = p.item;
 if (p.isData) {
 if (item != null) {
 if (x == null)
 x = new Object[4];
 else if (size == x.length)
 x = Arrays.copyOf(x, 2 * (size + 4));
 x[size++] = item;
 }
 } else if (item == null)
 break;
 if (p == (p = p.next))
 continue restartFromHead;
 }
 if (x == null)
 return new Object[0];
 else if (a != null && size <= a.length) {
 if (a != x)
 System.arraycopy(x, 0, a, 0, size);
 if (size < a.length)
 a[size] = null;
 return a;
 }
 return (size == x.length) ? x : Arrays.copyOf(x, size);
 }
 }

 /**
 * 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() {
 return toArrayInternal(null);
 }

 /**
 * 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) {
 Objects.requireNonNull(a);
 return (T[]) toArrayInternal(a);
 }

 /**
 * Weakly-consistent iterator.
 *
 * Lazily updated ancestor is expected to be amortized O(1) remove(),
 * but O(n) in the worst case, when lastRet is concurrently deleted.
 */
 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 ancestor; // Helps unlink lastRet on remove()

 /**
 * Moves to next node after pred, or first node if pred null.
 */
 @SuppressWarnings("unchecked")
 private void advance(Node pred) {
 for (Node p = (pred == null) ? head : pred.next, c = p;
 p != null; ) {
 final Object item;
 if ((item = p.item) != null && p.isData) {
 nextNode = p;
 nextItem = (E) item;
 if (c != p)
 tryCasSuccessor(pred, c, p);
 return;
 }
 else if (!p.isData && item == null)
 break;
 if (c != p && !tryCasSuccessor(pred, c, c = p)) {
 pred = p;
 c = p = p.next;
 }
 else if (p == (p = p.next)) {
 pred = null;
 c = p = head;
 }
 }
 nextItem = null;
 nextNode = null;
 }

 Itr() {
 advance(null);
 }

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

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

 public void forEachRemaining(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 Node q = null;
 for (Node p; (p = nextNode) != null; advance(q = p))
 action.accept(nextItem);
 if (q != null)
 lastRet = q;
 }

 public final void remove() {
 final Node lastRet = this.lastRet;
 if (lastRet == null)
 throw new IllegalStateException();
 this.lastRet = null;
 if (lastRet.item == null) // already deleted?
 return;
 // Advance ancestor, collapsing intervening dead nodes
 Node pred = ancestor;
 for (Node p = (pred == null) ? head : pred.next, c = p, q;
 p != null; ) {
 if (p == lastRet) {
 final Object item;
 if ((item = p.item) != null)
 p.tryMatch(item, null);
 if ((q = p.next) == null) q = p;
 if (c != q) tryCasSuccessor(pred, c, q);
 ancestor = pred;
 return;
 }
 final Object item; final boolean pAlive;
 if (pAlive = ((item = p.item) != null && p.isData)) {
 // exceptionally, nothing to do
 }
 else if (!p.isData && item == null)
 break;
 if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) {
 pred = p;
 c = p = p.next;
 }
 else if (p == (p = p.next)) {
 pred = null;
 c = p = head;
 }
 }
 // traversal failed to find lastRet; must have been deleted;
 // leave ancestor at original location to avoid overshoot;
 // better luck next time!

 // assert lastRet.isMatched();
 }
 }

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

 public Spliterator<E> trySplit() {
 Node p, q;
 if ((p = current()) == null || (q = p.next) == null)
 return null;
 int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
 Object[] a = null;
 do {
 final Object item = p.item;
 if (p.isData) {
 if (item != null) {
 if (a == null)
 a = new Object[n];
 a[i++] = item;
 }
 } else if (item == null) {
 p = null;
 break;
 }
 if (p == (p = q))
 p = firstDataNode();
 } while (p != null && (q = p.next) != null && i < n);
 setCurrent(p);
 return (i == 0) ? null :
 Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED |
 Spliterator.NONNULL |
 Spliterator.CONCURRENT));
 }

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

 @SuppressWarnings("unchecked")
 public boolean tryAdvance(Consumer<? super E> action) {
 Objects.requireNonNull(action);
 Node p;
 if ((p = current()) != null) {
 E e = null;
 do {
 final Object item = p.item;
 final boolean isData = p.isData;
 if (p == (p = p.next))
 p = head;
 if (isData) {
 if (item != null) {
 e = (E) item;
 break;
 }
 }
 else if (item == null)
 p = null;
 } while (p != null);
 setCurrent(p);
 if (e != null) {
 action.accept(e);
 return true;
 }
 }
 return false;
 }

 private void setCurrent(Node p) {
 if ((current = p) == null)
 exhausted = true;
 }

 private Node current() {
 Node p;
 if ((p = current) == null && !exhausted)
 setCurrent(p = firstDataNode());
 return p;
 }

 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();
 }

 /* -------------- 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
 * @param s the node to be unspliced
 */
 final void unsplice(Node pred, Node s) {
 // assert pred != null;
 // assert pred != s;
 // assert s != null;
 // assert s.isMatched();
 // assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0;
 s.waiter = null; // disable signals
 /*
 * 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.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)
 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.selfLink(); // advance head
 }
 // sweep every SWEEP_THRESHOLD votes
 if (pred.next != pred && s.next != s // recheck if offlist
 && (incSweepVotes() & (SWEEP_THRESHOLD - 1)) == 0)
 sweep();
 }
 }
 }

 /**
 * 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);
 }
 }

 /**
 * Creates an initially empty {@code LinkedTransferQueue}.
 */
 public LinkedTransferQueue() {
 head = tail = new Node();
 }

 /**
 * 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) {
 Node h = null, t = null;
 for (E e : c) {
 Node newNode = new Node(Objects.requireNonNull(e));
 if (h == null)
 h = t = newNode;
 else
 t.appendRelaxed(t = newNode);
 }
 if (h == null)
 h = t = new Node();
 head = h;
 tail = t;
 }

 /**
 * 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 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) {
 Objects.requireNonNull(c);
 if (c == this)
 throw new IllegalArgumentException();
 int n = 0;
 for (E e; (e = poll()) != null; n++)
 c.add(e);
 return n;
 }

 /**
 * @throws NullPointerException {@inheritDoc}
 * @throws IllegalArgumentException {@inheritDoc}
 */
 public int drainTo(Collection<? super E> c, int maxElements) {
 Objects.requireNonNull(c);
 if (c == this)
 throw new IllegalArgumentException();
 int n = 0;
 for (E e; n < maxElements && (e = poll()) != null; n++)
 c.add(e);
 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() {
 restartFromHead: for (;;) {
 for (Node p = head; p != null;) {
 Object item = p.item;
 if (p.isData) {
 if (item != null) {
 @SuppressWarnings("unchecked") E e = (E) item;
 return e;
 }
 }
 else if (item == null)
 break;
 if (p == (p = p.next))
 continue restartFromHead;
 }
 return null;
 }
 }

 /**
 * Returns {@code true} if this queue contains no elements.
 *
 * @return {@code true} if this queue contains no elements
 */
 public boolean isEmpty() {
 return firstDataNode() == null;
 }

 public boolean hasWaitingConsumer() {
 restartFromHead: for (;;) {
 for (Node p = head; p != null;) {
 Object item = p.item;
 if (p.isData) {
 if (item != null)
 break;
 }
 else if (item == null)
 return true;
 if (p == (p = p.next))
 continue restartFromHead;
 }
 return false;
 }
 }

 /**
 * 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) {
 if (o == null) return false;
 restartFromHead: for (;;) {
 for (Node p = head, pred = null; p != null; ) {
 Node q = p.next;
 final Object item;
 if ((item = p.item) != null) {
 if (p.isData) {
 if (o.equals(item) && p.tryMatch(item, null)) {
 skipDeadNodes(pred, p, p, q);
 return true;
 }
 pred = p; p = q; continue;
 }
 }
 else if (!p.isData)
 break;
 for (Node c = p;; q = p.next) {
 if (q == null || !q.isMatched()) {
 pred = skipDeadNodes(pred, c, p, q); p = q; break;
 }
 if (p == (p = q)) continue restartFromHead;
 }
 }
 return false;
 }
 }

 /**
 * 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;
 restartFromHead: for (;;) {
 for (Node p = head, pred = null; p != null; ) {
 Node q = p.next;
 final Object item;
 if ((item = p.item) != null) {
 if (p.isData) {
 if (o.equals(item))
 return true;
 pred = p; p = q; continue;
 }
 }
 else if (!p.isData)
 break;
 for (Node c = p;; q = p.next) {
 if (q == null || !q.isMatched()) {
 pred = skipDeadNodes(pred, c, p, q); p = q; break;
 }
 if (p == (p = q)) continue restartFromHead;
 }
 }
 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 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 {

 // Read in elements until trailing null sentinel found
 Node h = null, t = null;
 for (Object item; (item = s.readObject()) != null; ) {
 Node newNode = new Node(item);
 if (h == null)
 h = t = newNode;
 else
 t.appendRelaxed(t = newNode);
 }
 if (h == null)
 h = t = new Node();
 head = h;
 tail = t;
 }

 /**
 * @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));
 }

 public void clear() {
 bulkRemove(e -> true);
 }

 /**
 * Tolerate this many consecutive dead nodes before CAS-collapsing.
 * Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element.
 */
 private static final int MAX_HOPS = 8;

 /** Implementation of bulk remove methods. */
 @SuppressWarnings("unchecked")
 private boolean bulkRemove(Predicate<? super E> filter) {
 boolean removed = false;
 restartFromHead: for (;;) {
 int hops = MAX_HOPS;
 // c will be CASed to collapse intervening dead nodes between
 // pred (or head if null) and p.
 for (Node p = head, c = p, pred = null, q; p != null; p = q) {
 q = p.next;
 final Object item; boolean pAlive;
 if (pAlive = ((item = p.item) != null && p.isData)) {
 if (filter.test((E) item)) {
 if (p.tryMatch(item, null))
 removed = true;
 pAlive = false;
 }
 }
 else if (!p.isData && item == null)
 break;
 if (pAlive || q == null || --hops == 0) {
 // p might already be self-linked here, but if so:
 // - CASing head will surely fail
 // - CASing pred's next will be useless but harmless.
 if ((c != p && !tryCasSuccessor(pred, c, c = p))
 || pAlive) {
 // if CAS failed or alive, abandon old pred
 hops = MAX_HOPS;
 pred = p;
 c = q;
 }
 } else if (p == q)
 continue restartFromHead;
 }
 return removed;
 }
 }

 /**
 * Runs action on each element found during a traversal starting at p.
 * If p is null, the action is not run.
 */
 @SuppressWarnings("unchecked")
 void forEachFrom(Consumer<? super E> action, Node p) {
 for (Node pred = null; p != null; ) {
 Node q = p.next;
 final Object item;
 if ((item = p.item) != null) {
 if (p.isData) {
 action.accept((E) item);
 pred = p; p = q; continue;
 }
 }
 else if (!p.isData)
 break;
 for (Node c = p;; q = p.next) {
 if (q == null || !q.isMatched()) {
 pred = skipDeadNodes(pred, c, p, q); p = q; break;
 }
 if (p == (p = q)) { pred = null; p = head; break; }
 }
 }
 }

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

 // VarHandle mechanics
 private static final VarHandle HEAD;
 private static final VarHandle TAIL;
 private static final VarHandle SWEEPVOTES;
 static final VarHandle ITEM;
 static final VarHandle NEXT;
 static final VarHandle WAITER;
 static {
 try {
 MethodHandles.Lookup l = MethodHandles.lookup();
 HEAD = l.findVarHandle(LinkedTransferQueue.class, "head",
 Node.class);
 TAIL = l.findVarHandle(LinkedTransferQueue.class, "tail",
 Node.class);
 SWEEPVOTES = l.findVarHandle(LinkedTransferQueue.class, "sweepVotes",
 int.class);
 ITEM = l.findVarHandle(Node.class, "item", Object.class);
 NEXT = l.findVarHandle(Node.class, "next", Node.class);
 WAITER = l.findVarHandle(Node.class, "waiter", Thread.class);
 } catch (ReflectiveOperationException e) {
 throw new ExceptionInInitializerError(e);
 }

 // Reduce the risk of rare disastrous classloading in first call to
 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
 Class<?> ensureLoaded = LockSupport.class;
 }
}