/*

 * 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.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}),