/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent;

import java.util.AbstractQueue;

import java.util.Collection;

import java.util.Iterator;

import java.util.NoSuchElementException;

import java.util.Queue;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.locks.LockSupport;

/**

 * An unbounded {@link TransferQueue} based on linked nodes.

 * This queue orders elements FIFO (first-in-first-out) with respect

 * to any given producer.  The <em>head</em> of the queue is that

 * element that has been on the queue the longest time for some

 * producer.  The <em>tail</em> of the queue is that element that has

 * been on the queue the shortest time for some producer.

 *

 * <p>Beware that, unlike in most collections, the {@code size} method

 * is <em>NOT</em> a constant-time operation. Because of the

 * asynchronous nature of these queues, determining the current number

 * of elements requires a traversal of the elements, and so may report

 * inaccurate results if this collection is modified during traversal.

 * Additionally, the bulk operations {@code addAll},

 * {@code removeAll}, {@code retainAll}, {@code containsAll},

 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed

 * to be performed atomically. For example, an iterator operating

 * concurrently with an {@code addAll} operation might view only some

 * of the added elements.

 *

 * <p>This class and its iterator implement all of the

 * <em>optional</em> methods of the {@link Collection} and {@link

 * Iterator} interfaces.

 *

 * <p>Memory consistency effects: As with other concurrent

 * collections, actions in a thread prior to placing an object into a

 * {@code LinkedTransferQueue}

 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

 * actions subsequent to the access or removal of that element from

 * the {@code LinkedTransferQueue} in another thread.

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @since 1.7

 * @author Doug Lea

 * @param <E> the type of elements held in this collection

 */

public class LinkedTransferQueue<E> extends AbstractQueue<E>

    implements TransferQueue<E>, java.io.Serializable {

    private static final long serialVersionUID = -3223113410248163686L;

    /*

     * *** Overview of Dual Queues with Slack ***

     *

     * Dual Queues, introduced by Scherer and Scott

     * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are

     * (linked) queues in which nodes may represent either data or

     * requests.  When a thread tries to enqueue a data node, but

     * encounters a request node, it instead "matches" and removes it;

     * and vice versa for enqueuing requests. Blocking Dual Queues

     * arrange that threads enqueuing unmatched requests block until

     * other threads provide the match. Dual Synchronous Queues (see

     * Scherer, Lea, & Scott

     * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)

     * additionally arrange that threads enqueuing unmatched data also

     * block.  Dual Transfer Queues support all of these modes, as

     * dictated by callers.

     *

     * A FIFO dual queue may be implemented using a variation of the

     * Michael & Scott (M&S) lock-free queue algorithm

     * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).

     * It maintains two pointer fields, "head", pointing to a

     * (matched) node that in turn points to the first actual

     * (unmatched) queue node (or null if empty); and "tail" that

     * points to the last node on the queue (or again null if

     * empty). For example, here is a possible queue with four data

     * elements:

     *

     *  head                tail

     *    |                   |

     *    v                   v

     *    M -> U -> U -> U -> U

     *

     * The M&S queue algorithm is known to be prone to scalability and

     * overhead limitations when maintaining (via CAS) these head and

     * tail pointers. This has led to the development of

     * contention-reducing variants such as elimination arrays (see

     * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and

     * optimistic back pointers (see Ladan-Mozes & Shavit

     * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).

     * However, the nature of dual queues enables a simpler tactic for

     * improving M&S-style implementations when dual-ness is needed.

     *

     * In a dual queue, each node must atomically maintain its match

     * status. While there are other possible variants, we implement

     * this here as: for a data-mode node, matching entails CASing an

     * "item" field from a non-null data value to null upon match, and

     * vice-versa for request nodes, CASing from null to a data

     * value. (Note that the linearization properties of this style of

     * queue are easy to verify -- elements are made available by

     * linking, and unavailable by matching.) Compared to plain M&S

     * queues, this property of dual queues requires one additional

     * successful atomic operation per enq/deq pair. But it also

     * enables lower cost variants of queue maintenance mechanics. (A

     * variation of this idea applies even for non-dual queues that

     * support deletion of interior elements, such as

     * j.u.c.ConcurrentLinkedQueue.)

     *

     * Once a node is matched, its match status can never again

     * change.  We may thus arrange that the linked list of them

     * contain a prefix of zero or more matched nodes, followed by a

     * suffix of zero or more unmatched nodes. (Note that we allow

     * both the prefix and suffix to be zero length, which in turn

     * means that we do not use a dummy header.)  If we were not

     * concerned with either time or space efficiency, we could

     * correctly perform enqueue and dequeue operations by traversing

     * from a pointer to the initial node; CASing the item of the

     * first unmatched node on match and CASing the next field of the

     * trailing node on appends. (Plus some special-casing when

     * initially empty).  While this would be a terrible idea in

     * itself, it does have the benefit of not requiring ANY atomic

     * updates on head/tail fields.

     *

     * We introduce here an approach that lies between the extremes of

     * never versus always updating queue (head and tail) pointers.

     * This offers a tradeoff between sometimes requiring extra

     * traversal steps to locate the first and/or last unmatched

     * nodes, versus the reduced overhead and contention of fewer

     * updates to queue pointers. For example, a possible snapshot of

     * a queue is:

     *

     *  head           tail

     *    |              |

     *    v              v

     *    M -> M -> U -> U -> U -> U

     *

     * The best value for this "slack" (the targeted maximum distance

     * between the value of "head" and the first unmatched node, and

     * similarly for "tail") is an empirical matter. We have found

     * that using very small constants in the range of 1-3 work best

     * over a range of platforms. Larger values introduce increasing

     * costs of cache misses and risks of long traversal chains, while

     * smaller values increase CAS contention and overhead.

     *

     * Dual queues with slack differ from plain M&S dual queues by

     * virtue of only sometimes updating head or tail pointers when

     * matching, appending, or even traversing nodes; in order to

     * maintain a targeted slack.  The idea of "sometimes" may be

     * operationalized in several ways. The simplest is to use a

     * per-operation counter incremented on each traversal step, and

     * to try (via CAS) to update the associated queue pointer

     * whenever the count exceeds a threshold. Another, that requires

     * more overhead, is to use random number generators to update

     * with a given probability per traversal step.

     *

     * In any strategy along these lines, because CASes updating

     * fields may fail, the actual slack may exceed targeted

     * slack. However, they may be retried at any time to maintain

     * targets.  Even when using very small slack values, this

     * approach works well for dual queues because it allows all

     * operations up to the point of matching or appending an item

     * (hence potentially allowing progress by another thread) to be

     * read-only, thus not introducing any further contention. As

     * described below, we implement this by performing slack

     * maintenance retries only after these points.

     *

     * As an accompaniment to such techniques, traversal overhead can

     * be further reduced without increasing contention of head

     * pointer updates: Threads may sometimes shortcut the "next" link

     * path from the current "head" node to be closer to the currently

     * known first unmatched node, and similarly for tail. Again, this

     * may be triggered with using thresholds or randomization.

     *

     * These ideas must be further extended to avoid unbounded amounts

     * of costly-to-reclaim garbage caused by the sequential "next"

     * links of nodes starting at old forgotten head nodes: As first

     * described in detail by Boehm

     * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC

     * delays noticing that any arbitrarily old node has become

     * garbage, all newer dead nodes will also be unreclaimed.

     * (Similar issues arise in non-GC environments.)  To cope with

     * this in our implementation, upon CASing to advance the head

     * pointer, we set the "next" link of the previous head to point

     * only to itself; thus limiting the length of connected dead lists.

     * (We also take similar care to wipe out possibly garbage

     * retaining values held in other Node fields.)  However, doing so

     * adds some further complexity to traversal: If any "next"

     * pointer links to itself, it indicates that the current thread

     * has lagged behind a head-update, and so the traversal must

     * continue from the "head".  Traversals trying to find the

     * current tail starting from "tail" may also encounter

     * self-links, in which case they also continue at "head".

     *

     * It is tempting in slack-based scheme to not even use CAS for

     * updates (similarly to Ladan-Mozes & Shavit). However, this

     * cannot be done for head updates under the above link-forgetting

     * mechanics because an update may leave head at a detached node.

     * And while direct writes are possible for tail updates, they

     * increase the risk of long retraversals, and hence long garbage

     * chains, which can be much more costly than is worthwhile

     * considering that the cost difference of performing a CAS vs

     * write is smaller when they are not triggered on each operation

     * (especially considering that writes and CASes equally require

     * additional GC bookkeeping ("write barriers") that are sometimes

     * more costly than the writes themselves because of contention).

     *

     * *** Overview of implementation ***

     *

     * We use a threshold-based approach to updates, with a slack

     * threshold of two -- that is, we update head/tail when the

     * current pointer appears to be two or more steps away from the

     * first/last node. The slack value is hard-wired: a path greater

     * than one is naturally implemented by checking equality of

     * traversal pointers except when the list has only one element,

     * in which case we keep slack threshold at one. Avoiding tracking

     * explicit counts across method calls slightly simplifies an

     * already-messy implementation. Using randomization would

     * probably work better if there were a low-quality dirt-cheap

     * per-thread one available, but even ThreadLocalRandom is too

     * heavy for these purposes.

     *

     * With such a small slack threshold value, it is not worthwhile

     * to augment this with path short-circuiting (i.e., unsplicing

     * interior nodes) except in the case of cancellation/removal (see

     * below).

     *

     * We allow both the head and tail fields to be null before any

     * nodes are enqueued; initializing upon first append.  This

     * simplifies some other logic, as well as providing more

     * efficient explicit control paths instead of letting JVMs insert

     * implicit NullPointerExceptions when they are null.  While not

     * currently fully implemented, we also leave open the possibility

     * of re-nulling these fields when empty (which is complicated to

     * arrange, for little benefit.)

     *

     * All enqueue/dequeue operations are handled by the single method

     * "xfer" with parameters indicating whether to act as some form

     * of offer, put, poll, take, or transfer (each possibly with

     * timeout). The relative complexity of using one monolithic

     * method outweighs the code bulk and maintenance problems of

     * using separate methods for each case.

     *

     * Operation consists of up to three phases. The first is

     * implemented within method xfer, the second in tryAppend, and

     * the third in method awaitMatch.

     *

     * 1. Try to match an existing node

     *

     *    Starting at head, skip already-matched nodes until finding

     *    an unmatched node of opposite mode, if one exists, in which

     *    case matching it and returning, also if necessary updating

     *    head to one past the matched node (or the node itself if the

     *    list has no other unmatched nodes). If the CAS misses, then

     *    a loop retries advancing head by two steps until either

     *    success or the slack is at most two. By requiring that each

     *    attempt advances head by two (if applicable), we ensure that

     *    the slack does not grow without bound. Traversals also check

     *    if the initial head is now off-list, in which case they

     *    start at the new head.

     *

     *    If no candidates are found and the call was untimed

     *    poll/offer, (argument "how" is NOW) return.

     *

     * 2. Try to append a new node (method tryAppend)

     *

     *    Starting at current tail pointer, find the actual last node

     *    and try to append a new node (or if head was null, establish

     *    the first node). Nodes can be appended only if their

     *    predecessors are either already matched or are of the same

     *    mode. If we detect otherwise, then a new node with opposite

     *    mode must have been appended during traversal, so we must

     *    restart at phase 1. The traversal and update steps are

     *    otherwise similar to phase 1: Retrying upon CAS misses and

     *    checking for staleness.  In particular, if a self-link is

     *    encountered, then we can safely jump to a node on the list

     *    by continuing the traversal at current head.

     *

     *    On successful append, if the call was ASYNC, return.

     *

     * 3. Await match or cancellation (method awaitMatch)

     *

     *    Wait for another thread to match node; instead cancelling if

     *    the current thread was interrupted or the wait timed out. On

     *    multiprocessors, we use front-of-queue spinning: If a node

     *    appears to be the first unmatched node in the queue, it

     *    spins a bit before blocking. In either case, before blocking

     *    it tries to unsplice any nodes between the current "head"

     *    and the first unmatched node.

     *

     *    Front-of-queue spinning vastly improves performance of

     *    heavily contended queues. And so long as it is relatively

     *    brief and "quiet", spinning does not much impact performance

     *    of less-contended queues.  During spins threads check their

     *    interrupt status and generate a thread-local random number

     *    to decide to occasionally perform a Thread.yield. While

     *    yield has underdefined specs, we assume that might it help,

     *    and will not hurt in limiting impact of spinning on busy

     *    systems.  We also use smaller (1/2) spins for nodes that are

     *    not known to be front but whose predecessors have not

     *    blocked -- these "chained" spins avoid artifacts of

     *    front-of-queue rules which otherwise lead to alternating

     *    nodes spinning vs blocking. Further, front threads that

     *    represent phase changes (from data to request node or vice

     *    versa) compared to their predecessors receive additional

     *    chained spins, reflecting longer paths typically required to

     *    unblock threads during phase changes.

     *

     *

     * ** Unlinking removed interior nodes **

     *

     * In addition to minimizing garbage retention via self-linking

     * described above, we also unlink removed interior nodes. These

     * may arise due to timed out or interrupted waits, or calls to

     * remove(x) or Iterator.remove.  Normally, given a node that was

     * at one time known to be the predecessor of some node s that is

     * to be removed, we can unsplice s by CASing the next field of

     * its predecessor if it still points to s (otherwise s must

     * already have been removed or is now offlist). But there are two

     * situations in which we cannot guarantee to make node s

     * unreachable in this way: (1) If s is the trailing node of list

     * (i.e., with null next), then it is pinned as the target node

     * for appends, so can only be removed later after other nodes are

     * appended. (2) We cannot necessarily unlink s given a

     * predecessor node that is matched (including the case of being

     * cancelled): the predecessor may already be unspliced, in which

     * case some previous reachable node may still point to s.

     * (For further explanation see Herlihy & Shavit "The Art of

     * Multiprocessor Programming" chapter 9).  Although, in both

     * cases, we can rule out the need for further action if either s

     * or its predecessor are (or can be made to be) at, or fall off

     * from, the head of list.

     *

     * Without taking these into account, it would be possible for an

     * unbounded number of supposedly removed nodes to remain

     * reachable.  Situations leading to such buildup are uncommon but

     * can occur in practice; for example when a series of short timed

     * calls to poll repeatedly time out but never otherwise fall off

     * the list because of an untimed call to take at the front of the

     * queue.

     *

     * When these cases arise, rather than always retraversing the

     * entire list to find an actual predecessor to unlink (which

     * won't help for case (1) anyway), we record a conservative

     * estimate of possible unsplice failures (in "sweepVotes").

     * We trigger a full sweep when the estimate exceeds a threshold

     * ("SWEEP_THRESHOLD") indicating the maximum number of estimated

     * removal failures to tolerate before sweeping through, unlinking

     * cancelled nodes that were not unlinked upon initial removal.

     * We perform sweeps by the thread hitting threshold (rather than

     * background threads or by spreading work to other threads)

     * because in the main contexts in which removal occurs, the

     * caller is already timed-out, cancelled, or performing a

     * potentially O(n) operation (e.g. remove(x)), none of which are

     * time-critical enough to warrant the overhead that alternatives

     * would impose on other threads.

     *

     * Because the sweepVotes estimate is conservative, and because

     * nodes become unlinked "naturally" as they fall off the head of

     * the queue, and because we allow votes to accumulate even while

     * sweeps are in progress, there are typically significantly fewer

     * such nodes than estimated.  Choice of a threshold value

     * balances the likelihood of wasted effort and contention, versus

     * providing a worst-case bound on retention of interior nodes in

     * quiescent queues. The value defined below was chosen

     * empirically to balance these under various timeout scenarios.

     *

     * Note that we cannot self-link unlinked interior nodes during

     * sweeps. However, the associated garbage chains terminate when

     * some successor ultimately falls off the head of the list and is

     * self-linked.

     */

    /** True if on multiprocessor */

    private static final boolean MP =

        Runtime.getRuntime().availableProcessors() > 1;

    /**

     * The number of times to spin (with randomly interspersed calls

     * to Thread.yield) on multiprocessor before blocking when a node

     * is apparently the first waiter in the queue.  See above for

     * explanation. Must be a power of two. The value is empirically

     * derived -- it works pretty well across a variety of processors,

     * numbers of CPUs, and OSes.

     */

    private static final int FRONT_SPINS   = 1 << 7;

    /**

     * The number of times to spin before blocking when a node is

     * preceded by another node that is apparently spinning.  Also

     * serves as an increment to FRONT_SPINS on phase changes, and as

     * base average frequency for yielding during spins. Must be a

     * power of two.

     */

    private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;

    /**

     * The maximum number of estimated removal failures (sweepVotes)

     * to tolerate before sweeping through the queue unlinking

     * cancelled nodes that were not unlinked upon initial

     * removal. See above for explanation. The value must be at least

     * two to avoid useless sweeps when removing trailing nodes.

     */

    static final int SWEEP_THRESHOLD = 32;

    /**

     * Queue nodes. Uses Object, not E, for items to allow forgetting

     * them after use.  Relies heavily on Unsafe mechanics to minimize

     * unnecessary ordering constraints: Writes that are intrinsically

     * ordered wrt other accesses or CASes use simple relaxed forms.

     */

    static final class Node {

        final boolean isData;   // false if this is a request node

        volatile Object item;   // initially non-null if isData; CASed to match

        volatile Node next;

        volatile Thread waiter; // null until waiting

        // CAS methods for fields

        final boolean casNext(Node cmp, Node val) {

            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);

        }

        final boolean casItem(Object cmp, Object val) {

            // assert cmp == null || cmp.getClass() != Node.class;

            return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);

        }

        /**

         * Constructs a new node.  Uses relaxed write because item can

         * only be seen after publication via casNext.

         */

        Node(Object item, boolean isData) {

            UNSAFE.putObject(this, itemOffset, item); // relaxed write

            this.isData = isData;

        }

        /**

         * Links node to itself to avoid garbage retention.  Called

         * only after CASing head field, so uses relaxed write.

         */

        final void forgetNext() {

            UNSAFE.putObject(this, nextOffset, this);

        }

        /**

         * Sets item to self and waiter to null, to avoid garbage

         * retention after matching or cancelling. Uses relaxed writes

         * because order is already constrained in the only calling

         * contexts: item is forgotten only after volatile/atomic

         * mechanics that extract items.  Similarly, clearing waiter

         * follows either CAS or return from park (if ever parked;

         * else we don't care).

         */

        final void forgetContents() {

            UNSAFE.putObject(this, itemOffset, this);

            UNSAFE.putObject(this, waiterOffset, null);

        }

        /**

         * Returns true if this node has been matched, including the

         * case of artificial matches due to cancellation.

         */

        final boolean isMatched() {

            Object x = item;

            return (x == this) || ((x == null) == isData);

        }

        /**

         * Returns true if this is an unmatched request node.

         */

        final boolean isUnmatchedRequest() {

            return !isData && item == null;

        }

        /**

         * Returns true if a node with the given mode cannot be

         * appended to this node because this node is unmatched and

         * has opposite data mode.

         */

        final boolean cannotPrecede(boolean haveData) {

            boolean d = isData;

            Object x;

            return d != haveData && (x = item) != this && (x != null) == d;

        }

        /**

         * Tries to artificially match a data node -- used by remove.

         */

        final boolean tryMatchData() {

            // assert isData;

            Object x = item;

            if (x != null && x != this && casItem(x, null)) {

                LockSupport.unpark(waiter);

                return true;

            }

            return false;

        }

        private static final long serialVersionUID = -3375979862319811754L;

        // Unsafe mechanics

        private static final sun.misc.Unsafe UNSAFE;

        private static final long itemOffset;

        private static final long nextOffset;

        private static final long waiterOffset;

        static {

            try {

                UNSAFE = sun.misc.Unsafe.getUnsafe();

                Class k = Node.class;

                itemOffset = UNSAFE.objectFieldOffset

                    (k.getDeclaredField("item"));

                nextOffset = UNSAFE.objectFieldOffset

                    (k.getDeclaredField("next"));

                waiterOffset = UNSAFE.objectFieldOffset

                    (k.getDeclaredField("waiter"));

            } catch (Exception e) {

                throw new Error(e);

            }

        }

    }

    /** head of the queue; null until first enqueue */

    transient volatile Node head;

    /** tail of the queue; null until first append */

    private transient volatile Node tail;

    /** The number of apparent failures to unsplice removed nodes */

    private transient volatile int sweepVotes;

    // CAS methods for fields

    private boolean casTail(Node cmp, Node val) {

        return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);

    }

    private boolean casHead(Node cmp, Node val) {

        return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);

    }

    private boolean casSweepVotes(int cmp, int val) {

        return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);

    }

    /*

     * Possible values for "how" argument in xfer method.

     */

    private static final int NOW   = 0; // for untimed poll, tryTransfer

    private static final int ASYNC = 1; // for offer, put, add

    private static final int SYNC  = 2; // for transfer, take

    private static final int TIMED = 3; // for timed poll, tryTransfer

    @SuppressWarnings("unchecked")

    static <E> E cast(Object item) {

        // assert item == null || item.getClass() != Node.class;

        return (E) item;

    }

    /**

     * Implements all queuing methods. See above for explanation.

     *

     * @param e the item or null for take

     * @param haveData true if this is a put, else a take

     * @param how NOW, ASYNC, SYNC, or TIMED

     * @param nanos timeout in nanosecs, used only if mode is TIMED

     * @return an item if matched, else e

     * @throws NullPointerException if haveData mode but e is null

     */

    private E xfer(E e, boolean haveData, int how, long nanos) {

        if (haveData && (e == null))

            throw new NullPointerException();

        Node s = null;                        // the node to append, if needed

        retry:

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

            for (Node h = head, p = h; p != null;) { // find & match first node

                boolean isData = p.isData;

                Object item = p.item;

                if (item != p && (item != null) == isData) { // unmatched

                    if (isData == haveData)   // can't match

                        break;

                    if (p.casItem(item, e)) { // match

                        for (Node q = p; q != h;) {

                            Node n = q.next;  // update by 2 unless singleton

                            if (head == h && casHead(h, n == null ? q : n)) {

                                h.forgetNext();

                                break;

                            }                 // advance and retry

                            if ((h = head)   == null ||

                                (q = h.next) == null || !q.isMatched())

                                break;        // unless slack < 2

                        }

                        LockSupport.unpark(p.waiter);

                        return this.<E>cast(item);

                    }

                }

                Node n = p.next;

                p = (p != n) ? n : (h = head); // Use head if p offlist

            }

            if (how != NOW) {                 // No matches available

                if (s == null)

                    s = new Node(e, haveData);

                Node pred = tryAppend(s, haveData);

                if (pred == null)

                    continue retry;           // lost race vs opposite mode

                if (how != ASYNC)

                    return awaitMatch(s, pred, e, (how == TIMED), nanos);

            }

            return e; // not waiting

        }

    }

    /**

     * Tries to append node s as tail.

     *

     * @param s the node to append

     * @param haveData true if appending in data mode

     * @return null on failure due to losing race with append in

     * different mode, else s's predecessor, or s itself if no

     * predecessor

     */

    private Node tryAppend(Node s, boolean haveData) {

        for (Node t = tail, p = t;;) {        // move p to last node and append

            Node n, u;                        // temps for reads of next & tail

            if (p == null && (p = head) == null) {

                if (casHead(null, s))

                    return s;                 // initialize

            }

            else if (p.cannotPrecede(haveData))

                return null;                  // lost race vs opposite mode

            else if ((n = p.next) != null)    // not last; keep traversing

                p = p != t && t != (u = tail) ? (t = u) : // stale tail

                    (p != n) ? n : null;      // restart if off list

            else if (!p.casNext(null, s))

                p = p.next;                   // re-read on CAS failure

            else {

                if (p != t) {                 // update if slack now >= 2

                    while ((tail != t || !casTail(t, s)) &&

                           (t = tail)   != null &&

                           (s = t.next) != null && // advance and retry

                           (s = s.next) != null && s != t);

                }

                return p;

            }

        }

    }

    /**

     * Spins/yields/blocks until node s is matched or caller gives up.

     *

     * @param s the waiting node

     * @param pred the predecessor of s, or s itself if it has no

     * predecessor, or null if unknown (the null case does not occur

     * in any current calls but may in possible future extensions)

     * @param e the comparison value for checking match

     * @param timed if true, wait only until timeout elapses

     * @param nanos timeout in nanosecs, used only if timed is true

     * @return matched item, or e if unmatched on interrupt or timeout

     */

    private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {

        long lastTime = timed ? System.nanoTime() : 0L;

        Thread w = Thread.currentThread();

        int spins = -1; // initialized after first item and cancel checks

        ThreadLocalRandom randomYields = null; // bound if needed

        for (;;) {

            Object item = s.item;

            if (item != e) {                  // matched

                // assert item != s;

                s.forgetContents();           // avoid garbage

                return this.<E>cast(item);

            }

            if ((w.isInterrupted() || (timed && nanos <= 0)) &&

                    s.casItem(e, s)) {        // cancel

                unsplice(pred, s);

                return e;

            }

            if (spins < 0) {                  // establish spins at/near front

                if ((spins = spinsFor(pred, s.isData)) > 0)

                    randomYields = ThreadLocalRandom.current();

            }

            else if (spins > 0) {             // spin

                --spins;

                if (randomYields.nextInt(CHAINED_SPINS) == 0)

                    Thread.yield();           // occasionally yield

            }

            else if (s.waiter == null) {

                s.waiter = w;                 // request unpark then recheck

            }

            else if (timed) {

                long now = System.nanoTime();

                if ((nanos -= now - lastTime) > 0)

                    LockSupport.parkNanos(this, nanos);

                lastTime = now;

            }

            else {

                LockSupport.park(this);

            }

        }

    }

    /**

     * Returns spin/yield value for a node with given predecessor and

     * data mode. See above for explanation.

     */

    private static int spinsFor(Node pred, boolean haveData) {

        if (MP && pred != null) {

            if (pred.isData != haveData)      // phase change

                return FRONT_SPINS + CHAINED_SPINS;

            if (pred.isMatched())             // probably at front

                return FRONT_SPINS;

            if (pred.waiter == null)          // pred apparently spinning

                return CHAINED_SPINS;

        }

        return 0;

    }

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

    /**

     * Returns the successor of p, or the head node if p.next has been

     * linked to self, which will only be true if traversing with a

     * stale pointer that is now off the list.

     */

    final Node succ(Node p) {

        Node next = p.next;

        return (p == next) ? head : next;

    }

    /**

     * Returns the first unmatched node of the given mode, or null if

     * none.  Used by methods isEmpty, hasWaitingConsumer.

     */

    private Node firstOfMode(boolean isData) {

        for (Node p = head; p != null; p = succ(p)) {

            if (!p.isMatched())

                return (p.isData == isData) ? p : null;

        }

        return null;

    }

    /**

     * Returns the item in the first unmatched node with isData; or

     * null if none.  Used by peek.

     */

    private E firstDataItem() {

        for (Node p = head; p != null; p = succ(p)) {

            Object item = p.item;

            if (p.isData) {

                if (item != null && item != p)

                    return this.<E>cast(item);

            }

            else if (item == null)

                return null;

        }

        return null;

    }

    /**

     * Traverses and counts unmatched nodes of the given mode.

     * Used by methods size and getWaitingConsumerCount.

     */

    private int countOfMode(boolean data) {

        int count = 0;

        for (Node p = head; p != null; ) {

            if (!p.isMatched()) {

                if (p.isData != data)

                    return 0;

                if (++count == Integer.MAX_VALUE) // saturated

                    break;

            }

            Node n = p.next;

            if (n != p)

                p = n;

            else {

                count = 0;

                p = head;

            }

        }

        return count;

    }

    final class Itr implements Iterator<E> {

        private Node nextNode;   // next node to return item for

        private E nextItem;      // the corresponding item

        private Node lastRet;    // last returned node, to support remove

        private Node lastPred;   // predecessor to unlink lastRet

        /**

         * Moves to next node after prev, or first node if prev null.

         */

        private void advance(Node prev) {

            /*

             * To track and avoid buildup of deleted nodes in the face

             * of calls to both Queue.remove and Itr.remove, we must

             * include variants of unsplice and sweep upon each

             * advance: Upon Itr.remove, we may need to catch up links

             * from lastPred, and upon other removes, we might need to

             * skip ahead from stale nodes and unsplice deleted ones

             * found while advancing.

             */

            Node r, b; // reset lastPred upon possible deletion of lastRet

            if ((r = lastRet) != null && !r.isMatched())

                lastPred = r;    // next lastPred is old lastRet

            else if ((b = lastPred) == null || b.isMatched())

                lastPred = null; // at start of list

            else {

                Node s, n;       // help with removal of lastPred.next

                while ((s = b.next) != null &&

                       s != b && s.isMatched() &&

                       (n = s.next) != null && n != s)

                    b.casNext(s, n);

            }

            this.lastRet = prev;

            for (Node p = prev, s, n;;) {

                s = (p == null) ? head : p.next;

                if (s == null)

                    break;

                else if (s == p) {

                    p = null;

                    continue;

                }

                Object item = s.item;

                if (s.isData) {

                    if (item != null && item != s) {

                        nextItem = LinkedTransferQueue.<E>cast(item);

                        nextNode = s;

                        return;

                    }

                }

                else if (item == null)

                    break;

                // assert s.isMatched();

                if (p == null)

                    p = s;

                else if ((n = s.next) == null)

                    break;

                else if (s == n)

                    p = null;

                else

                    p.casNext(s, n);

            }

            nextNode = null;

            nextItem = null;

        }

        Itr() {

            advance(null);

        }

        public final boolean hasNext() {

            return nextNode != null;

        }

        public final E next() {

            Node p = nextNode;

            if (p == null) throw new NoSuchElementException();

            E e = nextItem;

            advance(p);

            return e;

        }

        public final void remove() {

            final Node lastRet = this.lastRet;

            if (lastRet == null)

                throw new IllegalStateException();

            this.lastRet = null;

            if (lastRet.tryMatchData())

                unsplice(lastPred, lastRet);

        }

    }

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

    /**

     * Unsplices (now or later) the given deleted/cancelled node with

     * the given predecessor.

     *

     * @param pred a node that was at one time known to be the

     * predecessor of s, or null or s itself if s is/was at head

     * @param s the node to be unspliced

     */

    final void unsplice(Node pred, Node s) {

        s.forgetContents(); // forget unneeded fields

        /*

         * See above for rationale. Briefly: if pred still points to

         * s, try to unlink s.  If s cannot be unlinked, because it is

         * trailing node or pred might be unlinked, and neither pred

         * nor s are head or offlist, add to sweepVotes, and if enough

         * votes have accumulated, sweep.

         */

        if (pred != null && pred != s && pred.next == s) {

            Node n = s.next;

            if (n == null ||

                (n != s && pred.casNext(s, n) && pred.isMatched())) {

                for (;;) {               // check if at, or could be, head

                    Node h = head;

                    if (h == pred || h == s || h == null)

                        return;          // at head or list empty

                    if (!h.isMatched())

                        break;

                    Node hn = h.next;

                    if (hn == null)

                        return;          // now empty

                    if (hn != h && casHead(h, hn))

                        h.forgetNext();  // advance head

                }

                if (pred.next != pred && s.next != s) { // recheck if offlist

                    for (;;) {           // sweep now if enough votes

                        int v = sweepVotes;

                        if (v < SWEEP_THRESHOLD) {

                            if (casSweepVotes(v, v + 1))

                                break;

                        }

                        else if (casSweepVotes(v, 0)) {

                            sweep();

                            break;

                        }

                    }

                }

            }

        }

    }

    /**

     * Unlinks matched (typically cancelled) nodes encountered in a

     * traversal from head.

     */

    private void sweep() {

        for (Node p = head, s, n; p != null && (s = p.next) != null; ) {

            if (!s.isMatched())

                // Unmatched nodes are never self-linked

                p = s;

            else if ((n = s.next) == null) // trailing node is pinned

                break;

            else if (s == n)    // stale

                // No need to also check for p == s, since that implies s == n

                p = head;

            else

                p.casNext(s, n);

        }

    }

    /**

     * Main implementation of remove(Object)

     */

    private boolean findAndRemove(Object e) {

        if (e != null) {

            for (Node pred = null, p = head; p != null; ) {

                Object item = p.item;

                if (p.isData) {

                    if (item != null && item != p && e.equals(item) &&