/*

  * 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 = false;

     /**

      * 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) {

             if (next == cmp) {

                 next = val;

                 return true;

             }

             return false;

         }

         final boolean casItem(Object cmp, Object val) {

             if (item == cmp) {

                 item = val;

                 return true;

             }

             return false;

         }

         /**

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

          * only be seen after publication via casNext.

          */

         Node(Object item, boolean isData) {

             this.item = item;

             this.isData = isData;

         }

         /**

          * Links node to itself to avoid garbage retention.  Called

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

          */

         final void forgetNext() {

             this.next = 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() {

             this.item = this;

             this.waiter = 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;

     }

     /** 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) {

         if (tail == cmp) {

             tail = val;

             return true;

         }

         return false;

     }

     private boolean casHead(Node cmp, Node val) {

         if (head == cmp) {

             head = val;

             return true;

         }

         return false;

     }

     private boolean casSweepVotes(int cmp, int val) {

         if (sweepVotes == cmp) {

             sweepVotes = val;

             return true;

         }

         return false;

     }

     /*

      * 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) &&

                         p.tryMatchData()) {

                         unsplice(pred, p);

                         return true;

                     }

                 }

                 else if (item == null)

                     break;

                 pred = p;

                 if ((p = p.next) == pred) { // stale

                     pred = null;

                     p = head;

                 }

             }

         }

         return false;

     }

     /**

      * Creates an initially empty {@code LinkedTransferQueue}.

      */

     public LinkedTransferQueue() {

     }

     /**

      * Creates a {@code LinkedTransferQueue}

      * initially containing the elements of the given collection,

      * added in traversal order of the collection's iterator.

      *

      * @param c the collection of elements to initially contain

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      */

     public LinkedTransferQueue(Collection<? extends E> c) {

         this();

         addAll(c);

     }

     /**

      * Inserts the specified element at the tail of this queue.

      * As the queue is unbounded, this method will never block.

      *

      * @throws NullPointerException if the specified element is null

      */

     public void put(E e) {

         xfer(e, true, ASYNC, 0);

     }

     /**

      * Inserts the specified element at the tail of this queue.

      * As the queue is unbounded, this method will never block or

      * return {@code false}.

      *

      * @return {@code true} (as specified by

      *  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})

      * @throws NullPointerException if the specified element is null

      */

     public boolean offer(E e, long timeout, TimeUnit unit) {

         xfer(e, true, ASYNC, 0);

         return true;

     }

     /**

      * Inserts the specified element at the tail of this queue.

      * As the queue is unbounded, this method will never return {@code false}.

      *

      * @return {@code true} (as specified by {@link Queue#offer})

      * @throws NullPointerException if the specified element is null

      */

     public boolean offer(E e) {

         xfer(e, true, ASYNC, 0);

         return true;

     }

     /**

      * Inserts the specified element at the tail of this queue.

      * As the queue is unbounded, this method will never throw

      * {@link IllegalStateException} or return {@code false}.

      *

      * @return {@code true} (as specified by {@link Collection#add})

      * @throws NullPointerException if the specified element is null

      */

     public boolean add(E e) {

         xfer(e, true, ASYNC, 0);

         return true;

     }

     /**

      * Transfers the element to a waiting consumer immediately, if possible.

      *

      * <p>More precisely, transfers the specified element immediately

      * if there exists a consumer already waiting to receive it (in

      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),

      * otherwise returning {@code false} without enqueuing the element.

      *

      * @throws NullPointerException if the specified element is null

      */

     public boolean tryTransfer(E e) {

         return xfer(e, true, NOW, 0) == null;

     }

     /**

      * Transfers the element to a consumer, waiting if necessary to do so.

      *

      * <p>More precisely, transfers the specified element immediately

      * if there exists a consumer already waiting to receive it (in

      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),

      * else inserts the specified element at the tail of this queue

      * and waits until the element is received by a consumer.

      *

      * @throws NullPointerException if the specified element is null

      */

     public void transfer(E e) throws InterruptedException {

         if (xfer(e, true, SYNC, 0) != null) {

             Thread.interrupted(); // failure possible only due to interrupt

             throw new InterruptedException();

         }

     }

     /**

      * Transfers the element to a consumer if it is possible to do so

      * before the timeout elapses.

      *

      * <p>More precisely, transfers the specified element immediately

      * if there exists a consumer already waiting to receive it (in

      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),

      * else inserts the specified element at the tail of this queue

      * and waits until the element is received by a consumer,

      * returning {@code false} if the specified wait time elapses

      * before the element can be transferred.

      *

      * @throws NullPointerException if the specified element is null

      */

     public boolean tryTransfer(E e, long timeout, TimeUnit unit)

         throws InterruptedException {

         if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)

             return true;

         if (!Thread.interrupted())

             return false;

         throw new InterruptedException();

     }

     public E take() throws InterruptedException {

         E e = xfer(null, false, SYNC, 0);

         if (e != null)

             return e;

         Thread.interrupted();

         throw new InterruptedException();

     }

     public E poll(long timeout, TimeUnit unit) throws InterruptedException {

         E e = xfer(null, false, TIMED, unit.toNanos(timeout));

         if (e != null || !Thread.interrupted())

             return e;

         throw new InterruptedException();

     }

     public E poll() {

         return xfer(null, false, NOW, 0);

     }

     /**

      * @throws NullPointerException     {@inheritDoc}

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public int drainTo(Collection<? super E> c) {

         if (c == null)

             throw new NullPointerException();

         if (c == this)

             throw new IllegalArgumentException();

         int n = 0;

         E e;

         while ( (e = poll()) != null) {

             c.add(e);

             ++n;

         }

         return n;

     }

     /**

      * @throws NullPointerException     {@inheritDoc}

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public int drainTo(Collection<? super E> c, int maxElements) {

         if (c == null)

             throw new NullPointerException();

         if (c == this)

             throw new IllegalArgumentException();

         int n = 0;

         E e;

         while (n < maxElements && (e = poll()) != null) {

             c.add(e);

             ++n;

         }

         return n;

     }

     /**

      * Returns an iterator over the elements in this queue in proper sequence.

      * The elements will be returned in order from first (head) to last (tail).

      *

      * <p>The returned iterator is a "weakly consistent" iterator that

      * will never throw {@link java.util.ConcurrentModificationException

      * ConcurrentModificationException}, and guarantees to traverse

      * elements as they existed upon construction of the iterator, and

      * may (but is not guaranteed to) reflect any modifications

      * subsequent to construction.

      *

      * @return an iterator over the elements in this queue in proper sequence

      */

     public Iterator<E> iterator() {

         return new Itr();

     }

     public E peek() {

         return firstDataItem();

     }

     /**

      * Returns {@code true} if this queue contains no elements.

      *

      * @return {@code true} if this queue contains no elements

      */

     public boolean isEmpty() {

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

             if (!p.isMatched())

                 return !p.isData;

         }

         return true;

     }

     public boolean hasWaitingConsumer() {

         return firstOfMode(false) != null;

     }

     /**

      * Returns the number of elements in this queue.  If this queue

      * contains more than {@code Integer.MAX_VALUE} elements, returns

      * {@code Integer.MAX_VALUE}.

      *

      * <p>Beware that, unlike in most collections, this method is

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

      * asynchronous nature of these queues, determining the current

      * number of elements requires an O(n) traversal.

      *

      * @return the number of elements in this queue

      */

     public int size() {

         return countOfMode(true);

     }

     public int getWaitingConsumerCount() {

         return countOfMode(false);

     }

     /**

      * Removes a single instance of the specified element from this queue,

      * if it is present.  More formally, removes an element {@code e} such

      * that {@code o.equals(e)}, if this queue contains one or more such

      * elements.

      * Returns {@code true} if this queue contained the specified element

      * (or equivalently, if this queue changed as a result of the call).

      *

      * @param o element to be removed from this queue, if present

      * @return {@code true} if this queue changed as a result of the call

      */

     public boolean remove(Object o) {

         return findAndRemove(o);

     }

     /**

      * Returns {@code true} if this queue contains the specified element.

      * More formally, returns {@code true} if and only if this queue contains

      * at least one element {@code e} such that {@code o.equals(e)}.

      *

      * @param o object to be checked for containment in this queue

      * @return {@code true} if this queue contains the specified element

      */

     public boolean contains(Object o) {

         if (o == null) return false;

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

             Object item = p.item;

             if (p.isData) {

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

                     return true;

             }

             else if (item == null)

                 break;

         }

         return false;

     }

     /**

      * Always returns {@code Integer.MAX_VALUE} because a

      * {@code LinkedTransferQueue} is not capacity constrained.

      *

      * @return {@code Integer.MAX_VALUE} (as specified by

      *         {@link BlockingQueue#remainingCapacity()})

      */

     public int remainingCapacity() {

         return Integer.MAX_VALUE;

     }

     /**

      * Saves the state to a stream (that is, serializes it).

      *

      * @serialData All of the elements (each an {@code E}) in

      * the proper order, followed by a null

      * @param s the stream

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException {

         s.defaultWriteObject();

         for (E e : this)

             s.writeObject(e);

         // Use trailing null as sentinel

         s.writeObject(null);

     }

     /**

      * Reconstitutes the Queue instance from a stream (that is,

      * deserializes it).

      *

      * @param s the stream

      */

     private void readObject(java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         s.defaultReadObject();

         for (;;) {

             @SuppressWarnings("unchecked") E item = (E) s.readObject();

             if (item == null)

                 break;

             else

                 offer(item);

         }

     }

 }