/*

  * 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, Bill Scherer, and Michael Scott 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.concurrent.locks.*;

 import java.util.*;

 /**

  * A {@linkplain BlockingQueue blocking queue} in which each insert

  * operation must wait for a corresponding remove operation by another

  * thread, and vice versa.  A synchronous queue does not have any

  * internal capacity, not even a capacity of one.  You cannot

  * <tt>peek</tt> at a synchronous queue because an element is only

  * present when you try to remove it; you cannot insert an element

  * (using any method) unless another thread is trying to remove it;

  * you cannot iterate as there is nothing to iterate.  The

  * <em>head</em> of the queue is the element that the first queued

  * inserting thread is trying to add to the queue; if there is no such

  * queued thread then no element is available for removal and

  * <tt>poll()</tt> will return <tt>null</tt>.  For purposes of other

  * <tt>Collection</tt> methods (for example <tt>contains</tt>), a

  * <tt>SynchronousQueue</tt> acts as an empty collection.  This queue

  * does not permit <tt>null</tt> elements.

  *

  * <p>Synchronous queues are similar to rendezvous channels used in

  * CSP and Ada. They are well suited for handoff designs, in which an

  * object running in one thread must sync up with an object running

  * in another thread in order to hand it some information, event, or

  * task.

  *

  * <p> This class supports an optional fairness policy for ordering

  * waiting producer and consumer threads.  By default, this ordering

  * is not guaranteed. However, a queue constructed with fairness set

  * to <tt>true</tt> grants threads access in FIFO order.

  *

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

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

  * Iterator} interfaces.

  *

  * <p>This class is a member of the

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

  * Java Collections Framework</a>.

  *

  * @since 1.5

  * @author Doug Lea and Bill Scherer and Michael Scott

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

  */

 public class SynchronousQueue<E> extends AbstractQueue<E>

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

     private static final long serialVersionUID = -3223113410248163686L;

     /*

      * This class implements extensions of the dual stack and dual

      * queue algorithms described in "Nonblocking Concurrent Objects

      * with Condition Synchronization", by W. N. Scherer III and

      * M. L. Scott.  18th Annual Conf. on Distributed Computing,

      * Oct. 2004 (see also

      * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).

      * The (Lifo) stack is used for non-fair mode, and the (Fifo)

      * queue for fair mode. The performance of the two is generally

      * similar. Fifo usually supports higher throughput under

      * contention but Lifo maintains higher thread locality in common

      * applications.

      *

      * A dual queue (and similarly stack) is one that at any given

      * time either holds "data" -- items provided by put operations,

      * or "requests" -- slots representing take operations, or is

      * empty. A call to "fulfill" (i.e., a call requesting an item

      * from a queue holding data or vice versa) dequeues a

      * complementary node.  The most interesting feature of these

      * queues is that any operation can figure out which mode the

      * queue is in, and act accordingly without needing locks.

      *

      * Both the queue and stack extend abstract class Transferer

      * defining the single method transfer that does a put or a

      * take. These are unified into a single method because in dual

      * data structures, the put and take operations are symmetrical,

      * so nearly all code can be combined. The resulting transfer

      * methods are on the long side, but are easier to follow than

      * they would be if broken up into nearly-duplicated parts.

      *

      * The queue and stack data structures share many conceptual

      * similarities but very few concrete details. For simplicity,

      * they are kept distinct so that they can later evolve

      * separately.

      *

      * The algorithms here differ from the versions in the above paper

      * in extending them for use in synchronous queues, as well as

      * dealing with cancellation. The main differences include:

      *

      *  1. The original algorithms used bit-marked pointers, but

      *     the ones here use mode bits in nodes, leading to a number

      *     of further adaptations.

      *  2. SynchronousQueues must block threads waiting to become

      *     fulfilled.

      *  3. Support for cancellation via timeout and interrupts,

      *     including cleaning out cancelled nodes/threads

      *     from lists to avoid garbage retention and memory depletion.

      *

      * Blocking is mainly accomplished using LockSupport park/unpark,

      * except that nodes that appear to be the next ones to become

      * fulfilled first spin a bit (on multiprocessors only). On very

      * busy synchronous queues, spinning can dramatically improve

      * throughput. And on less busy ones, the amount of spinning is

      * small enough not to be noticeable.

      *

      * Cleaning is done in different ways in queues vs stacks.  For

      * queues, we can almost always remove a node immediately in O(1)

      * time (modulo retries for consistency checks) when it is

      * cancelled. But if it may be pinned as the current tail, it must

      * wait until some subsequent cancellation. For stacks, we need a

      * potentially O(n) traversal to be sure that we can remove the

      * node, but this can run concurrently with other threads

      * accessing the stack.

      *

      * While garbage collection takes care of most node reclamation

      * issues that otherwise complicate nonblocking algorithms, care

      * is taken to "forget" references to data, other nodes, and

      * threads that might be held on to long-term by blocked

      * threads. In cases where setting to null would otherwise

      * conflict with main algorithms, this is done by changing a

      * node's link to now point to the node itself. This doesn't arise

      * much for Stack nodes (because blocked threads do not hang on to

      * old head pointers), but references in Queue nodes must be

      * aggressively forgotten to avoid reachability of everything any

      * node has ever referred to since arrival.

      */

     /**

      * Shared internal API for dual stacks and queues.

      */

     abstract static class Transferer {

         /**

          * Performs a put or take.

          *

          * @param e if non-null, the item to be handed to a consumer;

          *          if null, requests that transfer return an item

          *          offered by producer.

          * @param timed if this operation should timeout

          * @param nanos the timeout, in nanoseconds

          * @return if non-null, the item provided or received; if null,

          *         the operation failed due to timeout or interrupt --

          *         the caller can distinguish which of these occurred

          *         by checking Thread.interrupted.

          */

         abstract Object transfer(Object e, boolean timed, long nanos);

     }

     /** The number of CPUs, for spin control */

     static final int NCPUS = 1;

     /**

      * The number of times to spin before blocking in timed waits.

      * The value is empirically derived -- it works well across a

      * variety of processors and OSes. Empirically, the best value

      * seems not to vary with number of CPUs (beyond 2) so is just

      * a constant.

      */

     static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;

     /**

      * The number of times to spin before blocking in untimed waits.

      * This is greater than timed value because untimed waits spin

      * faster since they don't need to check times on each spin.

      */

     static final int maxUntimedSpins = maxTimedSpins * 16;

     /**

      * The number of nanoseconds for which it is faster to spin

      * rather than to use timed park. A rough estimate suffices.

      */

     static final long spinForTimeoutThreshold = 1000L;

     /** Dual stack */

     static final class TransferStack extends Transferer {

         /*

          * This extends Scherer-Scott dual stack algorithm, differing,

          * among other ways, by using "covering" nodes rather than

          * bit-marked pointers: Fulfilling operations push on marker

          * nodes (with FULFILLING bit set in mode) to reserve a spot

          * to match a waiting node.

          */

         /* Modes for SNodes, ORed together in node fields */

         /** Node represents an unfulfilled consumer */

         static final int REQUEST    = 0;

         /** Node represents an unfulfilled producer */

         static final int DATA       = 1;

         /** Node is fulfilling another unfulfilled DATA or REQUEST */

         static final int FULFILLING = 2;

         /** Return true if m has fulfilling bit set */

         static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }

         /** Node class for TransferStacks. */

         static final class SNode {

             volatile SNode next;        // next node in stack

             volatile SNode match;       // the node matched to this

             volatile Thread waiter;     // to control park/unpark

             Object item;                // data; or null for REQUESTs

             int mode;

             // Note: item and mode fields don't need to be volatile

             // since they are always written before, and read after,

             // other volatile/atomic operations.

             SNode(Object item) {

                 this.item = item;

             }

             boolean casNext(SNode cmp, SNode val) {

                 if (next == cmp) {

                     next = val;

                     return true;

                 }

                 return false;

             }

             /**

              * Tries to match node s to this node, if so, waking up thread.

              * Fulfillers call tryMatch to identify their waiters.

              * Waiters block until they have been matched.

              *

              * @param s the node to match

              * @return true if successfully matched to s

              */

             boolean tryMatch(SNode s) {

                 if (match == null) {

                     match = s;

                     Thread w = waiter;

                     if (w != null) {    // waiters need at most one unpark

                         waiter = null;

                         LockSupport.unpark(w);

                     }

                     return true;

                 }

                 return match == s;

             }

             /**

              * Tries to cancel a wait by matching node to itself.

              */

             void tryCancel() {

                 if (match == null) {

                     match = this;

                 }

             }

             boolean isCancelled() {

                 return match == this;

             }

         }

         /** The head (top) of the stack */

         volatile SNode head;

         boolean casHead(SNode h, SNode nh) {

             if (head == h) {

                 head = nh;

                 return true;

             }

             return false;

         }

         /**

          * Creates or resets fields of a node. Called only from transfer

          * where the node to push on stack is lazily created and

          * reused when possible to help reduce intervals between reads

          * and CASes of head and to avoid surges of garbage when CASes

          * to push nodes fail due to contention.

          */

         static SNode snode(SNode s, Object e, SNode next, int mode) {

             if (s == null) s = new SNode(e);

             s.mode = mode;

             s.next = next;

             return s;

         }

         /**

          * Puts or takes an item.

          */

         Object transfer(Object e, boolean timed, long nanos) {

             /*

              * Basic algorithm is to loop trying one of three actions:

              *

              * 1. If apparently empty or already containing nodes of same

              *    mode, try to push node on stack and wait for a match,

              *    returning it, or null if cancelled.

              *

              * 2. If apparently containing node of complementary mode,

              *    try to push a fulfilling node on to stack, match

              *    with corresponding waiting node, pop both from

              *    stack, and return matched item. The matching or

              *    unlinking might not actually be necessary because of

              *    other threads performing action 3:

              *

              * 3. If top of stack already holds another fulfilling node,

              *    help it out by doing its match and/or pop

              *    operations, and then continue. The code for helping

              *    is essentially the same as for fulfilling, except

              *    that it doesn't return the item.

              */

             SNode s = null; // constructed/reused as needed

             int mode = (e == null) ? REQUEST : DATA;

             for (;;) {

                 SNode h = head;

                 if (h == null || h.mode == mode) {  // empty or same-mode

                     if (timed && nanos <= 0) {      // can't wait

                         if (h != null && h.isCancelled())

                             casHead(h, h.next);     // pop cancelled node

                         else

                             return null;

                     } else if (casHead(h, s = snode(s, e, h, mode))) {

                         SNode m = awaitFulfill(s, timed, nanos);

                         if (m == s) {               // wait was cancelled

                             clean(s);

                             return null;

                         }

                         if ((h = head) != null && h.next == s)

                             casHead(h, s.next);     // help s's fulfiller

                         return (mode == REQUEST) ? m.item : s.item;

                     }

                 } else if (!isFulfilling(h.mode)) { // try to fulfill

                     if (h.isCancelled())            // already cancelled

                         casHead(h, h.next);         // pop and retry

                     else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {

                         for (;;) { // loop until matched or waiters disappear

                             SNode m = s.next;       // m is s's match

                             if (m == null) {        // all waiters are gone

                                 casHead(s, null);   // pop fulfill node

                                 s = null;           // use new node next time

                                 break;              // restart main loop

                             }

                             SNode mn = m.next;

                             if (m.tryMatch(s)) {

                                 casHead(s, mn);     // pop both s and m

                                 return (mode == REQUEST) ? m.item : s.item;

                             } else                  // lost match

                                 s.casNext(m, mn);   // help unlink

                         }

                     }

                 } else {                            // help a fulfiller

                     SNode m = h.next;               // m is h's match

                     if (m == null)                  // waiter is gone

                         casHead(h, null);           // pop fulfilling node

                     else {

                         SNode mn = m.next;

                         if (m.tryMatch(h))          // help match

                             casHead(h, mn);         // pop both h and m

                         else                        // lost match

                             h.casNext(m, mn);       // help unlink

                     }

                 }

             }

         }

         /**

          * Spins/blocks until node s is matched by a fulfill operation.

          *

          * @param s the waiting node

          * @param timed true if timed wait

          * @param nanos timeout value

          * @return matched node, or s if cancelled

          */

         SNode awaitFulfill(SNode s, boolean timed, long nanos) {

             /*

              * When a node/thread is about to block, it sets its waiter

              * field and then rechecks state at least one more time

              * before actually parking, thus covering race vs

              * fulfiller noticing that waiter is non-null so should be

              * woken.

              *

              * When invoked by nodes that appear at the point of call

              * to be at the head of the stack, calls to park are

              * preceded by spins to avoid blocking when producers and

              * consumers are arriving very close in time.  This can

              * happen enough to bother only on multiprocessors.

              *

              * The order of checks for returning out of main loop

              * reflects fact that interrupts have precedence over

              * normal returns, which have precedence over

              * timeouts. (So, on timeout, one last check for match is

              * done before giving up.) Except that calls from untimed

              * SynchronousQueue.{poll/offer} don't check interrupts

              * and don't wait at all, so are trapped in transfer

              * method rather than calling awaitFulfill.

              */

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

             Thread w = Thread.currentThread();

             SNode h = head;

             int spins = (shouldSpin(s) ?

                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);

             for (;;) {

                 if (w.isInterrupted())

                     s.tryCancel();

                 SNode m = s.match;

                 if (m != null)

                     return m;

                 if (timed) {

                     long now = System.nanoTime();

                     nanos -= now - lastTime;

                     lastTime = now;

                     if (nanos <= 0) {

                         s.tryCancel();

                         continue;

                     }

                 }

                 if (spins > 0)

                     spins = shouldSpin(s) ? (spins-1) : 0;

                 else if (s.waiter == null)

                     s.waiter = w; // establish waiter so can park next iter

                 else if (!timed)

                     LockSupport.park(this);

                 else if (nanos > spinForTimeoutThreshold)

                     LockSupport.parkNanos(this, nanos);

             }

         }

         /**

          * Returns true if node s is at head or there is an active

          * fulfiller.

          */

         boolean shouldSpin(SNode s) {

             SNode h = head;

             return (h == s || h == null || isFulfilling(h.mode));

         }

         /**

          * Unlinks s from the stack.

          */

         void clean(SNode s) {

             s.item = null;   // forget item

             s.waiter = null; // forget thread

             /*

              * At worst we may need to traverse entire stack to unlink

              * s. If there are multiple concurrent calls to clean, we

              * might not see s if another thread has already removed

              * it. But we can stop when we see any node known to

              * follow s. We use s.next unless it too is cancelled, in

              * which case we try the node one past. We don't check any

              * further because we don't want to doubly traverse just to

              * find sentinel.

              */

             SNode past = s.next;

             if (past != null && past.isCancelled())

                 past = past.next;

             // Absorb cancelled nodes at head

             SNode p;

             while ((p = head) != null && p != past && p.isCancelled())

                 casHead(p, p.next);

             // Unsplice embedded nodes

             while (p != null && p != past) {

                 SNode n = p.next;

                 if (n != null && n.isCancelled())

                     p.casNext(n, n.next);

                 else

                     p = n;

             }

         }

     }

     /** Dual Queue */

     static final class TransferQueue extends Transferer {

         /*

          * This extends Scherer-Scott dual queue algorithm, differing,

          * among other ways, by using modes within nodes rather than

          * marked pointers. The algorithm is a little simpler than

          * that for stacks because fulfillers do not need explicit

          * nodes, and matching is done by CAS'ing QNode.item field

          * from non-null to null (for put) or vice versa (for take).

          */

         /** Node class for TransferQueue. */

         static final class QNode {

             volatile QNode next;          // next node in queue

             volatile Object item;         // CAS'ed to or from null

             volatile Thread waiter;       // to control park/unpark

             final boolean isData;

             QNode(Object item, boolean isData) {

                 this.item = item;

                 this.isData = isData;

             }

             boolean casNext(QNode cmp, QNode val) {

                 if (next == cmp) {

                     next = val;

                     return true;

                 }

                 return false;

             }

             boolean casItem(Object cmp, Object val) {

                 if (item == cmp) {

                     item = val;

                     return true;

                 }

                 return false;

             }

             /**

              * Tries to cancel by CAS'ing ref to this as item.

              */

             void tryCancel(Object cmp) {

                 if (item == cmp) {

                     item = this;

                 }

             }

             boolean isCancelled() {

                 return item == this;

             }

             /**

              * Returns true if this node is known to be off the queue

              * because its next pointer has been forgotten due to

              * an advanceHead operation.

              */

             boolean isOffList() {

                 return next == this;

             }

         }

         /** Head of queue */

         transient volatile QNode head;

         /** Tail of queue */

         transient volatile QNode tail;

         /**

          * Reference to a cancelled node that might not yet have been

          * unlinked from queue because it was the last inserted node

          * when it cancelled.

          */

         transient volatile QNode cleanMe;

         TransferQueue() {

             QNode h = new QNode(null, false); // initialize to dummy node.

             head = h;

             tail = h;

         }

         /**

          * Tries to cas nh as new head; if successful, unlink

          * old head's next node to avoid garbage retention.

          */

         void advanceHead(QNode h, QNode nh) {

             if (head == h) {

                 head = nh;

                 h.next = h; // forget old next

             }

         }

         /**

          * Tries to cas nt as new tail.

          */

         void advanceTail(QNode t, QNode nt) {

             if (tail == t) {

                 tail = nt;

             }

         }

         /**

          * Tries to CAS cleanMe slot.

          */

         boolean casCleanMe(QNode cmp, QNode val) {

             if (cleanMe == cmp) {

                 cleanMe = val;

                 return true;

             }

             return false;

         }

         /**

          * Puts or takes an item.

          */

         Object transfer(Object e, boolean timed, long nanos) {

             /* Basic algorithm is to loop trying to take either of

              * two actions:

              *

              * 1. If queue apparently empty or holding same-mode nodes,

              *    try to add node to queue of waiters, wait to be

              *    fulfilled (or cancelled) and return matching item.

              *

              * 2. If queue apparently contains waiting items, and this

              *    call is of complementary mode, try to fulfill by CAS'ing

              *    item field of waiting node and dequeuing it, and then

              *    returning matching item.

              *

              * In each case, along the way, check for and try to help

              * advance head and tail on behalf of other stalled/slow

              * threads.

              *

              * The loop starts off with a null check guarding against

              * seeing uninitialized head or tail values. This never

              * happens in current SynchronousQueue, but could if

              * callers held non-volatile/final ref to the

              * transferer. The check is here anyway because it places

              * null checks at top of loop, which is usually faster

              * than having them implicitly interspersed.

              */

             QNode s = null; // constructed/reused as needed

             boolean isData = (e != null);

             for (;;) {

                 QNode t = tail;

                 QNode h = head;

                 if (t == null || h == null)         // saw uninitialized value

                     continue;                       // spin

                 if (h == t || t.isData == isData) { // empty or same-mode

                     QNode tn = t.next;

                     if (t != tail)                  // inconsistent read

                         continue;

                     if (tn != null) {               // lagging tail

                         advanceTail(t, tn);

                         continue;

                     }

                     if (timed && nanos <= 0)        // can't wait

                         return null;

                     if (s == null)

                         s = new QNode(e, isData);

                     if (!t.casNext(null, s))        // failed to link in

                         continue;

                     advanceTail(t, s);              // swing tail and wait

                     Object x = awaitFulfill(s, e, timed, nanos);

                     if (x == s) {                   // wait was cancelled

                         clean(t, s);

                         return null;

                     }

                     if (!s.isOffList()) {           // not already unlinked

                         advanceHead(t, s);          // unlink if head

                         if (x != null)              // and forget fields

                             s.item = s;

                         s.waiter = null;

                     }

                     return (x != null) ? x : e;

                 } else {                            // complementary-mode

                     QNode m = h.next;               // node to fulfill

                     if (t != tail || m == null || h != head)

                         continue;                   // inconsistent read

                     Object x = m.item;

                     if (isData == (x != null) ||    // m already fulfilled

                         x == m ||                   // m cancelled

                         !m.casItem(x, e)) {         // lost CAS

                         advanceHead(h, m);          // dequeue and retry

                         continue;

                     }

                     advanceHead(h, m);              // successfully fulfilled

                     LockSupport.unpark(m.waiter);

                     return (x != null) ? x : e;

                 }

             }

         }

         /**

          * Spins/blocks until node s is fulfilled.

          *

          * @param s the waiting node

          * @param e the comparison value for checking match

          * @param timed true if timed wait

          * @param nanos timeout value

          * @return matched item, or s if cancelled

          */

         Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {

             /* Same idea as TransferStack.awaitFulfill */

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

             Thread w = Thread.currentThread();

             int spins = ((head.next == s) ?

                          (timed ? maxTimedSpins : maxUntimedSpins) : 0);

             for (;;) {

                 if (w.isInterrupted())

                     s.tryCancel(e);

                 Object x = s.item;

                 if (x != e)

                     return x;

                 if (timed) {

                     long now = System.nanoTime();

                     nanos -= now - lastTime;

                     lastTime = now;

                     if (nanos <= 0) {

                         s.tryCancel(e);

                         continue;

                     }

                 }

                 if (spins > 0)

                     --spins;

                 else if (s.waiter == null)

                     s.waiter = w;

                 else if (!timed)

                     LockSupport.park(this);

                 else if (nanos > spinForTimeoutThreshold)

                     LockSupport.parkNanos(this, nanos);

             }

         }

         /**

          * Gets rid of cancelled node s with original predecessor pred.

          */

         void clean(QNode pred, QNode s) {

             s.waiter = null; // forget thread

             /*

              * At any given time, exactly one node on list cannot be

              * deleted -- the last inserted node. To accommodate this,

              * if we cannot delete s, we save its predecessor as

              * "cleanMe", deleting the previously saved version

              * first. At least one of node s or the node previously

              * saved can always be deleted, so this always terminates.

              */

             while (pred.next == s) { // Return early if already unlinked

                 QNode h = head;

                 QNode hn = h.next;   // Absorb cancelled first node as head

                 if (hn != null && hn.isCancelled()) {

                     advanceHead(h, hn);

                     continue;

                 }

                 QNode t = tail;      // Ensure consistent read for tail

                 if (t == h)

                     return;

                 QNode tn = t.next;

                 if (t != tail)

                     continue;

                 if (tn != null) {

                     advanceTail(t, tn);

                     continue;

                 }

                 if (s != t) {        // If not tail, try to unsplice

                     QNode sn = s.next;

                     if (sn == s || pred.casNext(s, sn))

                         return;

                 }

                 QNode dp = cleanMe;

                 if (dp != null) {    // Try unlinking previous cancelled node

                     QNode d = dp.next;

                     QNode dn;

                     if (d == null ||               // d is gone or

                         d == dp ||                 // d is off list or

                         !d.isCancelled() ||        // d not cancelled or

                         (d != t &&                 // d not tail and

                          (dn = d.next) != null &&  //   has successor

                          dn != d &&                //   that is on list

                          dp.casNext(d, dn)))       // d unspliced

                         casCleanMe(dp, null);

                     if (dp == pred)

                         return;      // s is already saved node

                 } else if (casCleanMe(null, pred))

                     return;          // Postpone cleaning s

             }

         }

     }

     /**

      * The transferer. Set only in constructor, but cannot be declared

      * as final without further complicating serialization.  Since

      * this is accessed only at most once per public method, there

      * isn't a noticeable performance penalty for using volatile

      * instead of final here.

      */

     private transient volatile Transferer transferer;

     /**

      * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.

      */

     public SynchronousQueue() {

         this(false);

     }

     /**

      * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.

      *

      * @param fair if true, waiting threads contend in FIFO order for

      *        access; otherwise the order is unspecified.

      */

     public SynchronousQueue(boolean fair) {

         transferer = fair ? new TransferQueue() : new TransferStack();

     }

     /**

      * Adds the specified element to this queue, waiting if necessary for

      * another thread to receive it.

      *

      * @throws InterruptedException {@inheritDoc}

      * @throws NullPointerException {@inheritDoc}

      */

     public void put(E o) throws InterruptedException {

         if (o == null) throw new NullPointerException();

         if (transferer.transfer(o, false, 0) == null) {

             Thread.interrupted();

             throw new InterruptedException();

         }

     }

     /**

      * Inserts the specified element into this queue, waiting if necessary

      * up to the specified wait time for another thread to receive it.

      *

      * @return <tt>true</tt> if successful, or <tt>false</tt> if the

      *         specified waiting time elapses before a consumer appears.

      * @throws InterruptedException {@inheritDoc}

      * @throws NullPointerException {@inheritDoc}

      */

     public boolean offer(E o, long timeout, TimeUnit unit)

         throws InterruptedException {

         if (o == null) throw new NullPointerException();

         if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)

             return true;

         if (!Thread.interrupted())

             return false;

         throw new InterruptedException();

     }

     /**

      * Inserts the specified element into this queue, if another thread is

      * waiting to receive it.

      *

      * @param e the element to add

      * @return <tt>true</tt> if the element was added to this queue, else

      *         <tt>false</tt>

      * @throws NullPointerException if the specified element is null

      */

     public boolean offer(E e) {

         if (e == null) throw new NullPointerException();

         return transferer.transfer(e, true, 0) != null;

     }

     /**

      * Retrieves and removes the head of this queue, waiting if necessary

      * for another thread to insert it.

      *

      * @return the head of this queue

      * @throws InterruptedException {@inheritDoc}

      */

     public E take() throws InterruptedException {

         Object e = transferer.transfer(null, false, 0);

         if (e != null)

             return (E)e;

         Thread.interrupted();

         throw new InterruptedException();

     }

     /**

      * Retrieves and removes the head of this queue, waiting

      * if necessary up to the specified wait time, for another thread

      * to insert it.

      *

      * @return the head of this queue, or <tt>null</tt> if the

      *         specified waiting time elapses before an element is present.

      * @throws InterruptedException {@inheritDoc}

      */

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

         Object e = transferer.transfer(null, true, unit.toNanos(timeout));

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

             return (E)e;

         throw new InterruptedException();

     }

     /**

      * Retrieves and removes the head of this queue, if another thread

      * is currently making an element available.

      *

      * @return the head of this queue, or <tt>null</tt> if no

      *         element is available.

      */

     public E poll() {

         return (E)transferer.transfer(null, true, 0);

     }

     /**

      * Always returns <tt>true</tt>.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @return <tt>true</tt>

      */

     public boolean isEmpty() {

         return true;

     }

     /**

      * Always returns zero.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @return zero.

      */

     public int size() {

         return 0;

     }

     /**

      * Always returns zero.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @return zero.

      */

     public int remainingCapacity() {

         return 0;

     }

     /**

      * Does nothing.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      */

     public void clear() {

     }

     /**

      * Always returns <tt>false</tt>.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @param o the element

      * @return <tt>false</tt>

      */

     public boolean contains(Object o) {

         return false;

     }

     /**

      * Always returns <tt>false</tt>.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @param o the element to remove

      * @return <tt>false</tt>

      */

     public boolean remove(Object o) {

         return false;

     }

     /**

      * Returns <tt>false</tt> unless the given collection is empty.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @param c the collection

      * @return <tt>false</tt> unless given collection is empty

      */

     public boolean containsAll(Collection<?> c) {

         return c.isEmpty();

     }

     /**

      * Always returns <tt>false</tt>.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @param c the collection

      * @return <tt>false</tt>

      */

     public boolean removeAll(Collection<?> c) {

         return false;

     }

     /**

      * Always returns <tt>false</tt>.

      * A <tt>SynchronousQueue</tt> has no internal capacity.

      *

      * @param c the collection

      * @return <tt>false</tt>

      */

     public boolean retainAll(Collection<?> c) {

         return false;

     }

     /**

      * Always returns <tt>null</tt>.

      * A <tt>SynchronousQueue</tt> does not return elements

      * unless actively waited on.

      *

      * @return <tt>null</tt>

      */

     public E peek() {

         return null;

     }

     /**

      * Returns an empty iterator in which <tt>hasNext</tt> always returns

      * <tt>false</tt>.

      *

      * @return an empty iterator

      */

     public Iterator<E> iterator() {

         return Collections.emptyIterator();

     }

     /**

      * Returns a zero-length array.

      * @return a zero-length array

      */

     public Object[] toArray() {

         return new Object[0];

     }

     /**

      * Sets the zeroeth element of the specified array to <tt>null</tt>

      * (if the array has non-zero length) and returns it.

      *

      * @param a the array

      * @return the specified array

      * @throws NullPointerException if the specified array is null

      */

     public <T> T[] toArray(T[] a) {

         if (a.length > 0)

             a[0] = null;

         return a;

     }

     /**

      * @throws UnsupportedOperationException {@inheritDoc}

      * @throws ClassCastException            {@inheritDoc}

      * @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 UnsupportedOperationException {@inheritDoc}

      * @throws ClassCastException            {@inheritDoc}

      * @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;

     }

     /*

      * To cope with serialization strategy in the 1.5 version of

      * SynchronousQueue, we declare some unused classes and fields

      * that exist solely to enable serializability across versions.

      * These fields are never used, so are initialized only if this

      * object is ever serialized or deserialized.

      */

     static class WaitQueue implements java.io.Serializable { }

     static class LifoWaitQueue extends WaitQueue {

         private static final long serialVersionUID = -3633113410248163686L;

     }

     static class FifoWaitQueue extends WaitQueue {

         private static final long serialVersionUID = -3623113410248163686L;

     }

     private ReentrantLock qlock;

     private WaitQueue waitingProducers;

     private WaitQueue waitingConsumers;

     /**

      * Save the state to a stream (that is, serialize it).

      *

      * @param s the stream

      */

     private void writeObject(java.io.ObjectOutputStream s)

         throws java.io.IOException {

         boolean fair = transferer instanceof TransferQueue;

         if (fair) {

             qlock = new ReentrantLock(true);

             waitingProducers = new FifoWaitQueue();

             waitingConsumers = new FifoWaitQueue();

         }

         else {

             qlock = new ReentrantLock();

             waitingProducers = new LifoWaitQueue();

             waitingConsumers = new LifoWaitQueue();

         }

         s.defaultWriteObject();

     }

     private void readObject(final java.io.ObjectInputStream s)

         throws java.io.IOException, ClassNotFoundException {

         s.defaultReadObject();

         if (waitingProducers instanceof FifoWaitQueue)

             transferer = new TransferQueue();

         else

             transferer = new TransferStack();

     }

 }