/*
    * Copyright 2009 Red Hat, Inc.
    *
    * Red Hat licenses this file to you under the Apache License, version 2.0
    * (the "License"); you may not use this file except in compliance with the
    * License.  You may obtain a copy of the License at:
    *
    *    http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  
  /*
   * 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/licenses/publicdomain
   */
  
  package org.jboss.netty.util.internal;
  
  import java.util.AbstractQueue;
  import java.util.Collection;
  import java.util.ConcurrentModificationException;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
  import java.util.concurrent.locks.LockSupport;
  /**
   * An unbounded {@link BlockingQueue} 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.
   *
   * <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>.
   *
   * @author <a href="http://www.jboss.org/netty/">The Netty Project</a>
   * @author Doug Lea
   * @author <a href="http://gleamynode.net/">Trustin Lee</a>
   * @version $Rev: 2373 $, $Date: 2010-10-20 20:33:23 +0900 (Wed, 20 Oct 2010) $ (Upstream: 1.79)
   *
   * @param <E> the type of elements held in this collection
   */
  public class LinkedTransferQueue<E> extends AbstractQueue<E>
      implements BlockingQueue<E>, java.io.Serializable {
      private static final long serialVersionUID = -3223113410248163686L;
  
      /*
       * *** Overview of Dual Queues with Slack ***
       *
       * Dual Queues, introduced by Scherer and Scott
       * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
       * (linked) queues in which nodes may represent either data or
       * requests.  When a thread tries to enqueue a data node, but
       * encounters a request node, it instead "matches" and removes it;
       * and vice versa for enqueuing requests. Blocking Dual Queues
       * arrange that threads enqueuing unmatched requests block until
       * other threads provide the match. Dual Synchronous Queues (see
       * Scherer, Lea, & Scott
       * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
       * additionally arrange that threads enqueuing unmatched data also
       * block.  Dual Transfer Queues support all of these modes, as
       * dictated by callers.
       *
       * A FIFO dual queue may be implemented using a variation of the
       * Michael & Scott (M&S) lock-free queue algorithm
       * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
       * It maintains two pointer fields, "head", pointing to a
       * (matched) node that in turn points to the first actual
       * (unmatched) queue node (or null if empty); and "tail" that
       * points to the last node on the queue (or again null if
       * empty). For example, here is a possible queue with four data
       * elements:
      *
      *  head                tail
      *    |                   |
      *    v                   v
      *    M -> U -> U -> U -> U
      *
      * The M&S queue algorithm is known to be prone to scalability and
      * overhead limitations when maintaining (via CAS) these head and
      * tail pointers. This has led to the development of
      * contention-reducing variants such as elimination arrays (see
      * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
      * optimistic back pointers (see Ladan-Mozes & Shavit
      * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
      * However, the nature of dual queues enables a simpler tactic for
      * improving M&S-style implementations when dual-ness is needed.
      *
      * In a dual queue, each node must atomically maintain its match
      * status. While there are other possible variants, we implement
      * this here as: for a data-mode node, matching entails CASing an
      * "item" field from a non-null data value to null upon match, and
      * vice-versa for request nodes, CASing from null to a data
      * value. (Note that the linearization properties of this style of
      * queue are easy to verify -- elements are made available by
      * linking, and unavailable by matching.) Compared to plain M&S
      * queues, this property of dual queues requires one additional
      * successful atomic operation per enq/deq pair. But it also
      * enables lower cost variants of queue maintenance mechanics. (A
      * variation of this idea applies even for non-dual queues that
      * support deletion of interior elements, such as
      * j.u.c.ConcurrentLinkedQueue.)
      *
      * Once a node is matched, its match status can never again
      * change.  We may thus arrange that the linked list of them
      * contain a prefix of zero or more matched nodes, followed by a
      * suffix of zero or more unmatched nodes. (Note that we allow
      * both the prefix and suffix to be zero length, which in turn
      * means that we do not use a dummy header.)  If we were not
      * concerned with either time or space efficiency, we could
      * correctly perform enqueue and dequeue operations by traversing
      * from a pointer to the initial node; CASing the item of the
      * first unmatched node on match and CASing the next field of the
      * trailing node on appends. (Plus some special-casing when
      * initially empty).  While this would be a terrible idea in
      * itself, it does have the benefit of not requiring ANY atomic
      * updates on head/tail fields.
      *
      * We introduce here an approach that lies between the extremes of
      * never versus always updating queue (head and tail) pointers.
      * This offers a tradeoff between sometimes requiring extra
      * traversal steps to locate the first and/or last unmatched
      * nodes, versus the reduced overhead and contention of fewer
      * updates to queue pointers. For example, a possible snapshot of
      * a queue is:
      *
      *  head           tail
      *    |              |
      *    v              v
      *    M -> M -> U -> U -> U -> U
      *
      * The best value for this "slack" (the targeted maximum distance
      * between the value of "head" and the first unmatched node, and
      * similarly for "tail") is an empirical matter. We have found
      * that using very small constants in the range of 1-3 work best
      * over a range of platforms. Larger values introduce increasing
      * costs of cache misses and risks of long traversal chains, while
      * smaller values increase CAS contention and overhead.
      *
      * Dual queues with slack differ from plain M&S dual queues by
      * virtue of only sometimes updating head or tail pointers when
      * matching, appending, or even traversing nodes; in order to
      * maintain a targeted slack.  The idea of "sometimes" may be
      * operationalized in several ways. The simplest is to use a
      * per-operation counter incremented on each traversal step, and
      * to try (via CAS) to update the associated queue pointer
      * whenever the count exceeds a threshold. Another, that requires
      * more overhead, is to use random number generators to update
      * with a given probability per traversal step.
      *
      * In any strategy along these lines, because CASes updating
      * fields may fail, the actual slack may exceed targeted
      * slack. However, they may be retried at any time to maintain
      * targets.  Even when using very small slack values, this
      * approach works well for dual queues because it allows all
      * operations up to the point of matching or appending an item
      * (hence potentially allowing progress by another thread) to be
      * read-only, thus not introducing any further contention. As
      * described below, we implement this by performing slack
      * maintenance retries only after these points.
      *
      * As an accompaniment to such techniques, traversal overhead can
      * be further reduced without increasing contention of head
      * pointer updates: Threads may sometimes shortcut the "next" link
      * path from the current "head" node to be closer to the currently
      * known first unmatched node, and similarly for tail. Again, this
      * may be triggered with using thresholds or randomization.
      *
      * These ideas must be further extended to avoid unbounded amounts
      * of costly-to-reclaim garbage caused by the sequential "next"
      * links of nodes starting at old forgotten head nodes: As first
      * described in detail by Boehm
      * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
      * delays noticing that any arbitrarily old node has become
      * garbage, all newer dead nodes will also be unreclaimed.
      * (Similar issues arise in non-GC environments.)  To cope with
      * this in our implementation, upon CASing to advance the head
      * pointer, we set the "next" link of the previous head to point
      * only to itself; thus limiting the length of connected dead lists.
      * (We also take similar care to wipe out possibly garbage
      * retaining values held in other Node fields.)  However, doing so
      * adds some further complexity to traversal: If any "next"
      * pointer links to itself, it indicates that the current thread
      * has lagged behind a head-update, and so the traversal must
      * continue from the "head".  Traversals trying to find the
      * current tail starting from "tail" may also encounter
      * self-links, in which case they also continue at "head".
      *
      * It is tempting in slack-based scheme to not even use CAS for
      * updates (similarly to Ladan-Mozes & Shavit). However, this
      * cannot be done for head updates under the above link-forgetting
      * mechanics because an update may leave head at a detached node.
      * And while direct writes are possible for tail updates, they
      * increase the risk of long retraversals, and hence long garbage
      * chains, which can be much more costly than is worthwhile
      * considering that the cost difference of performing a CAS vs
      * write is smaller when they are not triggered on each operation
      * (especially considering that writes and CASes equally require
      * additional GC bookkeeping ("write barriers") that are sometimes
      * more costly than the writes themselves because of contention).
      *
      * *** Overview of implementation ***
      *
      * We use a threshold-based approach to updates, with a slack
      * threshold of two -- that is, we update head/tail when the
      * current pointer appears to be two or more steps away from the
      * first/last node. The slack value is hard-wired: a path greater
      * than one is naturally implemented by checking equality of
      * traversal pointers except when the list has only one element,
      * in which case we keep slack threshold at one. Avoiding tracking
      * explicit counts across method calls slightly simplifies an
      * already-messy implementation. Using randomization would
      * probably work better if there were a low-quality dirt-cheap
      * per-thread one available, but even ThreadLocalRandom is too
      * heavy for these purposes.
      *
      * With such a small slack threshold value, it is not worthwhile
      * to augment this with path short-circuiting (i.e., unsplicing
      * interior nodes) except in the case of cancellation/removal (see
      * below).
      *
      * We allow both the head and tail fields to be null before any
      * nodes are enqueued; initializing upon first append.  This
      * simplifies some other logic, as well as providing more
      * efficient explicit control paths instead of letting JVMs insert
      * implicit NullPointerExceptions when they are null.  While not
      * currently fully implemented, we also leave open the possibility
      * of re-nulling these fields when empty (which is complicated to
      * arrange, for little benefit.)
      *
      * All enqueue/dequeue operations are handled by the single method
      * "xfer" with parameters indicating whether to act as some form
      * of offer, put, poll, take, or transfer (each possibly with
      * timeout). The relative complexity of using one monolithic
      * method outweighs the code bulk and maintenance problems of
      * using separate methods for each case.
      *
      * Operation consists of up to three phases. The first is
      * implemented within method xfer, the second in tryAppend, and
      * the third in method awaitMatch.
      *
      * 1. Try to match an existing node
      *
      *    Starting at head, skip already-matched nodes until finding
      *    an unmatched node of opposite mode, if one exists, in which
      *    case matching it and returning, also if necessary updating
      *    head to one past the matched node (or the node itself if the
      *    list has no other unmatched nodes). If the CAS misses, then
      *    a loop retries advancing head by two steps until either
      *    success or the slack is at most two. By requiring that each
      *    attempt advances head by two (if applicable), we ensure that
      *    the slack does not grow without bound. Traversals also check
      *    if the initial head is now off-list, in which case they
      *    start at the new head.
      *
      *    If no candidates are found and the call was untimed
      *    poll/offer, (argument "how" is NOW) return.
      *
      * 2. Try to append a new node (method tryAppend)
      *
      *    Starting at current tail pointer, find the actual last node
      *    and try to append a new node (or if head was null, establish
      *    the first node). Nodes can be appended only if their
      *    predecessors are either already matched or are of the same
      *    mode. If we detect otherwise, then a new node with opposite
      *    mode must have been appended during traversal, so we must
      *    restart at phase 1. The traversal and update steps are
      *    otherwise similar to phase 1: Retrying upon CAS misses and
      *    checking for staleness.  In particular, if a self-link is
      *    encountered, then we can safely jump to a node on the list
      *    by continuing the traversal at current head.
      *
      *    On successful append, if the call was ASYNC, return.
      *
      * 3. Await match or cancellation (method awaitMatch)
      *
      *    Wait for another thread to match node; instead cancelling if
      *    the current thread was interrupted or the wait timed out. On
      *    multiprocessors, we use front-of-queue spinning: If a node
      *    appears to be the first unmatched node in the queue, it
      *    spins a bit before blocking. In either case, before blocking
      *    it tries to unsplice any nodes between the current "head"
      *    and the first unmatched node.
      *
      *    Front-of-queue spinning vastly improves performance of
      *    heavily contended queues. And so long as it is relatively
      *    brief and "quiet", spinning does not much impact performance
      *    of less-contended queues.  During spins threads check their
      *    interrupt status and generate a thread-local random number
      *    to decide to occasionally perform a Thread.yield. While
      *    yield has underdefined specs, we assume that might it help,
      *    and will not hurt in limiting impact of spinning on busy
      *    systems.  We also use smaller (1/2) spins for nodes that are
      *    not known to be front but whose predecessors have not
      *    blocked -- these "chained" spins avoid artifacts of
      *    front-of-queue rules which otherwise lead to alternating
      *    nodes spinning vs blocking. Further, front threads that
      *    represent phase changes (from data to request node or vice
      *    versa) compared to their predecessors receive additional
      *    chained spins, reflecting longer paths typically required to
      *    unblock threads during phase changes.
      *
      *
      * ** Unlinking removed interior nodes **
      *
      * In addition to minimizing garbage retention via self-linking
      * described above, we also unlink removed interior nodes. These
      * may arise due to timed out or interrupted waits, or calls to
      * remove(x) or Iterator.remove.  Normally, given a node that was
      * at one time known to be the predecessor of some node s that is
      * to be removed, we can unsplice s by CASing the next field of
      * its predecessor if it still points to s (otherwise s must
      * already have been removed or is now offlist). But there are two
      * situations in which we cannot guarantee to make node s
      * unreachable in this way: (1) If s is the trailing node of list
      * (i.e., with null next), then it is pinned as the target node
      * for appends, so can only be removed later after other nodes are
      * appended. (2) We cannot necessarily unlink s given a
      * predecessor node that is matched (including the case of being
      * cancelled): the predecessor may already be unspliced, in which
      * case some previous reachable node may still point to s.
      * (For further explanation see Herlihy & Shavit "The Art of
      * Multiprocessor Programming" chapter 9).  Although, in both
      * cases, we can rule out the need for further action if either s
      * or its predecessor are (or can be made to be) at, or fall off
      * from, the head of list.
      *
      * Without taking these into account, it would be possible for an
      * unbounded number of supposedly removed nodes to remain
      * reachable.  Situations leading to such buildup are uncommon but
      * can occur in practice; for example when a series of short timed
      * calls to poll repeatedly time out but never otherwise fall off
      * the list because of an untimed call to take at the front of the
      * queue.
      *
      * When these cases arise, rather than always retraversing the
      * entire list to find an actual predecessor to unlink (which
      * won't help for case (1) anyway), we record a conservative
      * estimate of possible unsplice failures (in "sweepVotes").
      * We trigger a full sweep when the estimate exceeds a threshold
      * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
      * removal failures to tolerate before sweeping through, unlinking
      * cancelled nodes that were not unlinked upon initial removal.
      * We perform sweeps by the thread hitting threshold (rather than
      * background threads or by spreading work to other threads)
      * because in the main contexts in which removal occurs, the
      * caller is already timed-out, cancelled, or performing a
      * potentially O(n) operation (e.g. remove(x)), none of which are
      * time-critical enough to warrant the overhead that alternatives
      * would impose on other threads.
      *
      * Because the sweepVotes estimate is conservative, and because
      * nodes become unlinked "naturally" as they fall off the head of
      * the queue, and because we allow votes to accumulate even while
      * sweeps are in progress, there are typically significantly fewer
      * such nodes than estimated.  Choice of a threshold value
      * balances the likelihood of wasted effort and contention, versus
      * providing a worst-case bound on retention of interior nodes in
      * quiescent queues. The value defined below was chosen
      * empirically to balance these under various timeout scenarios.
      *
      * Note that we cannot self-link unlinked interior nodes during
      * sweeps. However, the associated garbage chains terminate when
      * some successor ultimately falls off the head of the list and is
      * self-linked.
      */
 
     /** True if on multiprocessor */
     private static final boolean MP =
         Runtime.getRuntime().availableProcessors() > 1;
 
     /**
      * The number of times to spin (with randomly interspersed calls
      * to Thread.yield) on multiprocessor before blocking when a node
      * is apparently the first waiter in the queue.  See above for
      * explanation. Must be a power of two. The value is empirically
      * derived -- it works pretty well across a variety of processors,
      * numbers of CPUs, and OSes.
      */
     private static final int FRONT_SPINS   = 1 << 7;
 
     /**
      * The number of times to spin before blocking when a node is
      * preceded by another node that is apparently spinning.  Also
      * serves as an increment to FRONT_SPINS on phase changes, and as
      * base average frequency for yielding during spins. Must be a
      * power of two.
      */
     private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
 
     /**
      * The maximum number of estimated removal failures (sweepVotes)
      * to tolerate before sweeping through the queue unlinking
      * cancelled nodes that were not unlinked upon initial
      * removal. See above for explanation. The value must be at least
      * two to avoid useless sweeps when removing trailing nodes.
      */
     static final int SWEEP_THRESHOLD = 32;
 
     /**
      * Queue nodes. Uses Object, not E, for items to allow forgetting
      * them after use.  Relies heavily on Unsafe mechanics to minimize
      * unnecessary ordering constraints: Writes that are intrinsically
      * ordered wrt other accesses or CASes use simple relaxed forms.
      */
     static final class Node {
         final boolean isData;   // false if this is a request node
         volatile Object item;   // initially non-null if isData; CASed to match
         volatile Node next;
         volatile Thread waiter; // null until waiting
 
         // CAS methods for fields
         final boolean casNext(Node cmp, Node val) {
             if (AtomicFieldUpdaterUtil.isAvailable()) {
                 return nextUpdater.compareAndSet(this, cmp, val);
             } else {
                 synchronized (this) {
                     if (next == cmp) {
                         next = val;
                         return true;
                     } else {
                         return false;
                     }
                 }
             }
         }
 
         final boolean casItem(Object cmp, Object val) {
             // assert cmp == null || cmp.getClass() != Node.class;
             if (AtomicFieldUpdaterUtil.isAvailable()) {
                 return itemUpdater.compareAndSet(this, cmp, val);
             } else {
                 synchronized (this) {
                     if (item == cmp) {
                         item = val;
                         return true;
                     } else {
                         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
          * bacause 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 AtomicReferenceFieldUpdater<Node, Node> nextUpdater =
             AtomicFieldUpdaterUtil.newRefUpdater(Node.class, Node.class, "next");
         private static final AtomicReferenceFieldUpdater<Node, Object> itemUpdater =
             AtomicFieldUpdaterUtil.newRefUpdater(Node.class, Object.class, "item");
 
         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 */
     transient volatile Node tail;
 
     /** The number of apparent failures to unsplice removed nodes */
     transient volatile int sweepVotes;
 
     // CAS methods for fields
     private boolean casTail(Node cmp, Node val) {
         if (AtomicFieldUpdaterUtil.isAvailable()) {
             return tailUpdater.compareAndSet(this, cmp, val);
         } else {
             synchronized (this) {
                 if (tail == cmp) {
                     tail = val;
                     return true;
                 } else {
                     return false;
                 }
             }
         }
     }
 
     private boolean casHead(Node cmp, Node val) {
         if (AtomicFieldUpdaterUtil.isAvailable()) {
             return headUpdater.compareAndSet(this, cmp, val);
         } else {
             synchronized (this) {
                 if (head == cmp) {
                     head = val;
                     return true;
                 } else {
                     return false;
                 }
             }
         }
     }
 
     private boolean casSweepVotes(int cmp, int val) {
         if (AtomicFieldUpdaterUtil.isAvailable()) {
             return sweepVotesUpdater.compareAndSet(this, cmp, val);
         } else {
             synchronized (this) {
                 if (sweepVotes == cmp) {
                     sweepVotes = val;
                     return true;
                 } else {
                     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 LinkedTransferQueue.<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) {
                         continue;
                     }
                 }
                 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 LinkedTransferQueue.<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(nanos);
                 }
                 lastTime = now;
             }
             else {
                 LockSupport.park();
             }
         }
     }
 
     /**
      * 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 LinkedTransferQueue.<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) {
             lastPred = lastRet;
             lastRet = prev;
             for (Node p = prev == null ? head : succ(prev);
                  p != null; p = succ(p)) {
                 Object item = p.item;
                 if (p.isData) {
                     if (item != null && item != p) {
                         nextItem = LinkedTransferQueue.<E>cast(item);
                         nextNode = p;
                         return;
                     }
                 }
                 else if (item == null) {
                     break;
                 }
             }
             nextNode = 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() {
             Node p = lastRet;
             if (p == null) {
                 throw new IllegalStateException();
             }
             if (p.tryMatchData()) {
                 unsplice(lastPred, p);
             }
         }
     }
 
     /* -------------- 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() {
         super();
     }
 
     /**
      * 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 BlockingQueue#offer(Object) BlockingQueue.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
      */
     @Override
     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, from head to tail.
      *
      * <p>The returned iterator is a "weakly consistent" iterator that
      * will never throw
      * {@link 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
      */
     @Override
     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
      */
     @Override
     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
      */
     @Override
     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
      */
     @Override
     public boolean remove(Object o) {
         return findAndRemove(o);
     }
 
     /**
      * 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);
             }
         }
     }
 
     @SuppressWarnings("rawtypes")
     private static final AtomicReferenceFieldUpdater<LinkedTransferQueue, Node> headUpdater =
         AtomicFieldUpdaterUtil.newRefUpdater(LinkedTransferQueue.class, Node.class, "head");
     @SuppressWarnings("rawtypes")
     private static final AtomicReferenceFieldUpdater<LinkedTransferQueue, Node> tailUpdater =
         AtomicFieldUpdaterUtil.newRefUpdater(LinkedTransferQueue.class, Node.class, "tail");
     @SuppressWarnings("rawtypes")
     private static final AtomicIntegerFieldUpdater<LinkedTransferQueue> sweepVotesUpdater =
         AtomicFieldUpdaterUtil.newIntUpdater(LinkedTransferQueue.class, "sweepVotes");
 }