content: /*
   *  Copyright Beijing 58 Information Technology Co.,Ltd.
   *
   *  Licensed to the Apache Software Foundation (ASF) under one
   *  or more contributor license agreements.  See the NOTICE file
   *  distributed with this work for additional information
   *  regarding copyright ownership.  The ASF 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 com.bj58.spat.gaea.server.util.jsr166;
  
  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.locks.LockSupport;
  
  import sun.misc.Unsafe;
  /**
   * An unbounded {@link TransferQueue} based on linked nodes.
   * This queue orders elements FIFO (first-in-first-out) with respect
   * to any given producer.  The <em>head</em> of the queue is that
   * element that has been on the queue the longest time for some
   * producer.  The <em>tail</em> of the queue is that element that has
   * been on the queue the shortest time for some producer.
   *
   * <p>Beware that, unlike in most collections, the {@code size}
   * method is <em>NOT</em> a constant-time operation. Because of the
   * asynchronous nature of these queues, determining the current number
   * of elements requires a traversal of the elements.
   *
   * <p>This class and its iterator implement all of the
   * <em>optional</em> methods of the {@link Collection} and {@link
   * Iterator} interfaces.
   *
   * <p>Memory consistency effects: As with other concurrent
   * collections, actions in a thread prior to placing an object into a
   * {@code LinkedTransferQueue}
   * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
   * actions subsequent to the access or removal of that element from
   * the {@code LinkedTransferQueue} in another thread.
   *
   * <p>This class is a member of the
   * <a href="{@docRoot}/../technotes/guides/collections/index.html">
   * Java Collections Framework</a>.
   *
   * @since 1.7
   * @author Doug Lea
   * @param <E> the type of elements held in this collection
   */
  @SuppressWarnings("all")
  public class LinkedTransferQueue<E> extends AbstractQueue<E>
      implements TransferQueue<E>, java.io.Serializable {
      private static final long serialVersionUID = -3223113410248163686L;
  
      /*
       * *** Overview of Dual Queues with Slack ***
       *
       * Dual Queues, introduced by Scherer and Scott
       * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
       * (linked) queues in which nodes may represent either data or
       * requests.  When a thread tries to enqueue a data node, but
       * encounters a request node, it instead "matches" and removes it;
       * and vice versa for enqueuing requests. Blocking Dual Queues
       * arrange that threads enqueuing unmatched requests block until
       * other threads provide the match. Dual Synchronous Queues (see
       * Scherer, Lea, & Scott
       * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
       * additionally arrange that threads enqueuing unmatched data also
       * block.  Dual Transfer Queues support all of these modes, as
       * dictated by callers.
       *
       * A FIFO dual queue may be implemented using a variation of the
       * Michael & Scott (M&S) lock-free queue algorithm
       * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
       * It maintains two pointer fields, "head", pointing to a
       * (matched) node that in turn points to the first actual
       * (unmatched) queue node (or null if empty); and "tail" that
       * points to the last node on the queue (or again null if
       * empty). For example, here is a possible queue with four data
       * elements:
       *
       *  head                tail
       *    |                   |
       *    v                   v
       *    M -> U -> U -> U -> U
       *
       * The M&S queue algorithm is known to be prone to scalability and
       * overhead limitations when maintaining (via CAS) these head and
       * tail pointers. This has led to the development of
       * contention-reducing variants such as elimination arrays (see
       * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
       * optimistic back pointers (see Ladan-Mozes & Shavit
       * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
       * However, the nature of dual queues enables a simpler tactic for
       * improving M&S-style implementations when dual-ness is needed.
       *
       * In a dual queue, each node must atomically maintain its match
       * status. While there are other possible variants, we implement
       * this here as: for a data-mode node, matching entails CASing an
       * "item" field from a non-null data value to null upon match, and
       * vice-versa for request nodes, CASing from null to a data
       * value. (Note that the linearization properties of this style of
       * queue are easy to verify -- elements are made available by
       * linking, and unavailable by matching.) Compared to plain M&S
       * queues, this property of dual queues requires one additional
       * successful atomic operation per enq/deq pair. But it also
       * enables lower cost variants of queue maintenance mechanics. (A
       * variation of this idea applies even for non-dual queues that
       * support deletion of interior elements, such as
       * j.u.c.ConcurrentLinkedQueue.)
       *
       * Once a node is matched, its match status can never again
       * change.  We may thus arrange that the linked list of them
       * contain a prefix of zero or more matched nodes, followed by a
       * suffix of zero or more unmatched nodes. (Note that we allow
       * both the prefix and suffix to be zero length, which in turn
       * means that we do not use a dummy header.)  If we were not
       * concerned with either time or space efficiency, we could
       * correctly perform enqueue and dequeue operations by traversing
       * from a pointer to the initial node; CASing the item of the
       * first unmatched node on match and CASing the next field of the
       * trailing node on appends. (Plus some special-casing when
       * initially empty).  While this would be a terrible idea in
       * itself, it does have the benefit of not requiring ANY atomic
       * updates on head/tail fields.
       *
       * We introduce here an approach that lies between the extremes of
       * never versus always updating queue (head and tail) pointers.
       * This offers a tradeoff between sometimes requiring extra
       * traversal steps to locate the first and/or last unmatched
       * nodes, versus the reduced overhead and contention of fewer
       * updates to queue pointers. For example, a possible snapshot of
       * a queue is:
       *
       *  head           tail
       *    |              |
       *    v              v
       *    M -> M -> U -> U -> U -> U
       *
       * The best value for this "slack" (the targeted maximum distance
       * between the value of "head" and the first unmatched node, and
       * similarly for "tail") is an empirical matter. We have found
       * that using very small constants in the range of 1-3 work best
       * over a range of platforms. Larger values introduce increasing
       * costs of cache misses and risks of long traversal chains, while
       * smaller values increase CAS contention and overhead.
       *
       * Dual queues with slack differ from plain M&S dual queues by
       * virtue of only sometimes updating head or tail pointers when
       * matching, appending, or even traversing nodes; in order to
       * maintain a targeted slack.  The idea of "sometimes" may be
       * operationalized in several ways. The simplest is to use a
       * per-operation counter incremented on each traversal step, and
       * to try (via CAS) to update the associated queue pointer
       * whenever the count exceeds a threshold. Another, that requires
       * more overhead, is to use random number generators to update
       * with a given probability per traversal step.
       *
       * In any strategy along these lines, because CASes updating
       * fields may fail, the actual slack may exceed targeted
       * slack. However, they may be retried at any time to maintain
       * targets.  Even when using very small slack values, this
       * approach works well for dual queues because it allows all
       * operations up to the point of matching or appending an item
       * (hence potentially allowing progress by another thread) to be
       * read-only, thus not introducing any further contention. As
       * described below, we implement this by performing slack
       * maintenance retries only after these points.
       *
       * As an accompaniment to such techniques, traversal overhead can
       * be further reduced without increasing contention of head
       * pointer updates: Threads may sometimes shortcut the "next" link
       * path from the current "head" node to be closer to the currently
       * known first unmatched node, and similarly for tail. Again, this
       * may be triggered with using thresholds or randomization.
       *
       * These ideas must be further extended to avoid unbounded amounts
       * of costly-to-reclaim garbage caused by the sequential "next"
       * links of nodes starting at old forgotten head nodes: As first
       * described in detail by Boehm
       * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
       * delays noticing that any arbitrarily old node has become
       * garbage, all newer dead nodes will also be unreclaimed.
       * (Similar issues arise in non-GC environments.)  To cope with
       * this in our implementation, upon CASing to advance the head
       * pointer, we set the "next" link of the previous head to point
       * only to itself; thus limiting the length of connected dead lists.
       * (We also take similar care to wipe out possibly garbage
       * retaining values held in other Node fields.)  However, doing so
       * adds some further complexity to traversal: If any "next"
       * pointer links to itself, it indicates that the current thread
       * has lagged behind a head-update, and so the traversal must
       * continue from the "head".  Traversals trying to find the
       * current tail starting from "tail" may also encounter
       * self-links, in which case they also continue at "head".
       *
       * It is tempting in slack-based scheme to not even use CAS for
       * updates (similarly to Ladan-Mozes & Shavit). However, this
       * cannot be done for head updates under the above link-forgetting
       * mechanics because an update may leave head at a detached node.
       * And while direct writes are possible for tail updates, they
       * increase the risk of long retraversals, and hence long garbage
       * chains, which can be much more costly than is worthwhile
       * considering that the cost difference of performing a CAS vs
       * write is smaller when they are not triggered on each operation
       * (especially considering that writes and CASes equally require
       * additional GC bookkeeping ("write barriers") that are sometimes
       * more costly than the writes themselves because of contention).
       *
       * *** Overview of implementation ***
       *
       * We use a threshold-based approach to updates, with a slack
       * threshold of two -- that is, we update head/tail when the
       * current pointer appears to be two or more steps away from the
       * first/last node. The slack value is hard-wired: a path greater
       * than one is naturally implemented by checking equality of
       * traversal pointers except when the list has only one element,
       * in which case we keep slack threshold at one. Avoiding tracking
       * explicit counts across method calls slightly simplifies an
       * already-messy implementation. Using randomization would
       * probably work better if there were a low-quality dirt-cheap
       * per-thread one available, but even ThreadLocalRandom is too
       * heavy for these purposes.
       *
       * With such a small slack threshold value, it is not worthwhile
       * to augment this with path short-circuiting (i.e., unsplicing
       * interior nodes) except in the case of cancellation/removal (see
       * below).
       *
       * We allow both the head and tail fields to be null before any
       * nodes are enqueued; initializing upon first append.  This
       * simplifies some other logic, as well as providing more
       * efficient explicit control paths instead of letting JVMs insert
       * implicit NullPointerExceptions when they are null.  While not
       * currently fully implemented, we also leave open the possibility
       * of re-nulling these fields when empty (which is complicated to
       * arrange, for little benefit.)
       *
       * All enqueue/dequeue operations are handled by the single method
       * "xfer" with parameters indicating whether to act as some form
       * of offer, put, poll, take, or transfer (each possibly with
       * timeout). The relative complexity of using one monolithic
       * method outweighs the code bulk and maintenance problems of
       * using separate methods for each case.
       *
       * Operation consists of up to three phases. The first is
       * implemented within method xfer, the second in tryAppend, and
       * the third in method awaitMatch.
       *
       * 1. Try to match an existing node
       *
       *    Starting at head, skip already-matched nodes until finding
       *    an unmatched node of opposite mode, if one exists, in which
       *    case matching it and returning, also if necessary updating
       *    head to one past the matched node (or the node itself if the
       *    list has no other unmatched nodes). If the CAS misses, then
       *    a loop retries advancing head by two steps until either
       *    success or the slack is at most two. By requiring that each
       *    attempt advances head by two (if applicable), we ensure that
       *    the slack does not grow without bound. Traversals also check
       *    if the initial head is now off-list, in which case they
       *    start at the new head.
       *
       *    If no candidates are found and the call was untimed
       *    poll/offer, (argument "how" is NOW) return.
       *
       * 2. Try to append a new node (method tryAppend)
       *
       *    Starting at current tail pointer, find the actual last node
       *    and try to append a new node (or if head was null, establish
       *    the first node). Nodes can be appended only if their
       *    predecessors are either already matched or are of the same
       *    mode. If we detect otherwise, then a new node with opposite
       *    mode must have been appended during traversal, so we must
       *    restart at phase 1. The traversal and update steps are
       *    otherwise similar to phase 1: Retrying upon CAS misses and
       *    checking for staleness.  In particular, if a self-link is
       *    encountered, then we can safely jump to a node on the list
       *    by continuing the traversal at current head.
       *
       *    On successful append, if the call was ASYNC, return.
       *
       * 3. Await match or cancellation (method awaitMatch)
       *
       *    Wait for another thread to match node; instead cancelling if
       *    the current thread was interrupted or the wait timed out. On
       *    multiprocessors, we use front-of-queue spinning: If a node
       *    appears to be the first unmatched node in the queue, it
       *    spins a bit before blocking. In either case, before blocking
       *    it tries to unsplice any nodes between the current "head"
       *    and the first unmatched node.
       *
       *    Front-of-queue spinning vastly improves performance of
       *    heavily contended queues. And so long as it is relatively
       *    brief and "quiet", spinning does not much impact performance
       *    of less-contended queues.  During spins threads check their
       *    interrupt status and generate a thread-local random number
       *    to decide to occasionally perform a Thread.yield. While
       *    yield has underdefined specs, we assume that might it help,
       *    and will not hurt in limiting impact of spinning on busy
       *    systems.  We also use smaller (1/2) spins for nodes that are
       *    not known to be front but whose predecessors have not
       *    blocked -- these "chained" spins avoid artifacts of
       *    front-of-queue rules which otherwise lead to alternating
       *    nodes spinning vs blocking. Further, front threads that
       *    represent phase changes (from data to request node or vice
       *    versa) compared to their predecessors receive additional
       *    chained spins, reflecting longer paths typically required to
       *    unblock threads during phase changes.
       *
       *
       * ** Unlinking removed interior nodes **
       *
       * In addition to minimizing garbage retention via self-linking
       * described above, we also unlink removed interior nodes. These
       * may arise due to timed out or interrupted waits, or calls to
       * remove(x) or Iterator.remove.  Normally, given a node that was
       * at one time known to be the predecessor of some node s that is
       * to be removed, we can unsplice s by CASing the next field of
       * its predecessor if it still points to s (otherwise s must
       * already have been removed or is now offlist). But there are two
       * situations in which we cannot guarantee to make node s
       * unreachable in this way: (1) If s is the trailing node of list
       * (i.e., with null next), then it is pinned as the target node
       * for appends, so can only be removed later when 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
       * 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 (i.e., remove(x)), none of which are time-critical
       * enough to warrant the overhead that alternatives would impose
       * on other threads.
       *
       * Because the sweepVotes estimate is conservative, and because
       * nodes become unlinked "naturally" as they fall off the head of
       * the queue, and because we allow votes to accumulate even while
       * sweeps are in progress, there are typically significantly fewer
       * such nodes than estimated.  Choice of a threshold value
       * balances the likelihood of wasted effort and contention, versus
       * providing a worst-case bound on retention of interior nodes in
       * quiescent queues. The value defined below was chosen
       * empirically to balance these under various timeout scenarios.
       *
       * Note that we cannot self-link unlinked interior nodes during
       * sweeps. However, the associated garbage chains terminate when
       * some successor ultimately falls off the head of the list and is
       * self-linked.
       */
  
      /** True if on multiprocessor */
      private static final boolean MP =
          Runtime.getRuntime().availableProcessors() > 1;
  
      /**
       * The number of times to spin (with randomly interspersed calls
       * to Thread.yield) on multiprocessor before blocking when a node
       * is apparently the first waiter in the queue.  See above for
       * explanation. Must be a power of two. The value is empirically
       * derived -- it works pretty well across a variety of processors,
       * numbers of CPUs, and OSes.
       */
      private static final int FRONT_SPINS   = 1 << 7;
  
      /**
       * The number of times to spin before blocking when a node is
       * preceded by another node that is apparently spinning.  Also
       * serves as an increment to FRONT_SPINS on phase changes, and as
       * base average frequency for yielding during spins. Must be a
       * power of two.
       */
      private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
  
      /**
       * The maximum number of estimated removal failures (sweepVotes)
       * to tolerate before sweeping through the queue unlinking
       * cancelled nodes that were not unlinked upon initial
       * removal. See above for explanation. The value must be at least
       * two to avoid useless sweeps when removing trailing nodes.
       */
      static final int SWEEP_THRESHOLD = 32;
  
      /**
       * Queue nodes. Uses Object, not E, for items to allow forgetting
       * them after use.  Relies heavily on Unsafe mechanics to minimize
       * unnecessary ordering constraints: Writes that are intrinsically
       * ordered wrt other accesses or CASes use simple relaxed forms.
       */
      static final class Node {
          final boolean isData;   // false if this is a request node
          volatile Object item;   // initially non-null if isData; CASed to match
          volatile Node next;
          volatile Thread waiter; // null until waiting
  
          // CAS methods for fields
          final boolean casNext(Node cmp, Node val) {
              return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
          }
  
          final boolean casItem(Object cmp, Object val) {
              assert cmp == null || cmp.getClass() != Node.class;
              return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
          }
  
          /**
           * Creates a new node. Uses relaxed write because item can only
           * be seen if followed by CAS.
           */
          Node(Object item, boolean isData) {
              UNSAFE.putObject(this, itemOffset, item); // relaxed write
              this.isData = isData;
          }
  
          /**
           * Links node to itself to avoid garbage retention.  Called
           * only after CASing head field, so uses relaxed write.
           */
          final void forgetNext() {
              UNSAFE.putObject(this, nextOffset, this);
          }
  
          /**
           * Sets item to self and waiter to null, to avoid garbage
           * retention after matching or cancelling. Uses relaxed writes
           * 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() {
              UNSAFE.putObject(this, itemOffset, this);
              UNSAFE.putObject(this, waiterOffset, null);
          }
  
          /**
           * Returns true if this node has been matched, including the
           * case of artificial matches due to cancellation.
           */
          final boolean isMatched() {
              Object x = item;
              return (x == this) || ((x == null) == isData);
          }
  
          /**
           * Returns true if this is an unmatched request node.
           */
          final boolean isUnmatchedRequest() {
              return !isData && item == null;
          }
  
          /**
           * Returns true if a node with the given mode cannot be
           * appended to this node because this node is unmatched and
           * has opposite data mode.
           */
          final boolean cannotPrecede(boolean haveData) {
              boolean d = isData;
              Object x;
              return d != haveData && (x = item) != this && (x != null) == d;
          }
  
          /**
           * Tries to artificially match a data node -- used by remove.
           */
          final boolean tryMatchData() {
              assert isData;
              Object x = item;
              if (x != null && x != this && casItem(x, null)) {
                  LockSupport.unpark(waiter);
                  return true;
              }
              return false;
          }
  
          // Unsafe mechanics
          
          private static final sun.misc.Unsafe UNSAFE = getUnsafe();
          private static final long nextOffset =
              objectFieldOffset(UNSAFE, "next", Node.class);
          private static final long itemOffset =
              objectFieldOffset(UNSAFE, "item", Node.class);
          private static final long waiterOffset =
              objectFieldOffset(UNSAFE, "waiter", Node.class);
  
          private static final long serialVersionUID = -3375979862319811754L;
      }
  
      /** head of the queue; null until first enqueue */
      transient volatile Node head;
  
      /** tail of the queue; null until first append */
      private transient volatile Node tail;
  
      /** The number of apparent failures to unsplice removed nodes */
      private transient volatile int sweepVotes;
  
      // CAS methods for fields
      private boolean casTail(Node cmp, Node val) {
          return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
      }
  
      private boolean casHead(Node cmp, Node val) {
          return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
      }
  
      private boolean casSweepVotes(int cmp, int val) {
          return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
      }
  
      /*
       * Possible values for "how" argument in xfer method.
       */
      private static final int NOW   = 0; // for untimed poll, tryTransfer
      private static final int ASYNC = 1; // for offer, put, add
      private static final int SYNC  = 2; // for transfer, take
      private static final int TIMED = 3; // for timed poll, tryTransfer
  
      @SuppressWarnings("unchecked")
      static <E> E cast(Object item) {
          assert item == null || item.getClass() != Node.class;
          return (E) item;
      }
  
      /**
       * Implements all queuing methods. See above for explanation.
       *
       * @param e the item or null for take
       * @param haveData true if this is a put, else a take
       * @param how NOW, ASYNC, SYNC, or TIMED
       * @param nanos timeout in nanosecs, used only if mode is TIMED
       * @return an item if matched, else e
       * @throws NullPointerException if haveData mode but e is null
       */
      private E xfer(E e, boolean haveData, int how, long nanos) {
          if (haveData && (e == null))
              throw new NullPointerException();
          Node s = null;                        // the node to append, if needed
  
          retry: for (;;) {                     // restart on append race
  
              for (Node h = head, p = h; p != null;) { // find & match first node
                  boolean isData = p.isData;
                  Object item = p.item;
                  if (item != p && (item != null) == isData) { // unmatched
                      if (isData == haveData)   // can't match
                          break;
                      if (p.casItem(item, e)) { // match
                          for (Node q = p; q != h;) {
                              Node n = q.next;  // update by 2 unless singleton
                              if (head == h && casHead(h, n == null? q : n)) {
                                  h.forgetNext();
                                  break;
                              }                 // advance and retry
                              if ((h = head)   == null ||
                                  (q = h.next) == null || !q.isMatched())
                                  break;        // unless slack < 2
                          }
                          LockSupport.unpark(p.waiter);
                          return this.<E>cast(item);
                      }
                  }
                  Node n = p.next;
                  p = (p != n) ? n : (h = head); // Use head if p offlist
              }
  
              if (how != NOW) {                 // No matches available
                  if (s == null)
                      s = new Node(e, haveData);
                  Node pred = tryAppend(s, haveData);
                  if (pred == null)
                      continue retry;           // lost race vs opposite mode
                  if (how != ASYNC)
                      return awaitMatch(s, pred, e, (how == TIMED), nanos);
              }
              return e; // not waiting
          }
      }
  
      /**
       * Tries to append node s as tail.
       *
       * @param s the node to append
       * @param haveData true if appending in data mode
       * @return null on failure due to losing race with append in
       * different mode, else s's predecessor, or s itself if no
       * predecessor
       */
      private Node tryAppend(Node s, boolean haveData) {
          for (Node t = tail, p = t;;) {        // move p to last node and append
              Node n, u;                        // temps for reads of next & tail
              if (p == null && (p = head) == null) {
                  if (casHead(null, s))
                      return s;                 // initialize
              }
              else if (p.cannotPrecede(haveData))
                  return null;                  // lost race vs opposite mode
              else if ((n = p.next) != null)    // not last; keep traversing
                  p = p != t && t != (u = tail) ? (t = u) : // stale tail
                      (p != n) ? n : null;      // restart if off list
              else if (!p.casNext(null, s))
                  p = p.next;                   // re-read on CAS failure
              else {
                  if (p != t) {                 // update if slack now >= 2
                      while ((tail != t || !casTail(t, s)) &&
                             (t = tail)   != null &&
                             (s = t.next) != null && // advance and retry
                             (s = s.next) != null && s != t);
                  }
                  return p;
              }
          }
      }
  
      /**
       * Spins/yields/blocks until node s is matched or caller gives up.
       *
       * @param s the waiting node
       * @param pred the predecessor of s, or s itself if it has no
       * predecessor, or null if unknown (the null case does not occur
       * in any current calls but may in possible future extensions)
       * @param e the comparison value for checking match
       * @param timed if true, wait only until timeout elapses
       * @param nanos timeout in nanosecs, used only if timed is true
       * @return matched item, or e if unmatched on interrupt or timeout
       */
      private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
          long lastTime = timed ? System.nanoTime() : 0L;
          Thread w = Thread.currentThread();
          int spins = -1; // initialized after first item and cancel checks
          ThreadLocalRandom randomYields = null; // bound if needed
  
          for (;;) {
              Object item = s.item;
              if (item != e) {                  // matched
                  assert item != s;
                  s.forgetContents();           // avoid garbage
                  return this.<E>cast(item);
              }
              if ((w.isInterrupted() || (timed && nanos <= 0)) &&
                      s.casItem(e, s)) {        // cancel
                  unsplice(pred, s);
                  return e;
              }
  
              if (spins < 0) {                  // establish spins at/near front
                  if ((spins = spinsFor(pred, s.isData)) > 0)
                      randomYields = ThreadLocalRandom.current();
              }
              else if (spins > 0) {             // spin
                  --spins;
                  if (randomYields.nextInt(CHAINED_SPINS) == 0)
                      Thread.yield();           // occasionally yield
              }
              else if (s.waiter == null) {
                  s.waiter = w;                 // request unpark then recheck
              }
              else if (timed) {
                  long now = System.nanoTime();
                  if ((nanos -= now - lastTime) > 0)
                  	/* #ifdef JDK5
                  	LockSupport.parkNanos(nanos);
                  	#endif JDK5 */
                  	// #ifdef JDK6
                      LockSupport.parkNanos(this, nanos);
                  	// #endif JDK6
                  lastTime = now;
              }
              else {
              	/* #ifdef JDK5
              	LockSupport.park();
              	#endif JDK5 */
              	// #ifdef JDK6
                  LockSupport.park(this);
                  // #endif JDK6
              }
          }
      }
  
      /**
       * Returns spin/yield value for a node with given predecessor and
       * data mode. See above for explanation.
       */
      private static int spinsFor(Node pred, boolean haveData) {
          if (MP && pred != null) {
              if (pred.isData != haveData)      // phase change
                  return FRONT_SPINS + CHAINED_SPINS;
              if (pred.isMatched())             // probably at front
                  return FRONT_SPINS;
              if (pred.waiter == null)          // pred apparently spinning
                  return CHAINED_SPINS;
          }
          return 0;
      }
  
      /* -------------- Traversal methods -------------- */
  
      /**
       * Returns the successor of p, or the head node if p.next has been
       * linked to self, which will only be true if traversing with a
       * stale pointer that is now off the list.
       */
      final Node succ(Node p) {
          Node next = p.next;
          return (p == next) ? head : next;
      }
  
      /**
       * Returns the first unmatched node of the given mode, or null if
       * none.  Used by methods isEmpty, hasWaitingConsumer.
       */
      private Node firstOfMode(boolean isData) {
          for (Node p = head; p != null; p = succ(p)) {
              if (!p.isMatched())
                  return (p.isData == isData) ? p : null;
          }
          return null;
      }
  
      /**
       * Returns the item in the first unmatched node with isData; or
       * null if none.  Used by peek.
       */
      private E firstDataItem() {
          for (Node p = head; p != null; p = succ(p)) {
              Object item = p.item;
              if (p.isData) {
                  if (item != null && item != p)
                      return this.<E>cast(item);
              }
              else if (item == null)
                  return null;
          }
          return null;
      }
  
      /**
       * Traverses and counts unmatched nodes of the given mode.
       * Used by methods size and getWaitingConsumerCount.
       */
      private int countOfMode(boolean data) {
          int count = 0;
          for (Node p = head; p != null; ) {
              if (!p.isMatched()) {
                  if (p.isData != data)
                      return 0;
                  if (++count == Integer.MAX_VALUE) // saturated
                      break;
              }
              Node n = p.next;
              if (n != p)
                  p = n;
              else {
                  count = 0;
                  p = head;
              }
          }
          return count;
      }
  
      final class Itr implements Iterator<E> {
          private Node nextNode;   // next node to return item for
          private E nextItem;      // the corresponding item
          private Node lastRet;    // last returned node, to support remove
          private Node lastPred;   // predecessor to unlink lastRet
  
          /**
           * Moves to next node after prev, or first node if prev null.
           */
          private void advance(Node prev) {
              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.this.<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 nodes encountered in a traversal from head.
       */
      private void sweep() {
          for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
              if (p == s)                    // stale
                  p = head;
              else if (!s.isMatched())
                  p = s;
              else if ((n = s.next) == null) // trailing node is pinned
                  break;
              else
                  p.casNext(s, n);
          }
      }
  
      /**
       * Main implementation of remove(Object)
       */
      private boolean findAndRemove(Object e) {
          if (e != null) {
              for (Node pred = null, p = head; p != null; ) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p && e.equals(item) &&
                          p.tryMatchData()) {
                          unsplice(pred, p);
                          return true;
                      }
                  }
                  else if (item == null)
                      break;
                  pred = p;
                  if ((p = p.next) == pred) { // stale
                      pred = null;
                      p = head;
                  }
              }
          }
          return false;
      }
  
  
      /**
       * Creates an initially empty {@code LinkedTransferQueue}.
       */
      public LinkedTransferQueue() {
      }
  
      /**
       * Creates a {@code LinkedTransferQueue}
       * initially containing the elements of the given collection,
       * added in traversal order of the collection's iterator.
       *
       * @param c the collection of elements to initially contain
       * @throws NullPointerException if the specified collection or any
       *         of its elements are null
       */
      public LinkedTransferQueue(Collection<? extends E> c) {
          this();
          addAll(c);
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never block.
       *
       * @throws NullPointerException if the specified element is null
       */
      public void put(E e) {
          xfer(e, true, ASYNC, 0);
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never block or
       * return {@code false}.
       *
       * @return {@code true} (as specified by
       *  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e, long timeout, TimeUnit unit) {
          xfer(e, true, ASYNC, 0);
          return true;
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never return {@code false}.
       *
       * @return {@code true} (as specified by
       *         {@link 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
       */
      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
       */
      public Iterator<E> iterator() {
          return new Itr();
      }
  
      public E peek() {
          return firstDataItem();
      }
  
      /**
       * Returns {@code true} if this queue contains no elements.
       *
       * @return {@code true} if this queue contains no elements
       */
      public boolean isEmpty() {
          for (Node p = head; p != null; p = succ(p)) {
              if (!p.isMatched())
                  return !p.isData;
          }
          return true;
      }
  
      public boolean hasWaitingConsumer() {
          return firstOfMode(false) != null;
      }
  
      /**
       * Returns the number of elements in this queue.  If this queue
       * contains more than {@code Integer.MAX_VALUE} elements, returns
       * {@code Integer.MAX_VALUE}.
       *
       * <p>Beware that, unlike in most collections, this method is
       * <em>NOT</em> a constant-time operation. Because of the
       * asynchronous nature of these queues, determining the current
       * number of elements requires an O(n) traversal.
       *
       * @return the number of elements in this queue
       */
      public int size() {
          return countOfMode(true);
      }
  
      public int getWaitingConsumerCount() {
          return countOfMode(false);
      }
  
      /**
       * Removes a single instance of the specified element from this queue,
       * if it is present.  More formally, removes an element {@code e} such
       * that {@code o.equals(e)}, if this queue contains one or more such
       * elements.
       * Returns {@code true} if this queue contained the specified element
       * (or equivalently, if this queue changed as a result of the call).
       *
       * @param o element to be removed from this queue, if present
       * @return {@code true} if this queue changed as a result of the call
       */
      public boolean remove(Object o) {
          return findAndRemove(o);
      }
  
      /**
       * 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);
          }
      }
  
      // Unsafe mechanics
  
      private static final Unsafe UNSAFE = getUnsafe();
      private static final long headOffset =
          objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
      private static final long tailOffset =
          objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
      private static final long sweepVotesOffset =
          objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class);
  
      static long objectFieldOffset(Unsafe UNSAFE,
                                    String field, Class<?> klazz) {
          try {
              return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
          } catch (NoSuchFieldException e) {
              // Convert Exception to corresponding Error
              NoSuchFieldError error = new NoSuchFieldError(field);
              error.initCause(e);
              throw error;
          }
      }
  
      /**
       * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
       * Replace with a simple call to Unsafe.getUnsafe when integrating
       * into a jdk.
       *
       * @return a sun.misc.Unsafe
       */
      static sun.misc.Unsafe getUnsafe() {
          try {
              return sun.misc.Unsafe.getUnsafe();
          } catch (SecurityException se) {
              try {
                  return java.security.AccessController.doPrivileged
                      (new java.security
                       .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                          public sun.misc.Unsafe run() throws Exception {
                              java.lang.reflect.Field f = sun.misc
                                  .Unsafe.class.getDeclaredField("theUnsafe");
                              f.setAccessible(true);
                              return (sun.misc.Unsafe) f.get(null);
                          }});
              } catch (java.security.PrivilegedActionException e) {
                  throw new RuntimeException("Could not initialize intrinsics",
                                             e.getCause());
              }
          }
      }
  
  }
  