content: /*
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
   * or visit www.oracle.com if you need additional information or have any
   * questions.
   */
  
  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Written by Doug Lea with assistance from members of JCP JSR-166
   * Expert Group and released to the public domain, as explained at
   * http://creativecommons.org/publicdomain/zero/1.0/
   */
  
  package java.util.concurrent;
  
  import java.util.AbstractQueue;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.Queue;
  import java.util.Spliterator;
  import java.util.Spliterators;
  import java.util.concurrent.locks.LockSupport;
  import java.util.function.Consumer;
  
  // BEGIN android-note
  // removed link to collections framework docs
  // END android-note
  
  /**
   * An unbounded {@link TransferQueue} based on linked nodes.
   * This queue orders elements FIFO (first-in-first-out) with respect
   * to any given producer.  The <em>head</em> of the queue is that
   * element that has been on the queue the longest time for some
   * producer.  The <em>tail</em> of the queue is that element that has
   * been on the queue the shortest time for some producer.
   *
   * <p>Beware that, unlike in most collections, the {@code size} method
   * is <em>NOT</em> a constant-time operation. Because of the
   * asynchronous nature of these queues, determining the current number
   * of elements requires a traversal of the elements, and so may report
   * inaccurate results if this collection is modified during traversal.
   * Additionally, the bulk operations {@code addAll},
   * {@code removeAll}, {@code retainAll}, {@code containsAll},
   * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
   * to be performed atomically. For example, an iterator operating
   * concurrently with an {@code addAll} operation might view only some
   * of the added elements.
   *
   * <p>This class and its iterator implement all of the
   * <em>optional</em> methods of the {@link Collection} and {@link
   * Iterator} interfaces.
   *
   * <p>Memory consistency effects: As with other concurrent
   * collections, actions in a thread prior to placing an object into a
   * {@code LinkedTransferQueue}
   * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
   * actions subsequent to the access or removal of that element from
   * the {@code LinkedTransferQueue} in another thread.
   *
   * @since 1.7
   * @author Doug Lea
   * @param <E> the type of elements held in this queue
   */
  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/~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 it might help,
       *    and will not hurt, in limiting impact of spinning on busy
       *    systems.  We also use smaller (1/2) spins for nodes that are
       *    not known to be front but whose predecessors have not
       *    blocked -- these "chained" spins avoid artifacts of
       *    front-of-queue rules which otherwise lead to alternating
       *    nodes spinning vs blocking. Further, front threads that
       *    represent phase changes (from data to request node or vice
       *    versa) compared to their predecessors receive additional
       *    chained spins, reflecting longer paths typically required to
       *    unblock threads during phase changes.
       *
       *
       * ** Unlinking removed interior nodes **
       *
       * In addition to minimizing garbage retention via self-linking
       * described above, we also unlink removed interior nodes. These
       * may arise due to timed out or interrupted waits, or calls to
       * remove(x) or Iterator.remove.  Normally, given a node that was
       * at one time known to be the predecessor of some node s that is
       * to be removed, we can unsplice s by CASing the next field of
       * its predecessor if it still points to s (otherwise s must
       * already have been removed or is now offlist). But there are two
       * situations in which we cannot guarantee to make node s
       * unreachable in this way: (1) If s is the trailing node of list
       * (i.e., with null next), then it is pinned as the target node
       * for appends, so can only be removed later after other nodes are
       * appended. (2) We cannot necessarily unlink s given a
       * predecessor node that is matched (including the case of being
       * cancelled): the predecessor may already be unspliced, in which
       * case some previous reachable node may still point to s.
       * (For further explanation see Herlihy & Shavit "The Art of
       * Multiprocessor Programming" chapter 9).  Although, in both
       * cases, we can rule out the need for further action if either s
       * or its predecessor are (or can be made to be) at, or fall off
       * from, the head of list.
       *
       * Without taking these into account, it would be possible for an
       * unbounded number of supposedly removed nodes to remain
       * reachable.  Situations leading to such buildup are uncommon but
       * can occur in practice; for example when a series of short timed
       * calls to poll repeatedly time out but never otherwise fall off
       * the list because of an untimed call to take at the front of the
       * queue.
       *
       * When these cases arise, rather than always retraversing the
       * entire list to find an actual predecessor to unlink (which
       * won't help for case (1) anyway), we record a conservative
       * estimate of possible unsplice failures (in "sweepVotes").
       * We trigger a full sweep when the estimate exceeds a threshold
       * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
       * removal failures to tolerate before sweeping through, unlinking
       * cancelled nodes that were not unlinked upon initial removal.
       * We perform sweeps by the thread hitting threshold (rather than
       * background threads or by spreading work to other threads)
       * because in the main contexts in which removal occurs, the
       * caller is already timed-out, cancelled, or performing a
       * potentially O(n) operation (e.g. remove(x)), none of which are
       * time-critical enough to warrant the overhead that alternatives
       * would impose on other threads.
       *
       * Because the sweepVotes estimate is conservative, and because
       * nodes become unlinked "naturally" as they fall off the head of
       * the queue, and because we allow votes to accumulate even while
       * sweeps are in progress, there are typically significantly fewer
       * such nodes than estimated.  Choice of a threshold value
       * balances the likelihood of wasted effort and contention, versus
       * providing a worst-case bound on retention of interior nodes in
       * quiescent queues. The value defined below was chosen
       * empirically to balance these under various timeout scenarios.
       *
       * Note that we cannot self-link unlinked interior nodes during
       * sweeps. However, the associated garbage chains terminate when
       * some successor ultimately falls off the head of the list and is
       * self-linked.
       */
  
      /** True if on multiprocessor */
      private static final boolean MP =
          Runtime.getRuntime().availableProcessors() > 1;
  
      /**
       * The number of times to spin (with randomly interspersed calls
       * to Thread.yield) on multiprocessor before blocking when a node
       * is apparently the first waiter in the queue.  See above for
       * explanation. Must be a power of two. The value is empirically
       * derived -- it works pretty well across a variety of processors,
       * numbers of CPUs, and OSes.
       */
      private static final int FRONT_SPINS   = 1 << 7;
  
      /**
       * The number of times to spin before blocking when a node is
       * preceded by another node that is apparently spinning.  Also
       * serves as an increment to FRONT_SPINS on phase changes, and as
       * base average frequency for yielding during spins. Must be a
       * power of two.
       */
      private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
  
      /**
       * The maximum number of estimated removal failures (sweepVotes)
       * to tolerate before sweeping through the queue unlinking
       * cancelled nodes that were not unlinked upon initial
       * removal. See above for explanation. The value must be at least
       * two to avoid useless sweeps when removing trailing nodes.
       */
      static final int SWEEP_THRESHOLD = 32;
  
      /**
       * Queue nodes. Uses Object, not E, for items to allow forgetting
       * them after use.  Relies heavily on Unsafe mechanics to minimize
       * unnecessary ordering constraints: Writes that are intrinsically
       * ordered wrt other accesses or CASes use simple relaxed forms.
       */
      static final class Node {
          final boolean isData;   // false if this is a request node
          volatile Object item;   // initially non-null if isData; CASed to match
          volatile Node next;
          volatile Thread waiter; // null until waiting
  
          // CAS methods for fields
          final boolean casNext(Node cmp, Node val) {
              return U.compareAndSwapObject(this, NEXT, cmp, val);
          }
  
          final boolean casItem(Object cmp, Object val) {
              // assert cmp == null || cmp.getClass() != Node.class;
              return U.compareAndSwapObject(this, ITEM, cmp, val);
          }
  
          /**
           * Constructs a new node.  Uses relaxed write because item can
           * only be seen after publication via casNext.
           */
          Node(Object item, boolean isData) {
              U.putObject(this, ITEM, 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() {
              U.putObject(this, NEXT, this);
          }
  
          /**
           * Sets item to self and waiter to null, to avoid garbage
           * retention after matching or cancelling. Uses relaxed writes
           * because order is already constrained in the only calling
           * contexts: item is forgotten only after volatile/atomic
           * mechanics that extract items.  Similarly, clearing waiter
           * follows either CAS or return from park (if ever parked;
           * else we don't care).
           */
          final void forgetContents() {
              U.putObject(this, ITEM, this);
              U.putObject(this, WAITER, null);
          }
  
          /**
           * Returns true if this node has been matched, including the
           * case of artificial matches due to cancellation.
           */
          final boolean isMatched() {
              Object x = item;
              return (x == this) || ((x == null) == isData);
          }
  
          /**
           * Returns true if this is an unmatched request node.
           */
          final boolean isUnmatchedRequest() {
              return !isData && item == null;
          }
  
          /**
           * Returns true if a node with the given mode cannot be
           * appended to this node because this node is unmatched and
           * has opposite data mode.
           */
          final boolean cannotPrecede(boolean haveData) {
              boolean d = isData;
              Object x;
              return d != haveData && (x = item) != this && (x != null) == d;
          }
  
          /**
           * Tries to artificially match a data node -- used by remove.
           */
          final boolean tryMatchData() {
              // assert isData;
              Object x = item;
              if (x != null && x != this && casItem(x, null)) {
                  LockSupport.unpark(waiter);
                  return true;
              }
              return false;
          }
  
          private static final long serialVersionUID = -3375979862319811754L;
  
          // Unsafe mechanics
          private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
          private static final long ITEM;
          private static final long NEXT;
          private static final long WAITER;
          static {
              try {
                  ITEM = U.objectFieldOffset
                      (Node.class.getDeclaredField("item"));
                  NEXT = U.objectFieldOffset
                      (Node.class.getDeclaredField("next"));
                  WAITER = U.objectFieldOffset
                      (Node.class.getDeclaredField("waiter"));
              } catch (ReflectiveOperationException e) {
                  throw new Error(e);
              }
          }
      }
  
      /** head of the queue; null until first enqueue */
      transient volatile Node head;
  
      /** tail of the queue; null until first append */
      private transient volatile Node tail;
  
      /** The number of apparent failures to unsplice removed nodes */
      private transient volatile int sweepVotes;
  
      // CAS methods for fields
      private boolean casTail(Node cmp, Node val) {
          return U.compareAndSwapObject(this, TAIL, cmp, val);
      }
  
      private boolean casHead(Node cmp, Node val) {
          return U.compareAndSwapObject(this, HEAD, cmp, val);
      }
  
      private boolean casSweepVotes(int cmp, int val) {
          return U.compareAndSwapInt(this, SWEEPVOTES, 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
  
      /**
       * 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);
                          @SuppressWarnings("unchecked") E itemE = (E) item;
                          return itemE;
                      }
                  }
                  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) {
          final long deadline = timed ? System.nanoTime() + nanos : 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
                  @SuppressWarnings("unchecked") E itemE = (E) item;
                  return itemE;
              }
              else if (w.isInterrupted() || (timed && nanos <= 0L)) {
                  unsplice(pred, s);           // try to unlink and cancel
                  if (s.casItem(e, s))         // return normally if lost CAS
                      return e;
              }
              else 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) {
                  nanos = deadline - System.nanoTime();
                  if (nanos > 0L)
                      LockSupport.parkNanos(this, nanos);
              }
              else {
                  LockSupport.park(this);
              }
          }
      }
  
      /**
       * Returns spin/yield value for a node with given predecessor and
       * data mode. See above for explanation.
       */
      private static int spinsFor(Node pred, boolean haveData) {
          if (MP && pred != null) {
              if (pred.isData != haveData)      // phase change
                  return FRONT_SPINS + CHAINED_SPINS;
              if (pred.isMatched())             // probably at front
                  return FRONT_SPINS;
              if (pred.waiter == null)          // pred apparently spinning
                  return CHAINED_SPINS;
          }
          return 0;
      }
  
      /* -------------- Traversal methods -------------- */
  
      /**
       * Returns the successor of p, or the head node if p.next has been
       * linked to self, which will only be true if traversing with a
       * stale pointer that is now off the list.
       */
      final Node succ(Node p) {
          Node next = p.next;
          return (p == next) ? head : next;
      }
  
      /**
       * Returns the first unmatched data node, or null if none.
       * Callers must recheck if the returned node's item field is null
       * or self-linked before using.
       */
      final Node firstDataNode() {
          restartFromHead: for (;;) {
              for (Node p = head; p != null;) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p)
                          return p;
                  }
                  else if (item == null)
                      break;
                  if (p == (p = p.next))
                      continue restartFromHead;
              }
              return null;
          }
      }
  
      /**
       * Traverses and counts unmatched nodes of the given mode.
       * Used by methods size and getWaitingConsumerCount.
       */
      private int countOfMode(boolean data) {
          restartFromHead: for (;;) {
              int count = 0;
              for (Node p = head; p != null;) {
                  if (!p.isMatched()) {
                      if (p.isData != data)
                          return 0;
                      if (++count == Integer.MAX_VALUE)
                          break;  // @see Collection.size()
                  }
                  if (p == (p = p.next))
                      continue restartFromHead;
              }
              return count;
          }
      }
  
      public String toString() {
          String[] a = null;
          restartFromHead: for (;;) {
              int charLength = 0;
              int size = 0;
              for (Node p = head; p != null;) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p) {
                          if (a == null)
                              a = new String[4];
                          else if (size == a.length)
                              a = Arrays.copyOf(a, 2 * size);
                          String s = item.toString();
                          a[size++] = s;
                          charLength += s.length();
                      }
                  } else if (item == null)
                      break;
                  if (p == (p = p.next))
                      continue restartFromHead;
              }
  
              if (size == 0)
                  return "[]";
  
              return Helpers.toString(a, size, charLength);
          }
      }
  
      private Object[] toArrayInternal(Object[] a) {
          Object[] x = a;
          restartFromHead: for (;;) {
              int size = 0;
              for (Node p = head; p != null;) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p) {
                          if (x == null)
                              x = new Object[4];
                          else if (size == x.length)
                              x = Arrays.copyOf(x, 2 * (size + 4));
                          x[size++] = item;
                      }
                  } else if (item == null)
                      break;
                  if (p == (p = p.next))
                      continue restartFromHead;
              }
              if (x == null)
                  return new Object[0];
              else if (a != null && size <= a.length) {
                  if (a != x)
                      System.arraycopy(x, 0, a, 0, size);
                  if (size < a.length)
                      a[size] = null;
                  return a;
              }
              return (size == x.length) ? x : Arrays.copyOf(x, size);
          }
      }
  
      /**
       * Returns an array containing all of the elements in this queue, in
       * proper sequence.
       *
       * <p>The returned array will be "safe" in that no references to it are
       * maintained by this queue.  (In other words, this method must allocate
       * a new array).  The caller is thus free to modify the returned array.
       *
       * <p>This method acts as bridge between array-based and collection-based
       * APIs.
       *
       * @return an array containing all of the elements in this queue
       */
      public Object[] toArray() {
          return toArrayInternal(null);
      }
  
      /**
       * Returns an array containing all of the elements in this queue, in
       * proper sequence; the runtime type of the returned array is that of
       * the specified array.  If the queue fits in the specified array, it
       * is returned therein.  Otherwise, a new array is allocated with the
       * runtime type of the specified array and the size of this queue.
       *
       * <p>If this queue fits in the specified array with room to spare
       * (i.e., the array has more elements than this queue), the element in
       * the array immediately following the end of the queue is set to
       * {@code null}.
       *
       * <p>Like the {@link #toArray()} method, this method acts as bridge between
       * array-based and collection-based APIs.  Further, this method allows
       * precise control over the runtime type of the output array, and may,
       * under certain circumstances, be used to save allocation costs.
       *
       * <p>Suppose {@code x} is a queue known to contain only strings.
       * The following code can be used to dump the queue into a newly
       * allocated array of {@code String}:
       *
       * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
       *
       * Note that {@code toArray(new Object[0])} is identical in function to
       * {@code toArray()}.
       *
       * @param a the array into which the elements of the queue are to
       *          be stored, if it is big enough; otherwise, a new array of the
       *          same runtime type is allocated for this purpose
       * @return an array containing all of the elements in this queue
       * @throws ArrayStoreException if the runtime type of the specified array
       *         is not a supertype of the runtime type of every element in
       *         this queue
       * @throws NullPointerException if the specified array is null
       */
      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
          if (a == null) throw new NullPointerException();
          return (T[]) toArrayInternal(a);
      }
  
      final class Itr implements Iterator<E> {
          private Node nextNode;   // next node to return item for
          private E nextItem;      // the corresponding item
          private Node lastRet;    // last returned node, to support remove
          private Node lastPred;   // predecessor to unlink lastRet
  
          /**
           * Moves to next node after prev, or first node if prev null.
           */
          private void advance(Node prev) {
              /*
               * To track and avoid buildup of deleted nodes in the face
               * of calls to both Queue.remove and Itr.remove, we must
               * include variants of unsplice and sweep upon each
               * advance: Upon Itr.remove, we may need to catch up links
               * from lastPred, and upon other removes, we might need to
               * skip ahead from stale nodes and unsplice deleted ones
               * found while advancing.
               */
  
              Node r, b; // reset lastPred upon possible deletion of lastRet
              if ((r = lastRet) != null && !r.isMatched())
                  lastPred = r;    // next lastPred is old lastRet
              else if ((b = lastPred) == null || b.isMatched())
                  lastPred = null; // at start of list
              else {
                  Node s, n;       // help with removal of lastPred.next
                  while ((s = b.next) != null &&
                         s != b && s.isMatched() &&
                         (n = s.next) != null && n != s)
                      b.casNext(s, n);
              }
  
              this.lastRet = prev;
  
              for (Node p = prev, s, n;;) {
                  s = (p == null) ? head : p.next;
                  if (s == null)
                      break;
                  else if (s == p) {
                      p = null;
                      continue;
                  }
                  Object item = s.item;
                  if (s.isData) {
                      if (item != null && item != s) {
                          @SuppressWarnings("unchecked") E itemE = (E) item;
                          nextItem = itemE;
                          nextNode = s;
                          return;
                      }
                  }
                  else if (item == null)
                      break;
                  // assert s.isMatched();
                  if (p == null)
                      p = s;
                  else if ((n = s.next) == null)
                      break;
                  else if (s == n)
                      p = null;
                  else
                      p.casNext(s, n);
              }
              nextNode = null;
              nextItem = null;
          }
  
          Itr() {
              advance(null);
          }
  
          public final boolean hasNext() {
              return nextNode != null;
          }
  
          public final E next() {
              Node p = nextNode;
              if (p == null) throw new NoSuchElementException();
              E e = nextItem;
              advance(p);
              return e;
          }
  
          public final void remove() {
              final Node lastRet = this.lastRet;
              if (lastRet == null)
                  throw new IllegalStateException();
              this.lastRet = null;
              if (lastRet.tryMatchData())
                  unsplice(lastPred, lastRet);
          }
      }
  
      /** A customized variant of Spliterators.IteratorSpliterator */
      final class LTQSpliterator<E> implements Spliterator<E> {
          static final int MAX_BATCH = 1 << 25;  // max batch array size;
          Node current;       // current node; null until initialized
          int batch;          // batch size for splits
          boolean exhausted;  // true when no more nodes
          LTQSpliterator() {}
  
          public Spliterator<E> trySplit() {
              Node p;
              int b = batch;
              int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
              if (!exhausted &&
                  ((p = current) != null || (p = firstDataNode()) != null) &&
                  p.next != null) {
                  Object[] a = new Object[n];
                  int i = 0;
                  do {
                      Object e = p.item;
                      if (e != p && (a[i] = e) != null)
                          ++i;
                      if (p == (p = p.next))
                          p = firstDataNode();
                  } while (p != null && i < n && p.isData);
                  if ((current = p) == null)
                      exhausted = true;
                  if (i > 0) {
                      batch = i;
                      return Spliterators.spliterator
                          (a, 0, i, (Spliterator.ORDERED |
                                     Spliterator.NONNULL |
                                     Spliterator.CONCURRENT));
                  }
              }
              return null;
          }
  
          @SuppressWarnings("unchecked")
          public void forEachRemaining(Consumer<? super E> action) {
              Node p;
              if (action == null) throw new NullPointerException();
              if (!exhausted &&
                  ((p = current) != null || (p = firstDataNode()) != null)) {
                  exhausted = true;
                  do {
                      Object e = p.item;
                      if (e != null && e != p)
                          action.accept((E)e);
                      if (p == (p = p.next))
                          p = firstDataNode();
                  } while (p != null && p.isData);
              }
          }
  
          @SuppressWarnings("unchecked")
          public boolean tryAdvance(Consumer<? super E> action) {
              Node p;
              if (action == null) throw new NullPointerException();
              if (!exhausted &&
                  ((p = current) != null || (p = firstDataNode()) != null)) {
                  Object e;
                  do {
                      if ((e = p.item) == p)
                          e = null;
                      if (p == (p = p.next))
                          p = firstDataNode();
                  } while (e == null && p != null && p.isData);
                  if ((current = p) == null)
                      exhausted = true;
                  if (e != null) {
                      action.accept((E)e);
                      return true;
                  }
              }
              return false;
          }
  
          public long estimateSize() { return Long.MAX_VALUE; }
  
          public int characteristics() {
              return Spliterator.ORDERED | Spliterator.NONNULL |
                  Spliterator.CONCURRENT;
          }
      }
  
      /**
       * Returns a {@link Spliterator} over the elements in this queue.
       *
       * <p>The returned spliterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
       * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
       *
       * @implNote
       * The {@code Spliterator} implements {@code trySplit} to permit limited
       * parallelism.
       *
       * @return a {@code Spliterator} over the elements in this queue
       * @since 1.8
       */
      public Spliterator<E> spliterator() {
          return new LTQSpliterator<E>();
      }
  
      /* -------------- 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.waiter = null; // disable signals
          /*
           * See above for rationale. Briefly: if pred still points to
           * s, try to unlink s.  If s cannot be unlinked, because it is
           * trailing node or pred might be unlinked, and neither pred
           * nor s are head or offlist, add to sweepVotes, and if enough
           * votes have accumulated, sweep.
           */
          if (pred != null && pred != s && pred.next == s) {
              Node n = s.next;
              if (n == null ||
                  (n != s && pred.casNext(s, n) && pred.isMatched())) {
                  for (;;) {               // check if at, or could be, head
                      Node h = head;
                      if (h == pred || h == s || h == null)
                          return;          // at head or list empty
                      if (!h.isMatched())
                          break;
                      Node hn = h.next;
                      if (hn == null)
                          return;          // now empty
                      if (hn != h && casHead(h, hn))
                          h.forgetNext();  // advance head
                  }
                  if (pred.next != pred && s.next != s) { // recheck if offlist
                      for (;;) {           // sweep now if enough votes
                          int v = sweepVotes;
                          if (v < SWEEP_THRESHOLD) {
                              if (casSweepVotes(v, v + 1))
                                  break;
                          }
                          else if (casSweepVotes(v, 0)) {
                              sweep();
                              break;
                          }
                      }
                  }
              }
          }
      }
  
      /**
       * Unlinks matched (typically cancelled) nodes encountered in a
       * traversal from head.
       */
      private void sweep() {
          for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
              if (!s.isMatched())
                  // Unmatched nodes are never self-linked
                  p = s;
              else if ((n = s.next) == null) // trailing node is pinned
                  break;
              else if (s == n)    // stale
                  // No need to also check for p == s, since that implies s == n
                  p = head;
              else
                  p.casNext(s, n);
          }
      }
  
      /**
       * Main implementation of remove(Object)
       */
      private boolean findAndRemove(Object e) {
          if (e != null) {
              for (Node pred = null, p = head; p != null; ) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p && e.equals(item) &&
                          p.tryMatchData()) {
                          unsplice(pred, p);
                          return true;
                      }
                  }
                  else if (item == null)
                      break;
                  pred = p;
                  if ((p = p.next) == pred) { // stale
                      pred = null;
                      p = head;
                  }
              }
          }
          return false;
      }
  
      /**
       * Creates an initially empty {@code LinkedTransferQueue}.
       */
      public LinkedTransferQueue() {
      }
  
      /**
       * Creates a {@code LinkedTransferQueue}
       * initially containing the elements of the given collection,
       * added in traversal order of the collection's iterator.
       *
       * @param c the collection of elements to initially contain
       * @throws NullPointerException if the specified collection or any
       *         of its elements are null
       */
      public LinkedTransferQueue(Collection<? extends E> c) {
          this();
          addAll(c);
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never block.
       *
       * @throws NullPointerException if the specified element is null
       */
      public void put(E e) {
          xfer(e, true, ASYNC, 0);
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never block or
       * return {@code false}.
       *
       * @return {@code true} (as specified by
       *  {@link BlockingQueue#offer(Object,long,TimeUnit)
       *  BlockingQueue.offer})
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e, long timeout, TimeUnit unit) {
          xfer(e, true, ASYNC, 0);
          return true;
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never return {@code false}.
       *
       * @return {@code true} (as specified by {@link Queue#offer})
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e) {
          xfer(e, true, ASYNC, 0);
          return true;
      }
  
      /**
       * Inserts the specified element at the tail of this queue.
       * As the queue is unbounded, this method will never throw
       * {@link IllegalStateException} or return {@code false}.
       *
       * @return {@code true} (as specified by {@link Collection#add})
       * @throws NullPointerException if the specified element is null
       */
      public boolean add(E e) {
          xfer(e, true, ASYNC, 0);
          return true;
      }
  
      /**
       * Transfers the element to a waiting consumer immediately, if possible.
       *
       * <p>More precisely, transfers the specified element immediately
       * if there exists a consumer already waiting to receive it (in
       * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
       * otherwise returning {@code false} without enqueuing the element.
       *
       * @throws NullPointerException if the specified element is null
       */
      public boolean tryTransfer(E e) {
          return xfer(e, true, NOW, 0) == null;
      }
  
      /**
       * Transfers the element to a consumer, waiting if necessary to do so.
       *
       * <p>More precisely, transfers the specified element immediately
       * if there exists a consumer already waiting to receive it (in
       * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
       * else inserts the specified element at the tail of this queue
       * and waits until the element is received by a consumer.
       *
       * @throws NullPointerException if the specified element is null
       */
      public void transfer(E e) throws InterruptedException {
          if (xfer(e, true, SYNC, 0) != null) {
              Thread.interrupted(); // failure possible only due to interrupt
              throw new InterruptedException();
          }
      }
  
      /**
       * Transfers the element to a consumer if it is possible to do so
       * before the timeout elapses.
       *
       * <p>More precisely, transfers the specified element immediately
       * if there exists a consumer already waiting to receive it (in
       * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
       * else inserts the specified element at the tail of this queue
       * and waits until the element is received by a consumer,
       * returning {@code false} if the specified wait time elapses
       * before the element can be transferred.
       *
       * @throws NullPointerException if the specified element is null
       */
      public boolean tryTransfer(E e, long timeout, TimeUnit unit)
          throws InterruptedException {
          if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
              return true;
          if (!Thread.interrupted())
              return false;
          throw new InterruptedException();
      }
  
      public E take() throws InterruptedException {
          E e = xfer(null, false, SYNC, 0);
          if (e != null)
              return e;
          Thread.interrupted();
          throw new InterruptedException();
      }
  
      public E poll(long timeout, TimeUnit unit) throws InterruptedException {
          E e = xfer(null, false, TIMED, unit.toNanos(timeout));
          if (e != null || !Thread.interrupted())
              return e;
          throw new InterruptedException();
      }
  
      public E poll() {
          return xfer(null, false, NOW, 0);
      }
  
      /**
       * @throws NullPointerException     {@inheritDoc}
       * @throws IllegalArgumentException {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c) {
          if (c == null)
              throw new NullPointerException();
          if (c == this)
              throw new IllegalArgumentException();
          int n = 0;
          for (E e; (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;
          for (E e; n < maxElements && (e = poll()) != null;) {
              c.add(e);
              ++n;
          }
          return n;
      }
  
      /**
       * Returns an iterator over the elements in this queue in proper sequence.
       * The elements will be returned in order from first (head) to last (tail).
       *
       * <p>The returned iterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * @return an iterator over the elements in this queue in proper sequence
       */
      public Iterator<E> iterator() {
          return new Itr();
      }
  
      public E peek() {
          restartFromHead: for (;;) {
              for (Node p = head; p != null;) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p) {
                          @SuppressWarnings("unchecked") E e = (E) item;
                          return e;
                      }
                  }
                  else if (item == null)
                      break;
                  if (p == (p = p.next))
                      continue restartFromHead;
              }
              return null;
          }
      }
  
      /**
       * Returns {@code true} if this queue contains no elements.
       *
       * @return {@code true} if this queue contains no elements
       */
      public boolean isEmpty() {
          return firstDataNode() == null;
      }
  
      public boolean hasWaitingConsumer() {
          restartFromHead: for (;;) {
              for (Node p = head; p != null;) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p)
                          break;
                  }
                  else if (item == null)
                      return true;
                  if (p == (p = p.next))
                      continue restartFromHead;
              }
              return false;
          }
      }
  
      /**
       * Returns the number of elements in this queue.  If this queue
       * contains more than {@code Integer.MAX_VALUE} elements, returns
       * {@code Integer.MAX_VALUE}.
       *
       * <p>Beware that, unlike in most collections, this method is
       * <em>NOT</em> a constant-time operation. Because of the
       * asynchronous nature of these queues, determining the current
       * number of elements requires an O(n) traversal.
       *
       * @return the number of elements in this queue
       */
      public int size() {
          return countOfMode(true);
      }
  
      public int getWaitingConsumerCount() {
          return countOfMode(false);
      }
  
      /**
       * Removes a single instance of the specified element from this queue,
       * if it is present.  More formally, removes an element {@code e} such
       * that {@code o.equals(e)}, if this queue contains one or more such
       * elements.
       * Returns {@code true} if this queue contained the specified element
       * (or equivalently, if this queue changed as a result of the call).
       *
       * @param o element to be removed from this queue, if present
       * @return {@code true} if this queue changed as a result of the call
       */
      public boolean remove(Object o) {
          return findAndRemove(o);
      }
  
      /**
       * Returns {@code true} if this queue contains the specified element.
       * More formally, returns {@code true} if and only if this queue contains
       * at least one element {@code e} such that {@code o.equals(e)}.
       *
       * @param o object to be checked for containment in this queue
       * @return {@code true} if this queue contains the specified element
       */
      public boolean contains(Object o) {
          if (o != null) {
              for (Node p = head; p != null; p = succ(p)) {
                  Object item = p.item;
                  if (p.isData) {
                      if (item != null && item != p && o.equals(item))
                          return true;
                  }
                  else if (item == null)
                      break;
              }
          }
          return false;
      }
  
      /**
       * Always returns {@code Integer.MAX_VALUE} because a
       * {@code LinkedTransferQueue} is not capacity constrained.
       *
       * @return {@code Integer.MAX_VALUE} (as specified by
       *         {@link BlockingQueue#remainingCapacity()
       *         BlockingQueue.remainingCapacity})
       */
      public int remainingCapacity() {
          return Integer.MAX_VALUE;
      }
  
      /**
       * Saves this queue to a stream (that is, serializes it).
       *
       * @param s the stream
       * @throws java.io.IOException if an I/O error occurs
       * @serialData All of the elements (each an {@code E}) in
       * the proper order, followed by a null
       */
      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 this queue from a stream (that is, deserializes it).
       * @param s the stream
       * @throws ClassNotFoundException if the class of a serialized object
       *         could not be found
       * @throws java.io.IOException if an I/O error occurs
       */
      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 sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
      private static final long HEAD;
      private static final long TAIL;
      private static final long SWEEPVOTES;
      static {
          try {
              HEAD = U.objectFieldOffset
                  (LinkedTransferQueue.class.getDeclaredField("head"));
              TAIL = U.objectFieldOffset
                  (LinkedTransferQueue.class.getDeclaredField("tail"));
              SWEEPVOTES = U.objectFieldOffset
                  (LinkedTransferQueue.class.getDeclaredField("sweepVotes"));
          } catch (ReflectiveOperationException e) {
              throw new Error(e);
          }
  
          // Reduce the risk of rare disastrous classloading in first call to
          // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
          Class<?> ensureLoaded = LockSupport.class;
      }
  }
  