/*

  * 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 and Martin Buchholz 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.AbstractCollection;

 import java.util.ArrayList;

 import java.util.Collection;

 import java.util.Deque;

 import java.util.Iterator;

 import java.util.NoSuchElementException;

 import java.util.Queue;

 /**

  * An unbounded concurrent {@linkplain Deque deque} based on linked nodes.

  * Concurrent insertion, removal, and access operations execute safely

  * across multiple threads.

  * A {@code ConcurrentLinkedDeque} is an appropriate choice when

  * many threads will share access to a common collection.

  * Like most other concurrent collection implementations, this class

  * does not permit the use of {@code null} elements.

  *

  * <p>Iterators are <i>weakly consistent</i>, returning elements

  * reflecting the state of the deque at some point at or since the

  * creation of the iterator.  They do <em>not</em> throw {@link

  * java.util.ConcurrentModificationException

  * ConcurrentModificationException}, and may proceed concurrently with

  * other operations.

  *

  * <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 deques, 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 Deque} 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 ConcurrentLinkedDeque}

  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

  * actions subsequent to the access or removal of that element from

  * the {@code ConcurrentLinkedDeque} 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

  * @author Martin Buchholz

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

  */

 public class ConcurrentLinkedDeque<E>

     extends AbstractCollection<E>

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

     /*

      * This is an implementation of a concurrent lock-free deque

      * supporting interior removes but not interior insertions, as

      * required to support the entire Deque interface.

      *

      * We extend the techniques developed for ConcurrentLinkedQueue and

      * LinkedTransferQueue (see the internal docs for those classes).

      * Understanding the ConcurrentLinkedQueue implementation is a

      * prerequisite for understanding the implementation of this class.

      *

      * The data structure is a symmetrical doubly-linked "GC-robust"

      * linked list of nodes.  We minimize the number of volatile writes

      * using two techniques: advancing multiple hops with a single CAS

      * and mixing volatile and non-volatile writes of the same memory

      * locations.

      *

      * A node contains the expected E ("item") and links to predecessor

      * ("prev") and successor ("next") nodes:

      *

      * class Node<E> { volatile Node<E> prev, next; volatile E item; }

      *

      * A node p is considered "live" if it contains a non-null item

      * (p.item != null).  When an item is CASed to null, the item is

      * atomically logically deleted from the collection.

      *

      * At any time, there is precisely one "first" node with a null

      * prev reference that terminates any chain of prev references

      * starting at a live node.  Similarly there is precisely one

      * "last" node terminating any chain of next references starting at

      * a live node.  The "first" and "last" nodes may or may not be live.

      * The "first" and "last" nodes are always mutually reachable.

      *

      * A new element is added atomically by CASing the null prev or

      * next reference in the first or last node to a fresh node

      * containing the element.  The element's node atomically becomes

      * "live" at that point.

      *

      * A node is considered "active" if it is a live node, or the

      * first or last node.  Active nodes cannot be unlinked.

      *

      * A "self-link" is a next or prev reference that is the same node:

      *   p.prev == p  or  p.next == p

      * Self-links are used in the node unlinking process.  Active nodes

      * never have self-links.

      *

      * A node p is active if and only if:

      *

      * p.item != null ||

      * (p.prev == null && p.next != p) ||

      * (p.next == null && p.prev != p)

      *

      * The deque object has two node references, "head" and "tail".

      * The head and tail are only approximations to the first and last

      * nodes of the deque.  The first node can always be found by

      * following prev pointers from head; likewise for tail.  However,

      * it is permissible for head and tail to be referring to deleted

      * nodes that have been unlinked and so may not be reachable from

      * any live node.

      *

      * There are 3 stages of node deletion;

      * "logical deletion", "unlinking", and "gc-unlinking".

      *

      * 1. "logical deletion" by CASing item to null atomically removes

      * the element from the collection, and makes the containing node

      * eligible for unlinking.

      *

      * 2. "unlinking" makes a deleted node unreachable from active

      * nodes, and thus eventually reclaimable by GC.  Unlinked nodes

      * may remain reachable indefinitely from an iterator.

      *

      * Physical node unlinking is merely an optimization (albeit a

      * critical one), and so can be performed at our convenience.  At

      * any time, the set of live nodes maintained by prev and next

      * links are identical, that is, the live nodes found via next

      * links from the first node is equal to the elements found via

      * prev links from the last node.  However, this is not true for

      * nodes that have already been logically deleted - such nodes may

      * be reachable in one direction only.

      *

      * 3. "gc-unlinking" takes unlinking further by making active

      * nodes unreachable from deleted nodes, making it easier for the

      * GC to reclaim future deleted nodes.  This step makes the data

      * structure "gc-robust", as first described in detail by Boehm

      * (http://portal.acm.org/citation.cfm?doid=503272.503282).

      *

      * GC-unlinked nodes may remain reachable indefinitely from an

      * iterator, but unlike unlinked nodes, are never reachable from

      * head or tail.

      *

      * Making the data structure GC-robust will eliminate the risk of

      * unbounded memory retention with conservative GCs and is likely

      * to improve performance with generational GCs.

      *

      * When a node is dequeued at either end, e.g. via poll(), we would

      * like to break any references from the node to active nodes.  We

      * develop further the use of self-links that was very effective in

      * other concurrent collection classes.  The idea is to replace

      * prev and next pointers with special values that are interpreted

      * to mean off-the-list-at-one-end.  These are approximations, but

      * good enough to preserve the properties we want in our

      * traversals, e.g. we guarantee that a traversal will never visit

      * the same element twice, but we don't guarantee whether a

      * traversal that runs out of elements will be able to see more

      * elements later after enqueues at that end.  Doing gc-unlinking

      * safely is particularly tricky, since any node can be in use

      * indefinitely (for example by an iterator).  We must ensure that

      * the nodes pointed at by head/tail never get gc-unlinked, since

      * head/tail are needed to get "back on track" by other nodes that

      * are gc-unlinked.  gc-unlinking accounts for much of the

      * implementation complexity.

      *

      * Since neither unlinking nor gc-unlinking are necessary for

      * correctness, there are many implementation choices regarding

      * frequency (eagerness) of these operations.  Since volatile

      * reads are likely to be much cheaper than CASes, saving CASes by

      * unlinking multiple adjacent nodes at a time may be a win.

      * gc-unlinking can be performed rarely and still be effective,

      * since it is most important that long chains of deleted nodes

      * are occasionally broken.

      *

      * The actual representation we use is that p.next == p means to

      * goto the first node (which in turn is reached by following prev

      * pointers from head), and p.next == null && p.prev == p means

      * that the iteration is at an end and that p is a (static final)

      * dummy node, NEXT_TERMINATOR, and not the last active node.

      * Finishing the iteration when encountering such a TERMINATOR is

      * good enough for read-only traversals, so such traversals can use

      * p.next == null as the termination condition.  When we need to

      * find the last (active) node, for enqueueing a new node, we need

      * to check whether we have reached a TERMINATOR node; if so,

      * restart traversal from tail.

      *

      * The implementation is completely directionally symmetrical,

      * except that most public methods that iterate through the list

      * follow next pointers ("forward" direction).

      *

      * We believe (without full proof) that all single-element deque

      * operations (e.g., addFirst, peekLast, pollLast) are linearizable

      * (see Herlihy and Shavit's book).  However, some combinations of

      * operations are known not to be linearizable.  In particular,

      * when an addFirst(A) is racing with pollFirst() removing B, it is

      * possible for an observer iterating over the elements to observe

      * A B C and subsequently observe A C, even though no interior

      * removes are ever performed.  Nevertheless, iterators behave

      * reasonably, providing the "weakly consistent" guarantees.

      *

      * Empirically, microbenchmarks suggest that this class adds about

      * 40% overhead relative to ConcurrentLinkedQueue, which feels as

      * good as we can hope for.

      */

     private static final long serialVersionUID = 876323262645176354L;

     /**

      * A node from which the first node on list (that is, the unique node p

      * with p.prev == null && p.next != p) can be reached in O(1) time.

      * Invariants:

      * - the first node is always O(1) reachable from head via prev links

      * - all live nodes are reachable from the first node via succ()

      * - head != null

      * - (tmp = head).next != tmp || tmp != head

      * - head is never gc-unlinked (but may be unlinked)

      * Non-invariants:

      * - head.item may or may not be null

      * - head may not be reachable from the first or last node, or from tail

      */

     private transient volatile Node<E> head;

     /**

      * A node from which the last node on list (that is, the unique node p

      * with p.next == null && p.prev != p) can be reached in O(1) time.

      * Invariants:

      * - the last node is always O(1) reachable from tail via next links

      * - all live nodes are reachable from the last node via pred()

      * - tail != null

      * - tail is never gc-unlinked (but may be unlinked)

      * Non-invariants:

      * - tail.item may or may not be null

      * - tail may not be reachable from the first or last node, or from head

      */

     private transient volatile Node<E> tail;

     private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR;

     @SuppressWarnings("unchecked")

     Node<E> prevTerminator() {

         return (Node<E>) PREV_TERMINATOR;

     }

     @SuppressWarnings("unchecked")

     Node<E> nextTerminator() {

         return (Node<E>) NEXT_TERMINATOR;

     }

     static final class Node<E> {

         volatile Node<E> prev;

         volatile E item;

         volatile Node<E> next;

         Node() {  // default constructor for NEXT_TERMINATOR, PREV_TERMINATOR

         }

         /**

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

          * only be seen after publication via casNext or casPrev.

          */

         Node(E item) {

             this.item = item;

         }

         boolean casItem(E cmp, E val) {

             if (item == cmp) {

                 item = val;

                 return true;

             }

             return false;

         }

         void lazySetNext(Node<E> val) {

             this.next = val;

         }

         boolean casNext(Node<E> cmp, Node<E> val) {

             if (next == cmp) {

                 next = val;

                 return true;

             }

             return false;

         }

         void lazySetPrev(Node<E> val) {

             this.prev = val;

         }

         boolean casPrev(Node<E> cmp, Node<E> val) {

             if (prev == cmp) {

                 prev = val;

                 return true;

             }

             return false;

         }

     }

     /**

      * Links e as first element.

      */

     private void linkFirst(E e) {

         checkNotNull(e);

         final Node<E> newNode = new Node<E>(e);

         restartFromHead:

         for (;;)

             for (Node<E> h = head, p = h, q;;) {

                 if ((q = p.prev) != null &&

                     (q = (p = q).prev) != null)

                     // Check for head updates every other hop.

                     // If p == q, we are sure to follow head instead.

                     p = (h != (h = head)) ? h : q;

                 else if (p.next == p) // PREV_TERMINATOR

                     continue restartFromHead;

                 else {

                     // p is first node

                     newNode.lazySetNext(p); // CAS piggyback

                     if (p.casPrev(null, newNode)) {

                         // Successful CAS is the linearization point

                         // for e to become an element of this deque,

                         // and for newNode to become "live".

                         if (p != h) // hop two nodes at a time

                             casHead(h, newNode);  // Failure is OK.

                         return;

                     }

                     // Lost CAS race to another thread; re-read prev

                 }

             }

     }

     /**

      * Links e as last element.

      */

     private void linkLast(E e) {

         checkNotNull(e);

         final Node<E> newNode = new Node<E>(e);

         restartFromTail:

         for (;;)

             for (Node<E> t = tail, p = t, q;;) {

                 if ((q = p.next) != null &&

                     (q = (p = q).next) != null)

                     // Check for tail updates every other hop.

                     // If p == q, we are sure to follow tail instead.

                     p = (t != (t = tail)) ? t : q;

                 else if (p.prev == p) // NEXT_TERMINATOR

                     continue restartFromTail;

                 else {

                     // p is last node

                     newNode.lazySetPrev(p); // CAS piggyback

                     if (p.casNext(null, newNode)) {

                         // Successful CAS is the linearization point

                         // for e to become an element of this deque,

                         // and for newNode to become "live".

                         if (p != t) // hop two nodes at a time

                             casTail(t, newNode);  // Failure is OK.

                         return;

                     }

                     // Lost CAS race to another thread; re-read next

                 }

             }

     }

     private static final int HOPS = 2;

     /**

      * Unlinks non-null node x.

      */

     void unlink(Node<E> x) {

         // assert x != null;

         // assert x.item == null;

         // assert x != PREV_TERMINATOR;

         // assert x != NEXT_TERMINATOR;

         final Node<E> prev = x.prev;

         final Node<E> next = x.next;

         if (prev == null) {

             unlinkFirst(x, next);

         } else if (next == null) {

             unlinkLast(x, prev);

         } else {

             // Unlink interior node.

             //

             // This is the common case, since a series of polls at the

             // same end will be "interior" removes, except perhaps for

             // the first one, since end nodes cannot be unlinked.

             //

             // At any time, all active nodes are mutually reachable by

             // following a sequence of either next or prev pointers.

             //

             // Our strategy is to find the unique active predecessor

             // and successor of x.  Try to fix up their links so that

             // they point to each other, leaving x unreachable from

             // active nodes.  If successful, and if x has no live

             // predecessor/successor, we additionally try to gc-unlink,

             // leaving active nodes unreachable from x, by rechecking

             // that the status of predecessor and successor are

             // unchanged and ensuring that x is not reachable from

             // tail/head, before setting x's prev/next links to their

             // logical approximate replacements, self/TERMINATOR.

             Node<E> activePred, activeSucc;

             boolean isFirst, isLast;

             int hops = 1;

             // Find active predecessor

             for (Node<E> p = prev; ; ++hops) {

                 if (p.item != null) {

                     activePred = p;

                     isFirst = false;

                     break;

                 }

                 Node<E> q = p.prev;

                 if (q == null) {

                     if (p.next == p)

                         return;

                     activePred = p;

                     isFirst = true;

                     break;

                 }

                 else if (p == q)

                     return;

                 else

                     p = q;

             }

             // Find active successor

             for (Node<E> p = next; ; ++hops) {

                 if (p.item != null) {

                     activeSucc = p;

                     isLast = false;

                     break;

                 }

                 Node<E> q = p.next;

                 if (q == null) {

                     if (p.prev == p)

                         return;

                     activeSucc = p;

                     isLast = true;

                     break;

                 }

                 else if (p == q)

                     return;

                 else

                     p = q;

             }

             // TODO: better HOP heuristics

             if (hops < HOPS

                 // always squeeze out interior deleted nodes

                 && (isFirst | isLast))

                 return;

             // Squeeze out deleted nodes between activePred and

             // activeSucc, including x.

             skipDeletedSuccessors(activePred);

             skipDeletedPredecessors(activeSucc);

             // Try to gc-unlink, if possible

             if ((isFirst | isLast) &&

                 // Recheck expected state of predecessor and successor

                 (activePred.next == activeSucc) &&

                 (activeSucc.prev == activePred) &&

                 (isFirst ? activePred.prev == null : activePred.item != null) &&

                 (isLast  ? activeSucc.next == null : activeSucc.item != null)) {

                 updateHead(); // Ensure x is not reachable from head

                 updateTail(); // Ensure x is not reachable from tail

                 // Finally, actually gc-unlink

                 x.lazySetPrev(isFirst ? prevTerminator() : x);

                 x.lazySetNext(isLast  ? nextTerminator() : x);

             }

         }

     }

     /**

      * Unlinks non-null first node.

      */

     private void unlinkFirst(Node<E> first, Node<E> next) {

         // assert first != null;

         // assert next != null;

         // assert first.item == null;

         for (Node<E> o = null, p = next, q;;) {

             if (p.item != null || (q = p.next) == null) {

                 if (o != null && p.prev != p && first.casNext(next, p)) {

                     skipDeletedPredecessors(p);

                     if (first.prev == null &&

                         (p.next == null || p.item != null) &&

                         p.prev == first) {

                         updateHead(); // Ensure o is not reachable from head

                         updateTail(); // Ensure o is not reachable from tail

                         // Finally, actually gc-unlink

                         o.lazySetNext(o);

                         o.lazySetPrev(prevTerminator());

                     }

                 }

                 return;

             }

             else if (p == q)

                 return;

             else {

                 o = p;

                 p = q;

             }

         }

     }

     /**

      * Unlinks non-null last node.

      */

     private void unlinkLast(Node<E> last, Node<E> prev) {

         // assert last != null;

         // assert prev != null;

         // assert last.item == null;

         for (Node<E> o = null, p = prev, q;;) {

             if (p.item != null || (q = p.prev) == null) {

                 if (o != null && p.next != p && last.casPrev(prev, p)) {

                     skipDeletedSuccessors(p);

                     if (last.next == null &&

                         (p.prev == null || p.item != null) &&

                         p.next == last) {

                         updateHead(); // Ensure o is not reachable from head

                         updateTail(); // Ensure o is not reachable from tail

                         // Finally, actually gc-unlink

                         o.lazySetPrev(o);

                         o.lazySetNext(nextTerminator());

                     }

                 }

                 return;

             }

             else if (p == q)

                 return;

             else {

                 o = p;

                 p = q;

             }

         }

     }

     /**

      * Guarantees that any node which was unlinked before a call to

      * this method will be unreachable from head after it returns.

      * Does not guarantee to eliminate slack, only that head will

      * point to a node that was active while this method was running.

      */

     private final void updateHead() {

         // Either head already points to an active node, or we keep

         // trying to cas it to the first node until it does.

         Node<E> h, p, q;

         restartFromHead:

         while ((h = head).item == null && (p = h.prev) != null) {

             for (;;) {

                 if ((q = p.prev) == null ||

                     (q = (p = q).prev) == null) {

                     // It is possible that p is PREV_TERMINATOR,

                     // but if so, the CAS is guaranteed to fail.

                     if (casHead(h, p))

                         return;

                     else

                         continue restartFromHead;

                 }

                 else if (h != head)

                     continue restartFromHead;

                 else

                     p = q;

             }

         }

     }

     /**

      * Guarantees that any node which was unlinked before a call to

      * this method will be unreachable from tail after it returns.

      * Does not guarantee to eliminate slack, only that tail will

      * point to a node that was active while this method was running.

      */

     private final void updateTail() {

         // Either tail already points to an active node, or we keep

         // trying to cas it to the last node until it does.

         Node<E> t, p, q;

         restartFromTail:

         while ((t = tail).item == null && (p = t.next) != null) {

             for (;;) {

                 if ((q = p.next) == null ||

                     (q = (p = q).next) == null) {

                     // It is possible that p is NEXT_TERMINATOR,

                     // but if so, the CAS is guaranteed to fail.

                     if (casTail(t, p))

                         return;

                     else

                         continue restartFromTail;

                 }

                 else if (t != tail)

                     continue restartFromTail;

                 else

                     p = q;

             }

         }

     }

     private void skipDeletedPredecessors(Node<E> x) {

         whileActive:

         do {

             Node<E> prev = x.prev;

             // assert prev != null;

             // assert x != NEXT_TERMINATOR;

             // assert x != PREV_TERMINATOR;

             Node<E> p = prev;

             findActive:

             for (;;) {

                 if (p.item != null)

                     break findActive;

                 Node<E> q = p.prev;

                 if (q == null) {

                     if (p.next == p)

                         continue whileActive;

                     break findActive;

                 }

                 else if (p == q)

                     continue whileActive;

                 else

                     p = q;

             }

             // found active CAS target

             if (prev == p || x.casPrev(prev, p))

                 return;

         } while (x.item != null || x.next == null);

     }

     private void skipDeletedSuccessors(Node<E> x) {

         whileActive:

         do {

             Node<E> next = x.next;

             // assert next != null;

             // assert x != NEXT_TERMINATOR;

             // assert x != PREV_TERMINATOR;

             Node<E> p = next;

             findActive:

             for (;;) {

                 if (p.item != null)

                     break findActive;

                 Node<E> q = p.next;

                 if (q == null) {

                     if (p.prev == p)

                         continue whileActive;

                     break findActive;

                 }

                 else if (p == q)

                     continue whileActive;

                 else

                     p = q;

             }

             // found active CAS target

             if (next == p || x.casNext(next, p))

                 return;

         } while (x.item != null || x.prev == null);

     }

     /**

      * Returns the successor of p, or the first 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<E> succ(Node<E> p) {

         // TODO: should we skip deleted nodes here?

         Node<E> q = p.next;

         return (p == q) ? first() : q;

     }

     /**

      * Returns the predecessor of p, or the last node if p.prev has been

      * linked to self, which will only be true if traversing with a

      * stale pointer that is now off the list.

      */

     final Node<E> pred(Node<E> p) {

         Node<E> q = p.prev;

         return (p == q) ? last() : q;

     }

     /**

      * Returns the first node, the unique node p for which:

      *     p.prev == null && p.next != p

      * The returned node may or may not be logically deleted.

      * Guarantees that head is set to the returned node.

      */

     Node<E> first() {

         restartFromHead:

         for (;;)

             for (Node<E> h = head, p = h, q;;) {

                 if ((q = p.prev) != null &&

                     (q = (p = q).prev) != null)

                     // Check for head updates every other hop.

                     // If p == q, we are sure to follow head instead.

                     p = (h != (h = head)) ? h : q;

                 else if (p == h

                          // It is possible that p is PREV_TERMINATOR,

                          // but if so, the CAS is guaranteed to fail.

                          || casHead(h, p))

                     return p;

                 else

                     continue restartFromHead;

             }

     }

     /**

      * Returns the last node, the unique node p for which:

      *     p.next == null && p.prev != p

      * The returned node may or may not be logically deleted.

      * Guarantees that tail is set to the returned node.

      */

     Node<E> last() {

         restartFromTail:

         for (;;)

             for (Node<E> t = tail, p = t, q;;) {

                 if ((q = p.next) != null &&

                     (q = (p = q).next) != null)

                     // Check for tail updates every other hop.

                     // If p == q, we are sure to follow tail instead.

                     p = (t != (t = tail)) ? t : q;

                 else if (p == t

                          // It is possible that p is NEXT_TERMINATOR,

                          // but if so, the CAS is guaranteed to fail.

                          || casTail(t, p))

                     return p;

                 else

                     continue restartFromTail;

             }

     }

     // Minor convenience utilities

     /**

      * Throws NullPointerException if argument is null.

      *

      * @param v the element

      */

     private static void checkNotNull(Object v) {

         if (v == null)

             throw new NullPointerException();

     }

     /**

      * Returns element unless it is null, in which case throws

      * NoSuchElementException.

      *

      * @param v the element

      * @return the element

      */

     private E screenNullResult(E v) {

         if (v == null)

             throw new NoSuchElementException();

         return v;

     }

     /**

      * Creates an array list and fills it with elements of this list.

      * Used by toArray.

      *

      * @return the arrayList

      */

     private ArrayList<E> toArrayList() {

         ArrayList<E> list = new ArrayList<E>();

         for (Node<E> p = first(); p != null; p = succ(p)) {

             E item = p.item;

             if (item != null)

                 list.add(item);

         }

         return list;

     }

     /**

      * Constructs an empty deque.

      */

     public ConcurrentLinkedDeque() {

         head = tail = new Node<E>(null);

     }

     /**

      * Constructs a deque 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 ConcurrentLinkedDeque(Collection<? extends E> c) {

         // Copy c into a private chain of Nodes

         Node<E> h = null, t = null;

         for (E e : c) {

             checkNotNull(e);

             Node<E> newNode = new Node<E>(e);

             if (h == null)

                 h = t = newNode;

             else {

                 t.lazySetNext(newNode);

                 newNode.lazySetPrev(t);

                 t = newNode;

             }

         }

         initHeadTail(h, t);

     }

     /**

      * Initializes head and tail, ensuring invariants hold.

      */

     private void initHeadTail(Node<E> h, Node<E> t) {

         if (h == t) {

             if (h == null)

                 h = t = new Node<E>(null);

             else {

                 // Avoid edge case of a single Node with non-null item.

                 Node<E> newNode = new Node<E>(null);

                 t.lazySetNext(newNode);

                 newNode.lazySetPrev(t);

                 t = newNode;

             }

         }

         head = h;

         tail = t;

     }

     /**

      * Inserts the specified element at the front of this deque.

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

      * {@link IllegalStateException}.

      *

      * @throws NullPointerException if the specified element is null

      */

     public void addFirst(E e) {

         linkFirst(e);

     }

     /**

      * Inserts the specified element at the end of this deque.

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

      * {@link IllegalStateException}.

      *

      * <p>This method is equivalent to {@link #add}.

      *

      * @throws NullPointerException if the specified element is null

      */

     public void addLast(E e) {

         linkLast(e);

     }

     /**

      * Inserts the specified element at the front of this deque.

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

      *

      * @return {@code true} (as specified by {@link Deque#offerFirst})

      * @throws NullPointerException if the specified element is null

      */

     public boolean offerFirst(E e) {

         linkFirst(e);

         return true;

     }

     /**

      * Inserts the specified element at the end of this deque.

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

      *

      * <p>This method is equivalent to {@link #add}.

      *

      * @return {@code true} (as specified by {@link Deque#offerLast})

      * @throws NullPointerException if the specified element is null

      */

     public boolean offerLast(E e) {

         linkLast(e);

         return true;

     }

     public E peekFirst() {

         for (Node<E> p = first(); p != null; p = succ(p)) {

             E item = p.item;

             if (item != null)

                 return item;

         }

         return null;

     }

     public E peekLast() {

         for (Node<E> p = last(); p != null; p = pred(p)) {

             E item = p.item;

             if (item != null)

                 return item;

         }

         return null;

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E getFirst() {

         return screenNullResult(peekFirst());

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E getLast() {

         return screenNullResult(peekLast());

     }

     public E pollFirst() {

         for (Node<E> p = first(); p != null; p = succ(p)) {

             E item = p.item;

             if (item != null && p.casItem(item, null)) {

                 unlink(p);

                 return item;

             }

         }

         return null;

     }

     public E pollLast() {

         for (Node<E> p = last(); p != null; p = pred(p)) {

             E item = p.item;

             if (item != null && p.casItem(item, null)) {

                 unlink(p);

                 return item;

             }

         }

         return null;

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E removeFirst() {

         return screenNullResult(pollFirst());

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E removeLast() {

         return screenNullResult(pollLast());

     }

     // *** Queue and stack methods ***

     /**

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

      * As the deque 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) {

         return offerLast(e);

     }

     /**

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

      * As the deque 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) {

         return offerLast(e);

     }

     public E poll()           { return pollFirst(); }

     public E remove()         { return removeFirst(); }

     public E peek()           { return peekFirst(); }

     public E element()        { return getFirst(); }

     public void push(E e)     { addFirst(e); }

     public E pop()            { return removeFirst(); }

     /**

      * Removes the first element {@code e} such that

      * {@code o.equals(e)}, if such an element exists in this deque.

      * If the deque does not contain the element, it is unchanged.

      *

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

      * @return {@code true} if the deque contained the specified element

      * @throws NullPointerException if the specified element is null

      */

     public boolean removeFirstOccurrence(Object o) {

         checkNotNull(o);

         for (Node<E> p = first(); p != null; p = succ(p)) {

             E item = p.item;

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

                 unlink(p);

                 return true;

             }

         }

         return false;

     }

     /**

      * Removes the last element {@code e} such that

      * {@code o.equals(e)}, if such an element exists in this deque.

      * If the deque does not contain the element, it is unchanged.

      *

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

      * @return {@code true} if the deque contained the specified element

      * @throws NullPointerException if the specified element is null

      */

     public boolean removeLastOccurrence(Object o) {

         checkNotNull(o);

         for (Node<E> p = last(); p != null; p = pred(p)) {

             E item = p.item;

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

                 unlink(p);

                 return true;

             }

         }

         return false;

     }

     /**

      * Returns {@code true} if this deque contains at least one

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

      *

      * @param o element whose presence in this deque is to be tested

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

      */

     public boolean contains(Object o) {

         if (o == null) return false;

         for (Node<E> p = first(); p != null; p = succ(p)) {

             E item = p.item;

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

                 return true;

         }

         return false;

     }

     /**

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

      *

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

      */

     public boolean isEmpty() {

         return peekFirst() == null;

     }

     /**

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

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

      * 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 deques, determining the current

      * number of elements requires traversing them all to count them.

      * Additionally, it is possible for the size to change during

      * execution of this method, in which case the returned result

      * will be inaccurate. Thus, this method is typically not very

      * useful in concurrent applications.

      *

      * @return the number of elements in this deque

      */

     public int size() {

         int count = 0;

         for (Node<E> p = first(); p != null; p = succ(p))

             if (p.item != null)

                 // Collection.size() spec says to max out

                 if (++count == Integer.MAX_VALUE)

                     break;

         return count;

     }

     /**

      * Removes the first element {@code e} such that

      * {@code o.equals(e)}, if such an element exists in this deque.

      * If the deque does not contain the element, it is unchanged.

      *

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

      * @return {@code true} if the deque contained the specified element

      * @throws NullPointerException if the specified element is null

      */

     public boolean remove(Object o) {

         return removeFirstOccurrence(o);

     }

     /**

      * Appends all of the elements in the specified collection to the end of

      * this deque, in the order that they are returned by the specified

      * collection's iterator.  Attempts to {@code addAll} of a deque to

      * itself result in {@code IllegalArgumentException}.

      *

      * @param c the elements to be inserted into this deque

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

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      * @throws IllegalArgumentException if the collection is this deque

      */

     public boolean addAll(Collection<? extends E> c) {

         if (c == this)

             // As historically specified in AbstractQueue#addAll

             throw new IllegalArgumentException();

         // Copy c into a private chain of Nodes

         Node<E> beginningOfTheEnd = null, last = null;

         for (E e : c) {

             checkNotNull(e);

             Node<E> newNode = new Node<E>(e);

             if (beginningOfTheEnd == null)

                 beginningOfTheEnd = last = newNode;

             else {

                 last.lazySetNext(newNode);

                 newNode.lazySetPrev(last);

                 last = newNode;

             }

         }

         if (beginningOfTheEnd == null)

             return false;

         // Atomically append the chain at the tail of this collection

         restartFromTail:

         for (;;)

             for (Node<E> t = tail, p = t, q;;) {

                 if ((q = p.next) != null &&

                     (q = (p = q).next) != null)

                     // Check for tail updates every other hop.

                     // If p == q, we are sure to follow tail instead.

                     p = (t != (t = tail)) ? t : q;

                 else if (p.prev == p) // NEXT_TERMINATOR

                     continue restartFromTail;

                 else {

                     // p is last node

                     beginningOfTheEnd.lazySetPrev(p); // CAS piggyback

                     if (p.casNext(null, beginningOfTheEnd)) {

                         // Successful CAS is the linearization point

                         // for all elements to be added to this deque.

                         if (!casTail(t, last)) {

                             // Try a little harder to update tail,

                             // since we may be adding many elements.

                             t = tail;

                             if (last.next == null)

                                 casTail(t, last);

                         }

                         return true;

                     }

                     // Lost CAS race to another thread; re-read next

                 }

             }

     }

     /**

      * Removes all of the elements from this deque.

      */

     public void clear() {

         while (pollFirst() != null)

             ;

     }

     /**

      * Returns an array containing all of the elements in this deque, in

      * proper sequence (from first to last element).

      *

      * <p>The returned array will be "safe" in that no references to it are

      * maintained by this deque.  (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 deque

      */

     public Object[] toArray() {

         return toArrayList().toArray();

     }

     /**

      * Returns an array containing all of the elements in this deque,

      * in proper sequence (from first to last element); the runtime

      * type of the returned array is that of the specified array.  If

      * the deque 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 deque.

      *

      * <p>If this deque fits in the specified array with room to spare

      * (i.e., the array has more elements than this deque), the element in

      * the array immediately following the end of the deque 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 deque known to contain only strings.

      * The following code can be used to dump the deque into a newly

      * allocated array of {@code String}:

      *

      * <pre>

      *     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 deque 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 deque

      * @throws ArrayStoreException if the runtime type of the specified array

      *         is not a supertype of the runtime type of every element in

      *         this deque

      * @throws NullPointerException if the specified array is null

      */

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

         return toArrayList().toArray(a);

     }

     /**

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

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

      *

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

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

      * ConcurrentModificationException}, and guarantees to traverse

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

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

      * subsequent to construction.

      *

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

      */

     public Iterator<E> iterator() {

         return new Itr();

     }

     /**

      * Returns an iterator over the elements in this deque in reverse

      * sequential order.  The elements will be returned in order from

      * last (tail) to first (head).

      *

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

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

      * ConcurrentModificationException}, and guarantees to traverse

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

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

      * subsequent to construction.

      *

      * @return an iterator over the elements in this deque in reverse order

      */

     public Iterator<E> descendingIterator() {

         return new DescendingItr();

     }

     private abstract class AbstractItr implements Iterator<E> {

         /**

          * Next node to return item for.

          */

         private Node<E> nextNode;

         /**

          * nextItem holds on to item fields because once we claim

          * that an element exists in hasNext(), we must return it in

          * the following next() call even if it was in the process of

          * being removed when hasNext() was called.

          */

         private E nextItem;

         /**

          * Node returned by most recent call to next. Needed by remove.

          * Reset to null if this element is deleted by a call to remove.

          */

         private Node<E> lastRet;

         abstract Node<E> startNode();

         abstract Node<E> nextNode(Node<E> p);

         AbstractItr() {

             advance();

         }

         /**

          * Sets nextNode and nextItem to next valid node, or to null

          * if no such.

          */

         private void advance() {

             lastRet = nextNode;

             Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode);

             for (;; p = nextNode(p)) {

                 if (p == null) {

                     // p might be active end or TERMINATOR node; both are OK

                     nextNode = null;

                     nextItem = null;

                     break;

                 }

                 E item = p.item;

                 if (item != null) {

                     nextNode = p;

                     nextItem = item;

                     break;

                 }

             }

         }

         public boolean hasNext() {

             return nextItem != null;

         }

         public E next() {

             E item = nextItem;

             if (item == null) throw new NoSuchElementException();

             advance();

             return item;

         }

         public void remove() {

             Node<E> l = lastRet;

             if (l == null) throw new IllegalStateException();

             l.item = null;

             unlink(l);

             lastRet = null;

         }

     }

     /** Forward iterator */

     private class Itr extends AbstractItr {

         Node<E> startNode() { return first(); }

         Node<E> nextNode(Node<E> p) { return succ(p); }

     }

     /** Descending iterator */

     private class DescendingItr extends AbstractItr {

         Node<E> startNode() { return last(); }

         Node<E> nextNode(Node<E> p) { return pred(p); }

     }

     /**

      * 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 {

         // Write out any hidden stuff

         s.defaultWriteObject();

         // Write out all elements in the proper order.

         for (Node<E> p = first(); p != null; p = succ(p)) {

             E item = p.item;

             if (item != null)

                 s.writeObject(item);

         }

         // Use trailing null as sentinel

         s.writeObject(null);

     }

     /**

      * Reconstitutes the 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();

         // Read in elements until trailing null sentinel found

         Node<E> h = null, t = null;

         Object item;

         while ((item = s.readObject()) != null) {

             @SuppressWarnings("unchecked")

             Node<E> newNode = new Node<E>((E) item);

             if (h == null)

                 h = t = newNode;

             else {

                 t.lazySetNext(newNode);

                 newNode.lazySetPrev(t);

                 t = newNode;

             }

         }

         initHeadTail(h, t);

     }

     private boolean casHead(Node<E> cmp, Node<E> val) {

         if (head == cmp) {

             head = val;

             return true;

         }

         return false;

     }

     private boolean casTail(Node<E> cmp, Node<E> val) {

         if (tail == cmp) {

             tail = val;

             return true;

         }

         return false;

     }

     static {

         PREV_TERMINATOR = new Node<Object>();

         PREV_TERMINATOR.next = PREV_TERMINATOR;

         NEXT_TERMINATOR = new Node<Object>();

         NEXT_TERMINATOR.prev = NEXT_TERMINATOR;

     }

 }