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From akuznet...@apache.org
Subject [05/14] incubator-ignite git commit: IGNITE-45 - Renamed jdk8.backport to jsr166
Date Mon, 23 Mar 2015 04:22:17 GMT
http://git-wip-us.apache.org/repos/asf/incubator-ignite/blob/2cb81f9b/modules/core/src/main/java/org/jsr166/ConcurrentLinkedDeque8.java
----------------------------------------------------------------------
diff --git a/modules/core/src/main/java/org/jsr166/ConcurrentLinkedDeque8.java b/modules/core/src/main/java/org/jsr166/ConcurrentLinkedDeque8.java
new file mode 100644
index 0000000..630db1c
--- /dev/null
+++ b/modules/core/src/main/java/org/jsr166/ConcurrentLinkedDeque8.java
@@ -0,0 +1,1954 @@
+/*
+ * 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/
+ */
+
+/*
+ * The initial version of this file was copied from JSR-166:
+ * http://gee.cs.oswego.edu/dl/concurrency-interest/
+ */
+
+package org.jsr166;
+
+import sun.misc.*;
+
+import java.lang.reflect.*;
+import java.security.*;
+import java.util.*;
+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>
+ * 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/
+ */
+@SuppressWarnings( {"ALL"})
+public class ConcurrentLinkedDeque8<E> extends AbstractCollection<E> implements Deque<E> {
+    /*
+     * 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.
+     */
+
+    /**
+     * 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 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 volatile Node<E> tail;
+
+    /** */
+    private final LongAdder8 size = new LongAdder8();
+
+    /** Previous and next terminators. */
+    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;
+    }
+
+    /**
+     * Internal node element.
+     *
+     * @param <E> Node item.
+     */
+    @SuppressWarnings( {"PackageVisibleField", "PackageVisibleInnerClass"})
+    public static final class Node<E> {
+        volatile Node<E> prev;
+        volatile E item;
+        volatile Node<E> next;
+
+        /**
+         * Default constructor for NEXT_TERMINATOR, PREV_TERMINATOR.
+         */
+        Node() {
+            // No-op.
+        }
+
+        /**
+         * Constructs a new node.  Uses relaxed write because item can
+         * only be seen after publication via casNext or casPrev.
+         *
+         * @param item Item to initialize.
+         */
+        Node(E item) {
+            UNSAFE.putObject(this, itemOffset, item);
+        }
+
+        /**
+         * @return Item or {@code null} if this node was removed from the queue.
+         */
+        public E item() {
+            return item;
+        }
+
+        /**
+         * @param cmp Compare value.
+         * @param val New value.
+         * @return {@code True} if set.
+         */
+        boolean casItem(E cmp, E val) {
+            return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
+        }
+
+        /**
+         * @param val New value.
+         */
+        void lazySetNext(Node<E> val) {
+            UNSAFE.putOrderedObject(this, nextOffset, val);
+        }
+
+        /**
+         * @param cmp Compare value.
+         * @param val New value.
+         * @return {@code True} if set.
+         */
+        boolean casNext(Node<E> cmp, Node<E> val) {
+            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
+        }
+
+        /**
+         * @param val New value.
+         */
+        void lazySetPrev(Node<E> val) {
+            UNSAFE.putOrderedObject(this, prevOffset, val);
+        }
+
+        /**
+         * @param cmp Compare value.
+         * @param val New value.
+         * @return {@code True} if set.
+         */
+        boolean casPrev(Node<E> cmp, Node<E> val) {
+            return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val);
+        }
+
+        /** Unsafe. */
+        private static final Unsafe UNSAFE;
+
+        /** Previous field offset. */
+        private static final long prevOffset;
+
+        /** Item field offset. */
+        private static final long itemOffset;
+
+        /** Next field offset. */
+        private static final long nextOffset;
+
+        /**
+         * Initialize offsets.
+         */
+        static {
+            try {
+                UNSAFE = unsafe();
+
+                Class k = Node.class;
+
+                prevOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("prev"));
+                itemOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("item"));
+                nextOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("next"));
+            }
+            catch (Exception e) {
+                throw new Error(e);
+            }
+        }
+    }
+
+    /**
+     * Links e as first element.
+     */
+    private void linkFirst(E e) {
+        checkNotNull(e);
+
+        size.increment();
+
+        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
+                }
+            }
+        }
+    }
+
+    /**
+     * Same as {@link #linkFirst(Object)}, but returns new {@link Node}.
+     *
+     * @param e Element to link.
+     * @return New node.
+     */
+    private Node<E> linkFirstx(E e) {
+        checkNotNull(e);
+
+        size.increment();
+
+        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 newNode;
+                    }
+                    // Lost CAS race to another thread; re-read prev
+                }
+            }
+        }
+    }
+
+    /**
+     * Links e as last element.
+     *
+     * @param e Element to link.
+     */
+    private void linkLast(E e) {
+        checkNotNull(e);
+
+        size.increment();
+
+        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
+                }
+            }
+        }
+    }
+
+    /**
+     * Links n as last node.
+     *
+     * @param n Node to link.
+     */
+    private void linkLast(Node<E> n) {
+        checkNotNull(n);
+
+        size.increment();
+
+        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
+                    n.lazySetPrev(p); // CAS piggyback.
+
+                    if (p.casNext(null, n)) {
+                        // 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, n);  // Failure is OK.
+
+                        return;
+                    }
+                    // Lost CAS race to another thread; re-read next
+                }
+            }
+        }
+    }
+
+    /**
+     * Same as {@link #linkLast(Object)}, but returns {@link Node}.
+     *
+     * @param e Element to link.
+     * @return New node.
+     */
+    private Node<E> linkLastx(E e) {
+        checkNotNull(e);
+
+        size.increment();
+
+        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 newNode;
+                    }
+                    // Lost CAS race to another thread; re-read next
+                }
+            }
+        }
+    }
+
+    /** Number of HOPs before unlinking head or tail. */
+    private static final int HOPS = 2;
+
+    /**
+     * Unlinks non-null node x, that has not yet been unlinked.
+     *
+     * @param x Node.
+     * @return {@code True} if node was unlinked by this call.
+     */
+    public boolean unlinkx(Node<E> x) {
+        assert x != null;
+
+        E item = x.item;
+
+        if (item != null && x.casItem(item, null)) {
+            unlink(x);
+
+            return true;
+        }
+
+        return false;
+    }
+
+    /**
+     * Unlinks non-null node x.
+     */
+    private 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;
+
+        // Unlink should not be called twice for the same node.
+        size.decrement();
+
+        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
+            // Always squeeze out interior deleted nodes.
+            if (hops < HOPS && (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;
+            }
+        }
+    }
+
+    /**
+     * @param x Node to start from.
+     */
+    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);
+    }
+
+    /**
+     * @param x Node to start from.
+     */
+    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.
+     *
+     * @param p Node to find successor for.
+     * @return Successor node.
+     */
+    final Node<E> successor(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.
+     *
+     * @param p Node to find predecessor for.
+     * @return Predecessor node.
+     */
+    final Node<E> predecessor(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.
+     *
+     * @return First node.
+     */
+    @SuppressWarnings( {"TooBroadScope"})
+    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.
+     *
+     * @return Last node.
+     */
+    @SuppressWarnings( {"TooBroadScope"})
+    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;
+            }
+    }
+
+    /**
+     * 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 = successor(p)) {
+            E item = p.item;
+
+            if (item != null)
+                list.add(item);
+        }
+
+        return list;
+    }
+
+    /**
+     * Constructs an empty deque.
+     */
+    public ConcurrentLinkedDeque8() {
+        head = tail = new Node<E>();
+    }
+
+    /**
+     * 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 ConcurrentLinkedDeque8(Iterable<? 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.
+     *
+     * @param h Head.
+     * @param t Tail.
+     */
+    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
+     */
+    @Override public void addFirst(E e) {
+        linkFirst(e);
+    }
+
+    /**
+     * Same as {@link #addFirst(Object)}, but returns new node.
+     *
+     * @param e Element to add.
+     * @return New node.
+     */
+    public Node<E> addFirstx(E e) {
+        return linkFirstx(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
+     */
+    @Override public void addLast(E e) {
+        linkLast(e);
+    }
+
+    /**
+     * Same as {@link #addLast(Object)}, but returns new node.
+     *
+     * @param e Element to add.
+     * @return New node.
+     */
+    public Node<E> addLastx(E e) {
+        return linkLastx(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
+     */
+    @Override public boolean offerFirst(E e) {
+        linkFirst(e);
+
+        return true;
+    }
+
+    /**
+     * Same as {@link #offerFirst(Object)}, but returns new {@link Node}.
+     *
+     * @param e Element to add.
+     * @return New node.
+     */
+    public Node<E> offerFirstx(E e) {
+        return linkFirstx(e);
+    }
+
+    /**
+     * 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
+     */
+    @Override public boolean offerLast(E e) {
+        linkLast(e);
+
+        return true;
+    }
+
+    /**
+     * Same as {@link #offerLast(Object)}, but returns new {@link Node}.
+     *
+     * @param e Element to add.
+     * @return New node.
+     */
+    public Node<E> offerLastx(E e) {
+        return linkLastx(e);
+    }
+
+    /** {@inheritDoc} */
+    @Override public E peekFirst() {
+        for (Node<E> p = first(); p != null; p = successor(p)) {
+            E item = p.item;
+
+            if (item != null)
+                return item;
+        }
+
+        return null;
+    }
+
+    /**
+     * Retrieves, but does not remove, the first node of this deque,
+     * or returns {@code null} if this deque is empty.
+     *
+     * @return The header node of this deque, or <tt>null</tt> if this deque is empty
+     */
+    public Node<E> peekFirstx() {
+        for (Node<E> p = first(); p != null; p = successor(p)) {
+            E item = p.item;
+
+            if (item != null)
+                return p;
+        }
+
+        return null;
+    }
+
+    /** {@inheritDoc} */
+    @Override public E peekLast() {
+        for (Node<E> p = last(); p != null; p = predecessor(p)) {
+            E item = p.item;
+
+            if (item != null)
+                return item;
+        }
+
+        return null;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    @Override public E getFirst() {
+        return screenNullResult(peekFirst());
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    @Override public E getLast() {
+        return screenNullResult(peekLast());
+    }
+
+    /** {@inheritDoc} */
+    @Override public E pollFirst() {
+        for (Node<E> p = first(); p != null; p = successor(p)) {
+            E item = p.item;
+
+            if (item != null && p.casItem(item, null)) {
+                unlink(p);
+
+                return item;
+            }
+        }
+
+        return null;
+    }
+
+    /** {@inheritDoc} */
+    @Override public E pollLast() {
+        for (Node<E> p = last(); p != null; p = predecessor(p)) {
+            E item = p.item;
+
+            if (item != null && p.casItem(item, null)) {
+                unlink(p);
+
+                return item;
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    @Override public E removeFirst() {
+        return screenNullResult(pollFirst());
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    @Override public E removeLast() {
+        return screenNullResult(pollLast());
+    }
+
+    /**
+     * 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
+     */
+    @Override public boolean offer(E e) {
+        return offerLast(e);
+    }
+
+    /**
+     * Same as {@link #offer(Object)}, but returns new {@link Node}.
+     *
+     * @param e Element to add.
+     * @return New node.
+     */
+    public Node<E> offerx(E e) {
+        return offerLastx(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
+     */
+    @Override public boolean add(E e) {
+        return offerLast(e);
+    }
+
+    /**
+     * Same as {@link #add(Object)}, but returns new node.
+     *
+     * @param e Element to add.
+     * @return New node.
+     */
+    public Node<E> addx(E e) {
+        return offerLastx(e);
+    }
+
+    /** {@inheritDoc} */
+    @Override public E poll() {
+        return pollFirst();
+    }
+
+    /** {@inheritDoc} */
+    @Override public E remove() {
+        return removeFirst();
+    }
+
+    /** {@inheritDoc} */
+    @Override public E peek() {
+        return peekFirst();
+    }
+
+    /**
+     * Retrieves, but does not remove, the header node of the queue represented by
+     * this deque (in other words, the first node of this deque), or
+     * returns {@code null} if this deque is empty.
+     * <p>
+     * This method is equivalent to {@link #peekFirst()}.
+     *
+     * @return The header node of the queue represented by this deque, or
+     *      {@code null} if this deque is empty
+     */
+    public Node<E> peekx() {
+        return peekFirstx();
+    }
+
+    /** {@inheritDoc} */
+    @Override public E element() {
+        return getFirst();
+    }
+
+    /** {@inheritDoc} */
+    @Override public void push(E e) {
+        addFirst(e);
+    }
+
+    /** {@inheritDoc} */
+    @Override 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
+     */
+    @Override public boolean removeFirstOccurrence(Object o) {
+        checkNotNull(o);
+
+        for (Node<E> p = first(); p != null; p = successor(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
+     */
+    @Override public boolean removeLastOccurrence(Object o) {
+        checkNotNull(o);
+
+        for (Node<E> p = last(); p != null; p = predecessor(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
+     */
+    @Override public boolean contains(Object o) {
+        if (o == null)
+            return false;
+
+        for (Node<E> p = first(); p != null; p = successor(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
+     */
+    @Override public boolean isEmpty() {
+        return peekFirst() == null;
+    }
+
+    /**
+     * Returns {@code true} if this collection contains no elements.
+     * <p>
+     * The difference from {@link #isEmpty()} method is that this method
+     * relies on {@link #sizex()} method.
+     *
+     * @return {@code True} if this collection contains no elements
+     */
+    public boolean isEmptyx() {
+        return sizex() == 0;
+    }
+
+    /**
+     * 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
+     */
+    @Override public int size() {
+        int cnt = 0;
+
+        for (Node<E> p = first(); p != null; p = successor(p))
+            if (p.item != null)
+                // Collection.size() spec says to max out
+                if (++cnt == Integer.MAX_VALUE)
+                    break;
+
+        return cnt;
+    }
+
+    /**
+     * @return Size based on performed operations.
+     */
+    public int sizex() {
+        return size.intValue();
+    }
+
+    /**
+     * 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
+     */
+    @Override 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
+     */
+    @SuppressWarnings( {"TooBroadScope"})
+    @Override 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;
+
+        int s = 0;
+
+        for (E e : c) {
+            checkNotNull(e);
+
+            Node<E> newNode = new Node<E>(e);
+
+            if (beginningOfTheEnd == null) {
+                beginningOfTheEnd = last = newNode;
+
+                s++;
+            }
+            else {
+                last.lazySetNext(newNode);
+
+                newNode.lazySetPrev(last);
+
+                last = newNode;
+
+                s++;
+            }
+        }
+
+        if (beginningOfTheEnd == null)
+            return false;
+
+        size.add(s);
+
+        // 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.
+     */
+    @Override public void clear() {
+        while (pollFirst() != null) {
+            // No-op.
+        }
+    }
+
+    /**
+     * 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
+     */
+    @Override 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
+     */
+    @SuppressWarnings( {"SuspiciousToArrayCall"})
+    @Override 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
+     */
+    @Override public Iterator<E> iterator() {
+        return new Iter();
+    }
+
+    /**
+     * 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
+     */
+    @Override public Iterator<E> descendingIterator() {
+        return new DescendingIter();
+    }
+
+    /**
+     * Extended iterator interface.
+     */
+    public interface IteratorEx<E> extends Iterator<E> {
+        /**
+         * Same semantics as iterator's remove, but will return {@code false} if remove did not happen.
+         *
+         * @return {@code True} if element was removed by this call, {@code false} otherwise.
+         */
+        public boolean removex();
+    }
+
+    /**
+     * Abstract iterator.
+     */
+    private abstract class AbstractIter implements IteratorEx<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;
+
+        /**
+         * @return Starting node.
+         */
+        abstract Node<E> startNode();
+
+        /**
+         * @param p Node.
+         * @return Next node.
+         */
+        abstract Node<E> nextNode(Node<E> p);
+
+        /**
+         * Advances to first element.
+         */
+        AbstractIter() {
+            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;
+                }
+            }
+        }
+
+        /** {@inheritDoc} */
+        @Override public boolean hasNext() {
+            return nextItem != null;
+        }
+
+        /** {@inheritDoc} */
+        @Override public E next() {
+            E item = nextItem;
+
+            if (item == null)
+                throw new NoSuchElementException();
+
+            advance();
+
+            return item;
+        }
+
+        /** {@inheritDoc} */
+        @Override public void remove() {
+            Node<E> l = lastRet;
+
+            if (l == null)
+                throw new IllegalStateException();
+
+            unlinkx(l);
+
+            lastRet = null;
+        }
+
+        /** {@inheritDoc} */
+        @Override public boolean removex() {
+            Node<E> l = lastRet;
+
+            if (l == null)
+                throw new IllegalStateException();
+
+            boolean res = unlinkx(l);
+
+            lastRet = null;
+
+            return res;
+        }
+    }
+
+    /**
+     * Forward iterator
+     */
+    private class Iter extends AbstractIter {
+        /** {@inheritDoc} */
+        @Override Node<E> startNode() {
+            return first();
+        }
+
+        /** {@inheritDoc} */
+        @Override Node<E> nextNode(Node<E> p) {
+            return successor(p);
+        }
+    }
+
+    /**
+     * Descending iterator.
+     */
+    private class DescendingIter extends AbstractIter {
+        /** {@inheritDoc} */
+        @Override Node<E> startNode() {
+            return last();
+        }
+
+        /** {@inheritDoc} */
+        @Override Node<E> nextNode(Node<E> p) {
+            return predecessor(p);
+        }
+    }
+
+    /**
+     * CAS for head.
+     *
+     * @param cmp Compare value.
+     * @param val New value.
+     * @return {@code True} if set.
+     */
+    private boolean casHead(Node<E> cmp, Node<E> val) {
+        return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
+    }
+
+    /**
+     * CAS for tail.
+     *
+     * @param cmp Compare value.
+     * @param val New value.
+     * @return {@code True} if set.
+     */
+    private boolean casTail(Node<E> cmp, Node<E> val) {
+        return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
+    }
+
+    /** Unsafe. */
+    private static final Unsafe UNSAFE;
+
+    /** Head offset. */
+    private static final long headOffset;
+
+    /** Tail offset. */
+    private static final long tailOffset;
+
+    /**
+     * Initialize terminators using unsafe semantics.
+     */
+    static {
+        PREV_TERMINATOR = new Node<Object>();
+        PREV_TERMINATOR.next = PREV_TERMINATOR;
+        NEXT_TERMINATOR = new Node<Object>();
+        NEXT_TERMINATOR.prev = NEXT_TERMINATOR;
+
+        try {
+            UNSAFE = unsafe();
+
+            Class cls = ConcurrentLinkedDeque8.class;
+
+            headOffset = UNSAFE.objectFieldOffset(cls.getDeclaredField("head"));
+            tailOffset = UNSAFE.objectFieldOffset(cls.getDeclaredField("tail"));
+        }
+        catch (Exception e) {
+            throw new Error(e);
+        }
+    }
+
+    /**
+     * @return Instance of Unsafe class.
+     */
+    static Unsafe unsafe() {
+        try {
+            return Unsafe.getUnsafe();
+        }
+        catch (SecurityException ignored) {
+            try {
+                return AccessController.doPrivileged
+                    (new PrivilegedExceptionAction<Unsafe>() {
+                        @Override
+                        public Unsafe run() throws Exception {
+                            Field f = Unsafe.class.getDeclaredField("theUnsafe");
+
+                            f.setAccessible(true);
+
+                            return (Unsafe) f.get(null);
+                        }
+                    });
+            }
+            catch (PrivilegedActionException e) {
+                throw new RuntimeException("Could not initialize intrinsics.", e.getCause());
+            }
+        }
+    }
+}


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