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/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/licenses/publicdomain

 */

package java.util.concurrent;

import java.util.*;

import java.util.concurrent.atomic.*;

/**

 * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.

 * This queue orders elements FIFO (first-in-first-out).

 * The <em>head</em> of the queue is that element that has been on the

 * queue the longest time.

 * The <em>tail</em> of the queue is that element that has been on the

 * queue the shortest time. New elements

 * are inserted at the tail of the queue, and the queue retrieval

 * operations obtain elements at the head of the queue.

 * A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when

 * many threads will share access to a common collection.

 * This queue does not permit <tt>null</tt> elements.

 *

 * <p>This implementation employs an efficient &quot;wait-free&quot;

 * algorithm based on one described in <a

 * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,

 * Fast, and Practical Non-Blocking and Blocking Concurrent Queue

 * Algorithms</a> by Maged M. Michael and Michael L. Scott.

 *

 * <p>Beware that, unlike in most collections, the <tt>size</tt> method

 * is <em>NOT</em> a constant-time operation. Because of the

 * asynchronous nature of these queues, determining the current number

 * of elements requires a traversal of the elements.

 *

 * <p>This class and its iterator implement all of the

 * <em>optional</em> methods of the {@link Collection} and {@link

 * Iterator} interfaces.

 *

 * <p>Memory consistency effects: As with other concurrent

 * collections, actions in a thread prior to placing an object into a

 * {@code ConcurrentLinkedQueue}

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

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

 * the {@code ConcurrentLinkedQueue} in another thread.

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @since 1.5

 * @author Doug Lea

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

 *

 */

public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>

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

    private static final long serialVersionUID = 196745693267521676L;

    /*

     * This is a straight adaptation of Michael & Scott algorithm.

     * For explanation, read the paper.  The only (minor) algorithmic

     * difference is that this version supports lazy deletion of

     * internal nodes (method remove(Object)) -- remove CAS'es item

     * fields to null. The normal queue operations unlink but then

     * pass over nodes with null item fields. Similarly, iteration

     * methods ignore those with nulls.

     *

     * Also note that like most non-blocking algorithms in this

     * package, this implementation relies on the fact that in garbage

     * collected systems, there is no possibility of ABA problems due

     * to recycled nodes, so there is no need to use "counted

     * pointers" or related techniques seen in versions used in

     * non-GC'ed settings.

     */

    private static class Node<E> {

        private volatile E item;

        private volatile Node<E> next;

        private static final

            AtomicReferenceFieldUpdater<Node, Node>

            nextUpdater =

            AtomicReferenceFieldUpdater.newUpdater

            (Node.class, Node.class, "next");

        private static final

            AtomicReferenceFieldUpdater<Node, Object>

            itemUpdater =

            AtomicReferenceFieldUpdater.newUpdater

            (Node.class, Object.class, "item");

        Node(E x) { item = x; }

        Node(E x, Node<E> n) { item = x; next = n; }

        E getItem() {

            return item;

        }

        boolean casItem(E cmp, E val) {

            return itemUpdater.compareAndSet(this, cmp, val);

        }

        void setItem(E val) {

            itemUpdater.set(this, val);

        }

        Node<E> getNext() {

            return next;

        }

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

            return nextUpdater.compareAndSet(this, cmp, val);

        }

        void setNext(Node<E> val) {

            nextUpdater.set(this, val);

        }

    }

    private static final

        AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>

        tailUpdater =

        AtomicReferenceFieldUpdater.newUpdater

        (ConcurrentLinkedQueue.class, Node.class, "tail");

    private static final

        AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>

        headUpdater =

        AtomicReferenceFieldUpdater.newUpdater

        (ConcurrentLinkedQueue.class,  Node.class, "head");

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

        return tailUpdater.compareAndSet(this, cmp, val);

    }

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

        return headUpdater.compareAndSet(this, cmp, val);

    }

    /**

     * Pointer to header node, initialized to a dummy node.  The first

     * actual node is at head.getNext().

     */

    private transient volatile Node<E> head = new Node<E>(null, null);

    /** Pointer to last node on list **/

    private transient volatile Node<E> tail = head;

    /**

     * Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.

     */

    public ConcurrentLinkedQueue() {}

    /**

     * Creates a <tt>ConcurrentLinkedQueue</tt>

     * 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 ConcurrentLinkedQueue(Collection<? extends E> c) {

        for (Iterator<? extends E> it = c.iterator(); it.hasNext();)

            add(it.next());

    }

    // Have to override just to update the javadoc

    /**

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

     *

     * @return <tt>true</tt> (as specified by {@link Collection#add})

     * @throws NullPointerException if the specified element is null

     */

    public boolean add(E e) {

        return offer(e);

    }

    /**

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

     *

     * @return <tt>true</tt> (as specified by {@link Queue#offer})

     * @throws NullPointerException if the specified element is null

     */

    public boolean offer(E e) {

        if (e == null) throw new NullPointerException();

        Node<E> n = new Node<E>(e, null);

        for (;;) {

            Node<E> t = tail;

            Node<E> s = t.getNext();

            if (t == tail) {

                if (s == null) {

                    if (t.casNext(s, n)) {

                        casTail(t, n);

                        return true;

                    }

                } else {

                    casTail(t, s);

                }

            }

        }

    }

    public E poll() {

        for (;;) {

            Node<E> h = head;

            Node<E> t = tail;

            Node<E> first = h.getNext();

            if (h == head) {

                if (h == t) {

                    if (first == null)

                        return null;

                    else

                        casTail(t, first);

                } else if (casHead(h, first)) {

                    E item = first.getItem();

                    if (item != null) {

                        first.setItem(null);

                        return item;

                    }

                    // else skip over deleted item, continue loop,

                }

            }

        }

    }

    public E peek() { // same as poll except don't remove item

        for (;;) {

            Node<E> h = head;

            Node<E> t = tail;

            Node<E> first = h.getNext();

            if (h == head) {

                if (h == t) {

                    if (first == null)

                        return null;

                    else

                        casTail(t, first);

                } else {

                    E item = first.getItem();

                    if (item != null)

                        return item;

                    else // remove deleted node and continue

                        casHead(h, first);

                }

            }

        }

    }

    /**

     * Returns the first actual (non-header) node on list.  This is yet

     * another variant of poll/peek; here returning out the first

     * node, not element (so we cannot collapse with peek() without

     * introducing race.)

     */

    Node<E> first() {

        for (;;) {

            Node<E> h = head;

            Node<E> t = tail;

            Node<E> first = h.getNext();

            if (h == head) {

                if (h == t) {

                    if (first == null)

                        return null;

                    else

                        casTail(t, first);

                } else {

                    if (first.getItem() != null)

                        return first;

                    else // remove deleted node and continue

                        casHead(h, first);

                }

            }

        }

    }

    /**

     * Returns <tt>true</tt> if this queue contains no elements.

     *

     * @return <tt>true</tt> if this queue contains no elements

     */

    public boolean isEmpty() {

        return first() == null;

    }

    /**

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

     * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns

     * <tt>Integer.MAX_VALUE</tt>.

     *

     * <p>Beware that, unlike in most collections, this method is

     * <em>NOT</em> a constant-time operation. Because of the

     * asynchronous nature of these queues, determining the current

     * number of elements requires an O(n) traversal.

     *

     * @return the number of elements in this queue

     */

    public int size() {

        int count = 0;

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

            if (p.getItem() != null) {

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

                if (++count == Integer.MAX_VALUE)

                    break;

            }

        }

        return count;

    }

    /**

     * Returns <tt>true</tt> if this queue contains the specified element.

     * More formally, returns <tt>true</tt> if and only if this queue contains

     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.

     *

     * @param o object to be checked for containment in this queue

     * @return <tt>true</tt> if this queue contains the specified element

     */

    public boolean contains(Object o) {

        if (o == null) return false;

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

            E item = p.getItem();

            if (item != null &&

                o.equals(item))

                return true;

        }

        return false;

    }

    /**

     * Removes a single instance of the specified element from this queue,

     * if it is present.  More formally, removes an element <tt>e</tt> such

     * that <tt>o.equals(e)</tt>, if this queue contains one or more such

     * elements.

     * Returns <tt>true</tt> if this queue contained the specified element

     * (or equivalently, if this queue changed as a result of the call).

     *

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

     * @return <tt>true</tt> if this queue changed as a result of the call

     */

    public boolean remove(Object o) {

        if (o == null) return false;

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

            E item = p.getItem();

            if (item != null &&

                o.equals(item) &&

                p.casItem(item, null))

                return true;

        }

        return false;

    }

    /**

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

     * The returned iterator is a "weakly consistent" iterator that

     * will never throw {@link ConcurrentModificationException},

     * and guarantees to traverse elements as they existed upon

     * construction of the iterator, and may (but is not guaranteed to)

     * reflect any modifications subsequent to construction.

     *

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

     */

    public Iterator<E> iterator() {

        return new Itr();

    }

    private class Itr 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 of the last returned item, to support remove.

         */

        private Node<E> lastRet;

        Itr() {

            advance();

        }

        /**

         * Moves to next valid node and returns item to return for

         * next(), or null if no such.

         */

        private E advance() {

            lastRet = nextNode;

            E x = nextItem;

            Node<E> p = (nextNode == null)? first() : nextNode.getNext();

            for (;;) {

                if (p == null) {

                    nextNode = null;

                    nextItem = null;

                    return x;

                }

                E item = p.getItem();

                if (item != null) {

                    nextNode = p;

                    nextItem = item;

                    return x;

                } else // skip over nulls

                    p = p.getNext();

            }

        }

        public boolean hasNext() {

            return nextNode != null;

        }

        public E next() {

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

            return advance();

        }

        public void remove() {

            Node<E> l = lastRet;

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

            // rely on a future traversal to relink.

            l.setItem(null);

            lastRet = null;

        }

    }

    /**

     * Save the state to a stream (that is, serialize it).

     *

     * @serialData All of the elements (each an <tt>E</tt>) 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 = p.getNext()) {

            Object item = p.getItem();

            if (item != null)

                s.writeObject(item);

        }

        // Use trailing null as sentinel

        s.writeObject(null);

    }

    /**

     * Reconstitute the Queue instance from a stream (that is,

     * deserialize it).

     * @param s the stream

     */

    private void readObject(java.io.ObjectInputStream s)

        throws java.io.IOException, ClassNotFoundException {

        // Read in capacity, and any hidden stuff

        s.defaultReadObject();

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

        tail = head;

        // Read in all elements and place in queue

        for (;;) {

            E item = (E)s.readObject();

            if (item == null)

                break;

            else

                offer(item);

        }

    }

}