/*

 * 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.lang.invoke.MethodHandles;

import java.lang.invoke.VarHandle;

import java.util.AbstractQueue;

import java.util.Arrays;

import java.util.Collection;

import java.util.Iterator;

import java.util.NoSuchElementException;

import java.util.Objects;

import java.util.Queue;

import java.util.Spliterator;

import java.util.Spliterators;

import java.util.function.Consumer;

import java.util.function.Predicate;

/**

 * 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 {@code ConcurrentLinkedQueue} 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>This implementation employs an efficient <em>non-blocking</em>

 * algorithm based on one described in

 * <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf">

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

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

 *

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

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

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

 * java.util.ConcurrentModificationException}, and may proceed concurrently

 * with other operations.  Elements contained in the queue since the creation

 * of the iterator will be returned exactly once.

 *

 * <p>Beware that, unlike in most collections, the {@code size} method

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

 * asynchronous nature of these queues, determining the current number

 * of elements requires a traversal of the elements, and so may report

 * inaccurate results if this collection is modified during traversal.

 *

 * <p>Bulk operations that add, remove, or examine multiple elements,

 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},

 * are <em>not</em> guaranteed to be performed atomically.

 * For example, a {@code forEach} traversal concurrent with an {@code

 * addAll} operation might observe only some of the added elements.

 *

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

 * methods of the {@link Queue} 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 queue

 */

public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>

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

    private static final long serialVersionUID = 196745693267521676L;

    /*

     * This is a modification of the Michael & Scott algorithm,

     * adapted for a garbage-collected environment, with support for

     * interior node deletion (to support e.g. remove(Object)).  For

     * explanation, read the paper.

     *

     * 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.

     *

     * The fundamental invariants are:

     * - There is exactly one (last) Node with a null next reference,

     *   which is CASed when enqueueing.  This last Node can be

     *   reached in O(1) time from tail, but tail is merely an

     *   optimization - it can always be reached in O(N) time from

     *   head as well.

     * - The elements contained in the queue are the non-null items in

     *   Nodes that are reachable from head.  CASing the item

     *   reference of a Node to null atomically removes it from the

     *   queue.  Reachability of all elements from head must remain

     *   true even in the case of concurrent modifications that cause

     *   head to advance.  A dequeued Node may remain in use

     *   indefinitely due to creation of an Iterator or simply a

     *   poll() that has lost its time slice.

     *

     * The above might appear to imply that all Nodes are GC-reachable

     * from a predecessor dequeued Node.  That would cause two problems:

     * - allow a rogue Iterator to cause unbounded memory retention

     * - cause cross-generational linking of old Nodes to new Nodes if

     *   a Node was tenured while live, which generational GCs have a

     *   hard time dealing with, causing repeated major collections.

     * However, only non-deleted Nodes need to be reachable from

     * dequeued Nodes, and reachability does not necessarily have to

     * be of the kind understood by the GC.  We use the trick of

     * linking a Node that has just been dequeued to itself.  Such a

     * self-link implicitly means to advance to head.

     *

     * Both head and tail are permitted to lag.  In fact, failing to

     * update them every time one could is a significant optimization

     * (fewer CASes). As with LinkedTransferQueue (see the internal

     * documentation for that class), we use a slack threshold of two;

     * that is, we update head/tail when the current pointer appears

     * to be two or more steps away from the first/last node.

     *

     * Since head and tail are updated concurrently and independently,

     * it is possible for tail to lag behind head (why not)?

     *

     * CASing a Node's item reference to null atomically removes the

     * element from the queue, leaving a "dead" node that should later

     * be unlinked (but unlinking is merely an optimization).

     * Interior element removal methods (other than Iterator.remove())

     * keep track of the predecessor node during traversal so that the

     * node can be CAS-unlinked.  Some traversal methods try to unlink

     * any deleted nodes encountered during traversal.  See comments

     * in bulkRemove.

     *

     * When constructing a Node (before enqueuing it) we avoid paying

     * for a volatile write to item.  This allows the cost of enqueue

     * to be "one-and-a-half" CASes.

     *

     * Both head and tail may or may not point to a Node with a

     * non-null item.  If the queue is empty, all items must of course

     * be null.  Upon creation, both head and tail refer to a dummy

     * Node with null item.  Both head and tail are only updated using

     * CAS, so they never regress, although again this is merely an

     * optimization.

     */

    static final class Node<E> {

        volatile E item;

        volatile Node<E> next;

        /**

         * Constructs a node holding item.  Uses relaxed write because

         * item can only be seen after piggy-backing publication via CAS.

         */

        Node(E item) {

            ITEM.set(this, item);

        }

        /** Constructs a dead dummy node. */

        Node() {}

        void appendRelaxed(Node<E> next) {

            // assert next != null;

            // assert this.next == null;

            NEXT.set(this, next);

        }

        boolean casItem(E cmp, E val) {

            // assert item == cmp || item == null;

            // assert cmp != null;

            // assert val == null;

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

        }

    }

    /**

     * A node from which the first live (non-deleted) node (if any)

     * can be reached in O(1) time.

     * Invariants:

     * - all live nodes are reachable from head via succ()

     * - head != null

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

     * Non-invariants:

     * - head.item may or may not be null.

     * - it is permitted for tail to lag behind head, that is, for tail

     *   to not be reachable from head!

     */

    transient volatile Node<E> head;

    /**

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

     * node with node.next == null) can be reached in O(1) time.

     * Invariants:

     * - the last node is always reachable from tail via succ()

     * - tail != null

     * Non-invariants:

     * - tail.item may or may not be null.

     * - it is permitted for tail to lag behind head, that is, for tail

     *   to not be reachable from head!

     * - tail.next may or may not be self-linked.

     */

    private transient volatile Node<E> tail;

    /**

     * Creates a {@code ConcurrentLinkedQueue} that is initially empty.

     */

    public ConcurrentLinkedQueue() {

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

    }

    /**

     * Creates a {@code ConcurrentLinkedQueue}

     * 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) {

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

        for (E e : c) {

            Node<E> newNode = new Node<E>(Objects.requireNonNull(e));

            if (h == null)

                h = t = newNode;

            else

                t.appendRelaxed(t = newNode);

        }

        if (h == null)

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

        head = h;

        tail = t;

    }

    // Have to override just to update the javadoc

    /**

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

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

     * {@link IllegalStateException} or return {@code false}.

     *

     * @return {@code true} (as specified by {@link Collection#add})

     * @throws NullPointerException if the specified element is null

     */

    public boolean add(E e) {

        return offer(e);

    }

    /**

     * Tries to CAS head to p. If successful, repoint old head to itself

     * as sentinel for succ(), below.

     */

    final void updateHead(Node<E> h, Node<E> p) {

        // assert h != null && p != null && (h == p || h.item == null);

        if (h != p && HEAD.compareAndSet(this, h, p))

            NEXT.setRelease(h, h);

    }

    /**

     * Returns the successor of p, or the head node if p.next has been

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

     * stale pointer that is now off the list.

     */

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

        if (p == (p = p.next))

            p = head;

        return p;

    }

    /**

     * Tries to CAS pred.next (or head, if pred is null) from c to p.

     * Caller must ensure that we're not unlinking the trailing node.

     */

    private boolean tryCasSuccessor(Node<E> pred, Node<E> c, Node<E> p) {

        // assert p != null;

        // assert c.item == null;

        // assert c != p;

        if (pred != null)

            return NEXT.compareAndSet(pred, c, p);

        if (HEAD.compareAndSet(this, c, p)) {

            NEXT.setRelease(c, c);

            return true;

        }

        return false;

    }

    /**

     * Collapse dead nodes between pred and q.

     * @param pred the last known live node, or null if none

     * @param c the first dead node

     * @param p the last dead node

     * @param q p.next: the next live node, or null if at end

     * @return either old pred or p if pred dead or CAS failed

     */

    private Node<E> skipDeadNodes(Node<E> pred, Node<E> c, Node<E> p, Node<E> q) {

        // assert pred != c;

        // assert p != q;

        // assert c.item == null;

        // assert p.item == null;

        if (q == null) {

            // Never unlink trailing node.

            if (c == p) return pred;

            q = p;

        }

        return (tryCasSuccessor(pred, c, q)

                && (pred == null || ITEM.get(pred) != null))

            ? pred : p;

    }

    /**

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

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

     *

     * @return {@code true} (as specified by {@link Queue#offer})

     * @throws NullPointerException if the specified element is null

     */

    public boolean offer(E e) {

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

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

            Node<E> q = p.next;

            if (q == null) {

                // p is last node

                if (NEXT.compareAndSet(p, null, newNode)) {

                    // Successful CAS is the linearization point

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

                    // and for newNode to become "live".

                    if (p != t) // hop two nodes at a time; failure is OK

                        TAIL.weakCompareAndSet(this, t, newNode);

                    return true;

                }

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

            }

            else if (p == q)

                // We have fallen off list.  If tail is unchanged, it

                // will also be off-list, in which case we need to

                // jump to head, from which all live nodes are always

                // reachable.  Else the new tail is a better bet.

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

            else

                // Check for tail updates after two hops.

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

        }

    }

    public E poll() {

        restartFromHead: for (;;) {

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

                final E item;

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

                    // Successful CAS is the linearization point

                    // for item to be removed from this queue.

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

                        updateHead(h, ((q = p.next) != null) ? q : p);

                    return item;

                }

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

                    updateHead(h, p);

                    return null;

                }

                else if (p == q)

                    continue restartFromHead;

            }

        }

    }

    public E peek() {

        restartFromHead: for (;;) {

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

                final E item;

                if ((item = p.item) != null

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

                    updateHead(h, p);

                    return item;

                }

                else if (p == q)

                    continue restartFromHead;

            }

        }

    }

    /**

     * Returns the first live (non-deleted) node on list, or null if none.

     * This is yet another variant of poll/peek; here returning the

     * first node, not element.  We could make peek() a wrapper around

     * first(), but that would cost an extra volatile read of item,

     * and the need to add a retry loop to deal with the possibility

     * of losing a race to a concurrent poll().

     */

    Node<E> first() {

        restartFromHead: for (;;) {

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

                boolean hasItem = (p.item != null);

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

                    updateHead(h, p);

                    return hasItem ? p : null;

                }

                else if (p == q)

                    continue restartFromHead;

            }

        }

    }

    /**

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

     *

     * @return {@code true} 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 {@code Integer.MAX_VALUE} elements, returns

     * {@code Integer.MAX_VALUE}.

     *

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

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

     * asynchronous nature of these queues, determining the current

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

     * Additionally, if elements are added or removed during execution

     * of this method, the returned result may be inaccurate.  Thus,

     * this method is typically not very useful in concurrent

     * applications.

     *

     * @return the number of elements in this queue

     */

    public int size() {

        restartFromHead: for (;;) {

            int count = 0;

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

                if (p.item != null)

                    if (++count == Integer.MAX_VALUE)

                        break;  // @see Collection.size()

                if (p == (p = p.next))

                    continue restartFromHead;

            }

            return count;

        }

    }

    /**

     * Returns {@code true} if this queue contains the specified element.

     * More formally, returns {@code true} if and only if this queue contains

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

     *

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

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

     */

    public boolean contains(Object o) {

        if (o == null) return false;

        restartFromHead: for (;;) {

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

                Node<E> q = p.next;

                final E item;

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

                    if (o.equals(item))

                        return true;