/*

 * 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

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/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

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

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

 * http://creativecommons.org/publicdomain/zero/1.0/

 */

package java.util.concurrent;

import java.util.concurrent.locks.*;

import java.util.*;

/**

 * An unbounded {@linkplain BlockingQueue blocking queue} of

 * <tt>Delayed</tt> elements, in which an element can only be taken

 * when its delay has expired.  The <em>head</em> of the queue is that

 * <tt>Delayed</tt> element whose delay expired furthest in the

 * past.  If no delay has expired there is no head and <tt>poll</tt>

 * will return <tt>null</tt>. Expiration occurs when an element's

 * <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less

 * than or equal to zero.  Even though unexpired elements cannot be

 * removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise

 * treated as normal elements. For example, the <tt>size</tt> method

 * returns the count of both expired and unexpired elements.

 * This queue does not permit null elements.

 *

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

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

 * Iterator} interfaces.

 *

 * <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 DelayQueue<E extends Delayed> extends AbstractQueue<E>

    implements BlockingQueue<E> {

    private transient final ReentrantLock lock = new ReentrantLock();

    private final PriorityQueue<E> q = new PriorityQueue<E>();

    /**

     * Thread designated to wait for the element at the head of

     * the queue.  This variant of the Leader-Follower pattern

     * (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to

     * minimize unnecessary timed waiting.  When a thread becomes

     * the leader, it waits only for the next delay to elapse, but

     * other threads await indefinitely.  The leader thread must

     * signal some other thread before returning from take() or

     * poll(...), unless some other thread becomes leader in the

     * interim.  Whenever the head of the queue is replaced with

     * an element with an earlier expiration time, the leader

     * field is invalidated by being reset to null, and some

     * waiting thread, but not necessarily the current leader, is

     * signalled.  So waiting threads must be prepared to acquire

     * and lose leadership while waiting.

     */

    private Thread leader = null;

    /**

     * Condition signalled when a newer element becomes available

     * at the head of the queue or a new thread may need to

     * become leader.

     */

    private final Condition available = lock.newCondition();

    /**

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

     */

    public DelayQueue() {}

    /**

     * Creates a <tt>DelayQueue</tt> initially containing the elements of the

     * given collection of {@link Delayed} instances.

     *

     * @param c the collection of elements to initially contain

     * @throws NullPointerException if the specified collection or any

     *         of its elements are null

     */

    public DelayQueue(Collection<? extends E> c) {

        this.addAll(c);

    }

    /**

     * Inserts the specified element into this delay queue.

     *

     * @param e the element to add

     * @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 into this delay queue.

     *

     * @param e the element to add

     * @return <tt>true</tt>

     * @throws NullPointerException if the specified element is null

     */

    public boolean offer(E e) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            q.offer(e);

            if (q.peek() == e) {

                leader = null;

                available.signal();

            }

            return true;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Inserts the specified element into this delay queue. As the queue is

     * unbounded this method will never block.

     *

     * @param e the element to add

     * @throws NullPointerException {@inheritDoc}

     */

    public void put(E e) {

        offer(e);

    }

    /**

     * Inserts the specified element into this delay queue. As the queue is

     * unbounded this method will never block.

     *

     * @param e the element to add

     * @param timeout This parameter is ignored as the method never blocks

     * @param unit This parameter is ignored as the method never blocks

     * @return <tt>true</tt>

     * @throws NullPointerException {@inheritDoc}

     */

    public boolean offer(E e, long timeout, TimeUnit unit) {

        return offer(e);

    }

    /**

     * Retrieves and removes the head of this queue, or returns <tt>null</tt>

     * if this queue has no elements with an expired delay.

     *

     * @return the head of this queue, or <tt>null</tt> if this

     *         queue has no elements with an expired delay

     */

    public E poll() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            E first = q.peek();

            if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)

                return null;

            else

                return q.poll();

        } finally {

            lock.unlock();

        }

    }

    /**

     * Retrieves and removes the head of this queue, waiting if necessary

     * until an element with an expired delay is available on this queue.

     *

     * @return the head of this queue

     * @throws InterruptedException {@inheritDoc}

     */

    public E take() throws InterruptedException {

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            for (;;) {

                E first = q.peek();

                if (first == null)

                    available.await();

                else {

                    long delay = first.getDelay(TimeUnit.NANOSECONDS);

                    if (delay <= 0)

                        return q.poll();

                    else if (leader != null)

                        available.await();

                    else {

                        Thread thisThread = Thread.currentThread();

                        leader = thisThread;

                        try {

                            available.awaitNanos(delay);

                        } finally {

                            if (leader == thisThread)

                                leader = null;

                        }

                    }

                }

            }

        } finally {

            if (leader == null && q.peek() != null)

                available.signal();

            lock.unlock();

        }

    }

    /**

     * Retrieves and removes the head of this queue, waiting if necessary

     * until an element with an expired delay is available on this queue,

     * or the specified wait time expires.

     *

     * @return the head of this queue, or <tt>null</tt> if the

     *         specified waiting time elapses before an element with

     *         an expired delay becomes available

     * @throws InterruptedException {@inheritDoc}

     */

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {

        long nanos = unit.toNanos(timeout);

        final ReentrantLock lock = this.lock;

        lock.lockInterruptibly();

        try {

            for (;;) {

                E first = q.peek();

                if (first == null) {

                    if (nanos <= 0)

                        return null;

                    else

                        nanos = available.awaitNanos(nanos);

                } else {

                    long delay = first.getDelay(TimeUnit.NANOSECONDS);

                    if (delay <= 0)

                        return q.poll();

                    if (nanos <= 0)

                        return null;

                    if (nanos < delay || leader != null)

                        nanos = available.awaitNanos(nanos);

                    else {

                        Thread thisThread = Thread.currentThread();

                        leader = thisThread;

                        try {

                            long timeLeft = available.awaitNanos(delay);

                            nanos -= delay - timeLeft;

                        } finally {

                            if (leader == thisThread)

                                leader = null;

                        }

                    }

                }

            }

        } finally {

            if (leader == null && q.peek() != null)

                available.signal();

            lock.unlock();

        }

    }

    /**

     * Retrieves, but does not remove, the head of this queue, or

     * returns <tt>null</tt> if this queue is empty.  Unlike

     * <tt>poll</tt>, if no expired elements are available in the queue,

     * this method returns the element that will expire next,

     * if one exists.

     *

     * @return the head of this queue, or <tt>null</tt> if this

     *         queue is empty.

     */

    public E peek() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.peek();

        } finally {

            lock.unlock();

        }

    }

    public int size() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.size();

        } finally {

            lock.unlock();

        }

    }

    /**

     * @throws UnsupportedOperationException {@inheritDoc}

     * @throws ClassCastException            {@inheritDoc}

     * @throws NullPointerException          {@inheritDoc}

     * @throws IllegalArgumentException      {@inheritDoc}

     */

    public int drainTo(Collection<? super E> c) {

        if (c == null)

            throw new NullPointerException();

        if (c == this)

            throw new IllegalArgumentException();

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int n = 0;

            for (;;) {

                E first = q.peek();

                if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)

                    break;

                c.add(q.poll());

                ++n;

            }

            return n;

        } finally {

            lock.unlock();

        }

    }

    /**

     * @throws UnsupportedOperationException {@inheritDoc}

     * @throws ClassCastException            {@inheritDoc}

     * @throws NullPointerException          {@inheritDoc}

     * @throws IllegalArgumentException      {@inheritDoc}

     */

    public int drainTo(Collection<? super E> c, int maxElements) {

        if (c == null)

            throw new NullPointerException();

        if (c == this)

            throw new IllegalArgumentException();

        if (maxElements <= 0)

            return 0;

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            int n = 0;

            while (n < maxElements) {

                E first = q.peek();

                if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)

                    break;

                c.add(q.poll());

                ++n;

            }

            return n;

        } finally {

            lock.unlock();

        }

    }

    /**

     * Atomically removes all of the elements from this delay queue.

     * The queue will be empty after this call returns.

     * Elements with an unexpired delay are not waited for; they are

     * simply discarded from the queue.

     */

    public void clear() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            q.clear();

        } finally {

            lock.unlock();

        }

    }

    /**

     * Always returns <tt>Integer.MAX_VALUE</tt> because

     * a <tt>DelayQueue</tt> is not capacity constrained.

     *

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

     */

    public int remainingCapacity() {

        return Integer.MAX_VALUE;

    }

    /**

     * Returns an array containing all of the elements in this queue.

     * The returned array elements are in no particular order.

     *

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

     * maintained by this queue.  (In other words, this method must allocate

     * a new array).  The caller is thus free to modify the returned array.

     *

     * <p>This method acts as bridge between array-based and collection-based

     * APIs.

     *

     * @return an array containing all of the elements in this queue

     */

    public Object[] toArray() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.toArray();

        } finally {

            lock.unlock();

        }

    }

    /**

     * Returns an array containing all of the elements in this queue; the

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

     * The returned array elements are in no particular order.

     * If the queue fits in the specified array, it is returned therein.

     * Otherwise, a new array is allocated with the runtime type of the

     * specified array and the size of this queue.

     *

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

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

     * the array immediately following the end of the queue is set to

     * <tt>null</tt>.

     *

     * <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>The following code can be used to dump a delay queue into a newly

     * allocated array of <tt>Delayed</tt>:

     *

     * <pre>

     *     Delayed[] a = q.toArray(new Delayed[0]);</pre>

     *

     * Note that <tt>toArray(new Object[0])</tt> is identical in function to

     * <tt>toArray()</tt>.

     *

     * @param a the array into which the elements of the queue are to

     *          be stored, if it is big enough; otherwise, a new array of the

     *          same runtime type is allocated for this purpose

     * @return an array containing all of the elements in this queue

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

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

     *         this queue

     * @throws NullPointerException if the specified array is null

     */

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

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.toArray(a);

        } finally {

            lock.unlock();

        }

    }

    /**

     * Removes a single instance of the specified element from this

     * queue, if it is present, whether or not it has expired.

     */

    public boolean remove(Object o) {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return q.remove(o);

        } finally {

            lock.unlock();

        }

    }

    /**

     * Returns an iterator over all the elements (both expired and

     * unexpired) in this queue. The iterator does not return the

     * elements in any particular order.

     *

     * <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 queue

     */

    public Iterator<E> iterator() {

        return new Itr(toArray());

    }

    /**

     * Snapshot iterator that works off copy of underlying q array.

     */

    private class Itr implements Iterator<E> {

        final Object[] array; // Array of all elements

        int cursor;           // index of next element to return;

        int lastRet;          // index of last element, or -1 if no such

        Itr(Object[] array) {

            lastRet = -1;

            this.array = array;

        }

        public boolean hasNext() {

            return cursor < array.length;

        }

        @SuppressWarnings("unchecked")

        public E next() {

            if (cursor >= array.length)

                throw new NoSuchElementException();

            lastRet = cursor;

            return (E)array[cursor++];

        }

        public void remove() {

            if (lastRet < 0)

                throw new IllegalStateException();

            Object x = array[lastRet];

            lastRet = -1;

            // Traverse underlying queue to find == element,

            // not just a .equals element.

            lock.lock();

            try {

                for (Iterator it = q.iterator(); it.hasNext(); ) {

                    if (it.next() == x) {

                        it.remove();

                        return;

                    }

                }

            } finally {

                lock.unlock();

            }

        }

    }

}