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	/*
	* 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 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 static java.util.concurrent.TimeUnit.NANOSECONDS;

	import java.util.AbstractQueue;
	import java.util.Collection;
	import java.util.Iterator;
	import java.util.NoSuchElementException;
	import java.util.Objects;
	import java.util.PriorityQueue;
	import java.util.concurrent.locks.Condition;
	import java.util.concurrent.locks.ReentrantLock;

	/**
	* An unbounded {@linkplain BlockingQueue blocking queue} of
	* {@code Delayed} elements, in which an element can only be taken
	* when its delay has expired. The <em>head</em> of the queue is that
	* {@code Delayed} element whose delay expired furthest in the
	* past. If no delay has expired there is no head and {@code poll}
	* will return {@code null}. Expiration occurs when an element's
	* {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
	* than or equal to zero. Even though unexpired elements cannot be
	* removed using {@code take} or {@code poll}, they are otherwise
	* treated as normal elements. For example, the {@code size} 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.
	* The Iterator provided in method {@link #iterator()} is <em>not</em>
	* guaranteed to traverse the elements of the DelayQueue in any
	* particular order.
	*
	* <p>This class is a member of the
	* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
	* Java Collections Framework</a>.
	*
	* @since 1.5
	* @author Doug Lea
	* @param <E> the type of elements held in this queue
	*/
	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
	implements BlockingQueue<E> {

	private final transient 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;

	/**
	* 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 {@code DelayQueue} that is initially empty.
	*/
	public DelayQueue() {}

	/**
	* Creates a {@code DelayQueue} 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 {@code true} (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 {@code true}
	* @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 {@code true}
	* @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 {@code null}
	* if this queue has no elements with an expired delay.
	*
	* @return the head of this queue, or {@code null} 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();
	return (first == null \|\| first.getDelay(NANOSECONDS) > 0)
	? null
	: 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(NANOSECONDS);
	if (delay <= 0L)
	return q.poll();
	first = null; // don't retain ref while waiting
	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 {@code null} 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 <= 0L)
	return null;
	else
	nanos = available.awaitNanos(nanos);
	} else {
	long delay = first.getDelay(NANOSECONDS);
	if (delay <= 0L)
	return q.poll();
	if (nanos <= 0L)
	return null;
	first = null; // don't retain ref while waiting
	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 {@code null} if this queue is empty. Unlike
	* {@code poll}, 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 {@code null} 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) {
	return drainTo(c, Integer.MAX_VALUE);
	}

	/**
	* @throws UnsupportedOperationException {@inheritDoc}
	* @throws ClassCastException {@inheritDoc}
	* @throws NullPointerException {@inheritDoc}
	* @throws IllegalArgumentException {@inheritDoc}
	*/
	public int drainTo(Collection<? super E> c, int maxElements) {
	Objects.requireNonNull(c);
	if (c == this)
	throw new IllegalArgumentException();
	if (maxElements <= 0)
	return 0;
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	int n = 0;
	for (E first;
	n < maxElements
	&& (first = q.peek()) != null
	&& first.getDelay(NANOSECONDS) <= 0;) {
	c.add(first); // In this order, in case add() throws.
	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 {@code Integer.MAX_VALUE} because
	* a {@code DelayQueue} is not capacity constrained.
	*
	* @return {@code Integer.MAX_VALUE}
	*/
	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
	* {@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>The following code can be used to dump a delay queue into a newly
	* allocated array of {@code Delayed}:
	*
	* <pre> {@code Delayed[] a = q.toArray(new Delayed[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 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();
	}
	}

	/**
	* Identity-based version for use in Itr.remove.
	*/
	void removeEQ(Object o) {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
	if (o == it.next()) {
	it.remove();
	break;
	}
	}
	} 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 href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* @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();
	return (E)array[lastRet = cursor++];
	}

	public void remove() {
	if (lastRet < 0)
	throw new IllegalStateException();
	removeEQ(array[lastRet]);
	lastRet = -1;
	}
	}

	}

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