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	/*
	* Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
	* 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.
	*/

	package java.lang.ref;

	import java.util.function.Consumer;
	import jdk.internal.misc.VM;

	/**
	* Reference queues, to which registered reference objects are appended by the
	* garbage collector after the appropriate reachability changes are detected.
	*
	* @author Mark Reinhold
	* @since 1.2
	*/

	public class ReferenceQueue<T> {

	/**
	* Constructs a new reference-object queue.
	*/
	public ReferenceQueue() { }

	private static class Null extends ReferenceQueue<Object> {
	boolean enqueue(Reference<?> r) {
	return false;
	}
	}

	static final ReferenceQueue<Object> NULL = new Null();
	static final ReferenceQueue<Object> ENQUEUED = new Null();

	private static class Lock { };
	private final Lock lock = new Lock();
	private volatile Reference<? extends T> head;
	private long queueLength = 0;

	boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
	synchronized (lock) {
	// Check that since getting the lock this reference hasn't already been
	// enqueued (and even then removed)
	ReferenceQueue<?> queue = r.queue;
	if ((queue == NULL) \|\| (queue == ENQUEUED)) {
	return false;
	}
	assert queue == this;
	// Self-loop end, so if a FinalReference it remains inactive.
	r.next = (head == null) ? r : head;
	head = r;
	queueLength++;
	// Update r.queue after adding to list, to avoid race
	// with concurrent enqueued checks and fast-path poll().
	// Volatiles ensure ordering.
	r.queue = ENQUEUED;
	if (r instanceof FinalReference) {
	VM.addFinalRefCount(1);
	}
	lock.notifyAll();
	return true;
	}
	}

	private Reference<? extends T> reallyPoll() { /* Must hold lock */
	Reference<? extends T> r = head;
	if (r != null) {
	r.queue = NULL;
	// Update r.queue before removing from list, to avoid
	// race with concurrent enqueued checks and fast-path
	// poll(). Volatiles ensure ordering.
	@SuppressWarnings("unchecked")
	Reference<? extends T> rn = r.next;
	// Handle self-looped next as end of list designator.
	head = (rn == r) ? null : rn;
	// Self-loop next rather than setting to null, so if a
	// FinalReference it remains inactive.
	r.next = r;
	queueLength--;
	if (r instanceof FinalReference) {
	VM.addFinalRefCount(-1);
	}
	return r;
	}
	return null;
	}

	/**
	* Polls this queue to see if a reference object is available. If one is
	* available without further delay then it is removed from the queue and
	* returned. Otherwise this method immediately returns {@code null}.
	*
	* @return A reference object, if one was immediately available,
	* otherwise {@code null}
	*/
	public Reference<? extends T> poll() {
	if (head == null)
	return null;
	synchronized (lock) {
	return reallyPoll();
	}
	}

	/**
	* Removes the next reference object in this queue, blocking until either
	* one becomes available or the given timeout period expires.
	*
	* <p> This method does not offer real-time guarantees: It schedules the
	* timeout as if by invoking the {@link Object#wait(long)} method.
	*
	* @param timeout If positive, block for up to {@code timeout}
	* milliseconds while waiting for a reference to be
	* added to this queue. If zero, block indefinitely.
	*
	* @return A reference object, if one was available within the specified
	* timeout period, otherwise {@code null}
	*
	* @throws IllegalArgumentException
	* If the value of the timeout argument is negative
	*
	* @throws InterruptedException
	* If the timeout wait is interrupted
	*/
	public Reference<? extends T> remove(long timeout)
	throws IllegalArgumentException, InterruptedException
	{
	if (timeout < 0) {
	throw new IllegalArgumentException("Negative timeout value");
	}
	synchronized (lock) {
	Reference<? extends T> r = reallyPoll();
	if (r != null) return r;
	long start = (timeout == 0) ? 0 : System.nanoTime();
	for (;;) {
	lock.wait(timeout);
	r = reallyPoll();
	if (r != null) return r;
	if (timeout != 0) {
	long end = System.nanoTime();
	timeout -= (end - start) / 1000_000;
	if (timeout <= 0) return null;
	start = end;
	}
	}
	}
	}

	/**
	* Removes the next reference object in this queue, blocking until one
	* becomes available.
	*
	* @return A reference object, blocking until one becomes available
	* @throws InterruptedException If the wait is interrupted
	*/
	public Reference<? extends T> remove() throws InterruptedException {
	return remove(0);
	}

	/**
	* Iterate queue and invoke given action with each Reference.
	* Suitable for diagnostic purposes.
	* WARNING: any use of this method should make sure to not
	* retain the referents of iterated references (in case of
	* FinalReference(s)) so that their life is not prolonged more
	* than necessary.
	*/
	void forEach(Consumer<? super Reference<? extends T>> action) {
	for (Reference<? extends T> r = head; r != null;) {
	action.accept(r);
	@SuppressWarnings("unchecked")
	Reference<? extends T> rn = r.next;
	if (rn == r) {
	if (r.queue == ENQUEUED) {
	// still enqueued -> we reached end of chain
	r = null;
	} else {
	// already dequeued: r.queue == NULL; ->
	// restart from head when overtaken by queue poller(s)
	r = head;
	}
	} else {
	// next in chain
	r = rn;
	}
	}
	}
	}

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