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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 java.util.concurrent.locks.AbstractQueuedSynchronizer;

	/**
	* A synchronization aid that allows one or more threads to wait until
	* a set of operations being performed in other threads completes.
	*
	* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
	* The {@link #await await} methods block until the current count reaches
	* zero due to invocations of the {@link #countDown} method, after which
	* all waiting threads are released and any subsequent invocations of
	* {@link #await await} return immediately. This is a one-shot phenomenon
	* -- the count cannot be reset. If you need a version that resets the
	* count, consider using a {@link CyclicBarrier}.
	*
	* <p>A {@code CountDownLatch} is a versatile synchronization tool
	* and can be used for a number of purposes. A
	* {@code CountDownLatch} initialized with a count of one serves as a
	* simple on/off latch, or gate: all threads invoking {@link #await await}
	* wait at the gate until it is opened by a thread invoking {@link
	* #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
	* can be used to make one thread wait until <em>N</em> threads have
	* completed some action, or some action has been completed N times.
	*
	* <p>A useful property of a {@code CountDownLatch} is that it
	* doesn't require that threads calling {@code countDown} wait for
	* the count to reach zero before proceeding, it simply prevents any
	* thread from proceeding past an {@link #await await} until all
	* threads could pass.
	*
	* <p><b>Sample usage:</b> Here is a pair of classes in which a group
	* of worker threads use two countdown latches:
	* <ul>
	* <li>The first is a start signal that prevents any worker from proceeding
	* until the driver is ready for them to proceed;
	* <li>The second is a completion signal that allows the driver to wait
	* until all workers have completed.
	* </ul>
	*
	* <pre> {@code
	* class Driver { // ...
	* void main() throws InterruptedException {
	* CountDownLatch startSignal = new CountDownLatch(1);
	* CountDownLatch doneSignal = new CountDownLatch(N);
	*
	* for (int i = 0; i < N; ++i) // create and start threads
	* new Thread(new Worker(startSignal, doneSignal)).start();
	*
	* doSomethingElse(); // don't let run yet
	* startSignal.countDown(); // let all threads proceed
	* doSomethingElse();
	* doneSignal.await(); // wait for all to finish
	* }
	* }
	*
	* class Worker implements Runnable {
	* private final CountDownLatch startSignal;
	* private final CountDownLatch doneSignal;
	* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
	* this.startSignal = startSignal;
	* this.doneSignal = doneSignal;
	* }
	* public void run() {
	* try {
	* startSignal.await();
	* doWork();
	* doneSignal.countDown();
	* } catch (InterruptedException ex) {} // return;
	* }
	*
	* void doWork() { ... }
	* }}</pre>
	*
	* <p>Another typical usage would be to divide a problem into N parts,
	* describe each part with a Runnable that executes that portion and
	* counts down on the latch, and queue all the Runnables to an
	* Executor. When all sub-parts are complete, the coordinating thread
	* will be able to pass through await. (When threads must repeatedly
	* count down in this way, instead use a {@link CyclicBarrier}.)
	*
	* <pre> {@code
	* class Driver2 { // ...
	* void main() throws InterruptedException {
	* CountDownLatch doneSignal = new CountDownLatch(N);
	* Executor e = ...
	*
	* for (int i = 0; i < N; ++i) // create and start threads
	* e.execute(new WorkerRunnable(doneSignal, i));
	*
	* doneSignal.await(); // wait for all to finish
	* }
	* }
	*
	* class WorkerRunnable implements Runnable {
	* private final CountDownLatch doneSignal;
	* private final int i;
	* WorkerRunnable(CountDownLatch doneSignal, int i) {
	* this.doneSignal = doneSignal;
	* this.i = i;
	* }
	* public void run() {
	* try {
	* doWork(i);
	* doneSignal.countDown();
	* } catch (InterruptedException ex) {} // return;
	* }
	*
	* void doWork() { ... }
	* }}</pre>
	*
	* <p>Memory consistency effects: Until the count reaches
	* zero, actions in a thread prior to calling
	* {@code countDown()}
	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	* actions following a successful return from a corresponding
	* {@code await()} in another thread.
	*
	* @since 1.5
	* @author Doug Lea
	*/
	public class CountDownLatch {
	/**
	* Synchronization control For CountDownLatch.
	* Uses AQS state to represent count.
	*/
	private static final class Sync extends AbstractQueuedSynchronizer {
	private static final long serialVersionUID = 4982264981922014374L;

	Sync(int count) {
	setState(count);
	}

	int getCount() {
	return getState();
	}

	protected int tryAcquireShared(int acquires) {
	return (getState() == 0) ? 1 : -1;
	}

	protected boolean tryReleaseShared(int releases) {
	// Decrement count; signal when transition to zero
	for (;;) {
	int c = getState();
	if (c == 0)
	return false;
	int nextc = c-1;
	if (compareAndSetState(c, nextc))
	return nextc == 0;
	}
	}
	}

	private final Sync sync;

	/**
	* Constructs a {@code CountDownLatch} initialized with the given count.
	*
	* @param count the number of times {@link #countDown} must be invoked
	* before threads can pass through {@link #await}
	* @throws IllegalArgumentException if {@code count} is negative
	*/
	public CountDownLatch(int count) {
	if (count < 0) throw new IllegalArgumentException("count < 0");
	this.sync = new Sync(count);
	}

	/**
	* Causes the current thread to wait until the latch has counted down to
	* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
	*
	* <p>If the current count is zero then this method returns immediately.
	*
	* <p>If the current count is greater than zero then the current
	* thread becomes disabled for thread scheduling purposes and lies
	* dormant until one of two things happen:
	* <ul>
	* <li>The count reaches zero due to invocations of the
	* {@link #countDown} method; or
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread.
	* </ul>
	*
	* <p>If the current thread:
	* <ul>
	* <li>has its interrupted status set on entry to this method; or
	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
	* </ul>
	* then {@link InterruptedException} is thrown and the current thread's
	* interrupted status is cleared.
	*
	* @throws InterruptedException if the current thread is interrupted
	* while waiting
	*/
	public void await() throws InterruptedException {
	sync.acquireSharedInterruptibly(1);
	}

	/**
	* Causes the current thread to wait until the latch has counted down to
	* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
	* or the specified waiting time elapses.
	*
	* <p>If the current count is zero then this method returns immediately
	* with the value {@code true}.
	*
	* <p>If the current count is greater than zero then the current
	* thread becomes disabled for thread scheduling purposes and lies
	* dormant until one of three things happen:
	* <ul>
	* <li>The count reaches zero due to invocations of the
	* {@link #countDown} method; or
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	* <li>The specified waiting time elapses.
	* </ul>
	*
	* <p>If the count reaches zero then the method returns with the
	* value {@code true}.
	*
	* <p>If the current thread:
	* <ul>
	* <li>has its interrupted status set on entry to this method; or
	* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
	* </ul>
	* then {@link InterruptedException} is thrown and the current thread's
	* interrupted status is cleared.
	*
	* <p>If the specified waiting time elapses then the value {@code false}
	* is returned. If the time is less than or equal to zero, the method
	* will not wait at all.
	*
	* @param timeout the maximum time to wait
	* @param unit the time unit of the {@code timeout} argument
	* @return {@code true} if the count reached zero and {@code false}
	* if the waiting time elapsed before the count reached zero
	* @throws InterruptedException if the current thread is interrupted
	* while waiting
	*/
	public boolean await(long timeout, TimeUnit unit)
	throws InterruptedException {
	return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
	}

	/**
	* Decrements the count of the latch, releasing all waiting threads if
	* the count reaches zero.
	*
	* <p>If the current count is greater than zero then it is decremented.
	* If the new count is zero then all waiting threads are re-enabled for
	* thread scheduling purposes.
	*
	* <p>If the current count equals zero then nothing happens.
	*/
	public void countDown() {
	sync.releaseShared(1);
	}

	/**
	* Returns the current count.
	*
	* <p>This method is typically used for debugging and testing purposes.
	*
	* @return the current count
	*/
	public long getCount() {
	return sync.getCount();
	}

	/**
	* Returns a string identifying this latch, as well as its state.
	* The state, in brackets, includes the String {@code "Count ="}
	* followed by the current count.
	*
	* @return a string identifying this latch, as well as its state
	*/
	public String toString() {
	return super.toString() + "[Count = " + sync.getCount() + "]";
	}
	}

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