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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.Condition;
	import java.util.concurrent.locks.ReentrantLock;

	/**
	* A synchronization aid that allows a set of threads to all wait for
	* each other to reach a common barrier point. CyclicBarriers are
	* useful in programs involving a fixed sized party of threads that
	* must occasionally wait for each other. The barrier is called
	* <em>cyclic</em> because it can be re-used after the waiting threads
	* are released.
	*
	* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command
	* that is run once per barrier point, after the last thread in the party
	* arrives, but before any threads are released.
	* This <em>barrier action</em> is useful
	* for updating shared-state before any of the parties continue.
	*
	* <p><b>Sample usage:</b> Here is an example of using a barrier in a
	* parallel decomposition design:
	*
	* <pre> {@code
	* class Solver {
	* final int N;
	* final float[][] data;
	* final CyclicBarrier barrier;
	*
	* class Worker implements Runnable {
	* int myRow;
	* Worker(int row) { myRow = row; }
	* public void run() {
	* while (!done()) {
	* processRow(myRow);
	*
	* try {
	* barrier.await();
	* } catch (InterruptedException ex) {
	* return;
	* } catch (BrokenBarrierException ex) {
	* return;
	* }
	* }
	* }
	* }
	*
	* public Solver(float[][] matrix) {
	* data = matrix;
	* N = matrix.length;
	* Runnable barrierAction = () -> mergeRows(...);
	* barrier = new CyclicBarrier(N, barrierAction);
	*
	* List<Thread> threads = new ArrayList<>(N);
	* for (int i = 0; i < N; i++) {
	* Thread thread = new Thread(new Worker(i));
	* threads.add(thread);
	* thread.start();
	* }
	*
	* // wait until done
	* for (Thread thread : threads)
	* thread.join();
	* }
	* }}</pre>
	*
	* Here, each worker thread processes a row of the matrix then waits at the
	* barrier until all rows have been processed. When all rows are processed
	* the supplied {@link Runnable} barrier action is executed and merges the
	* rows. If the merger
	* determines that a solution has been found then {@code done()} will return
	* {@code true} and each worker will terminate.
	*
	* <p>If the barrier action does not rely on the parties being suspended when
	* it is executed, then any of the threads in the party could execute that
	* action when it is released. To facilitate this, each invocation of
	* {@link #await} returns the arrival index of that thread at the barrier.
	* You can then choose which thread should execute the barrier action, for
	* example:
	* <pre> {@code
	* if (barrier.await() == 0) {
	* // log the completion of this iteration
	* }}</pre>
	*
	* <p>The {@code CyclicBarrier} uses an all-or-none breakage model
	* for failed synchronization attempts: If a thread leaves a barrier
	* point prematurely because of interruption, failure, or timeout, all
	* other threads waiting at that barrier point will also leave
	* abnormally via {@link BrokenBarrierException} (or
	* {@link InterruptedException} if they too were interrupted at about
	* the same time).
	*
	* <p>Memory consistency effects: Actions in a thread prior to calling
	* {@code await()}
	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	* actions that are part of the barrier action, which in turn
	* <i>happen-before</i> actions following a successful return from the
	* corresponding {@code await()} in other threads.
	*
	* @see CountDownLatch
	*
	* @author Doug Lea
	* @since 1.5
	*/
	public class CyclicBarrier {
	/**
	* Each use of the barrier is represented as a generation instance.
	* The generation changes whenever the barrier is tripped, or
	* is reset. There can be many generations associated with threads
	* using the barrier - due to the non-deterministic way the lock
	* may be allocated to waiting threads - but only one of these
	* can be active at a time (the one to which {@code count} applies)
	* and all the rest are either broken or tripped.
	* There need not be an active generation if there has been a break
	* but no subsequent reset.
	*/
	private static class Generation {
	Generation() {} // prevent access constructor creation
	boolean broken; // initially false
	}

	/** The lock for guarding barrier entry */
	private final ReentrantLock lock = new ReentrantLock();
	/** Condition to wait on until tripped */
	private final Condition trip = lock.newCondition();
	/** The number of parties */
	private final int parties;
	/** The command to run when tripped */
	private final Runnable barrierCommand;
	/** The current generation */
	private Generation generation = new Generation();

	/**
	* Number of parties still waiting. Counts down from parties to 0
	* on each generation. It is reset to parties on each new
	* generation or when broken.
	*/
	private int count;

	/**
	* Updates state on barrier trip and wakes up everyone.
	* Called only while holding lock.
	*/
	private void nextGeneration() {
	// signal completion of last generation
	trip.signalAll();
	// set up next generation
	count = parties;
	generation = new Generation();
	}

	/**
	* Sets current barrier generation as broken and wakes up everyone.
	* Called only while holding lock.
	*/
	private void breakBarrier() {
	generation.broken = true;
	count = parties;
	trip.signalAll();
	}

	/**
	* Main barrier code, covering the various policies.
	*/
	private int dowait(boolean timed, long nanos)
	throws InterruptedException, BrokenBarrierException,
	TimeoutException {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	final Generation g = generation;

	if (g.broken)
	throw new BrokenBarrierException();

	if (Thread.interrupted()) {
	breakBarrier();
	throw new InterruptedException();
	}

	int index = --count;
	if (index == 0) { // tripped
	boolean ranAction = false;
	try {
	final Runnable command = barrierCommand;
	if (command != null)
	command.run();
	ranAction = true;
	nextGeneration();
	return 0;
	} finally {
	if (!ranAction)
	breakBarrier();
	}
	}

	// loop until tripped, broken, interrupted, or timed out
	for (;;) {
	try {
	if (!timed)
	trip.await();
	else if (nanos > 0L)
	nanos = trip.awaitNanos(nanos);
	} catch (InterruptedException ie) {
	if (g == generation && ! g.broken) {
	breakBarrier();
	throw ie;
	} else {
	// We're about to finish waiting even if we had not
	// been interrupted, so this interrupt is deemed to
	// "belong" to subsequent execution.
	Thread.currentThread().interrupt();
	}
	}

	if (g.broken)
	throw new BrokenBarrierException();

	if (g != generation)
	return index;

	if (timed && nanos <= 0L) {
	breakBarrier();
	throw new TimeoutException();
	}
	}
	} finally {
	lock.unlock();
	}
	}

	/**
	* Creates a new {@code CyclicBarrier} that will trip when the
	* given number of parties (threads) are waiting upon it, and which
	* will execute the given barrier action when the barrier is tripped,
	* performed by the last thread entering the barrier.
	*
	* @param parties the number of threads that must invoke {@link #await}
	* before the barrier is tripped
	* @param barrierAction the command to execute when the barrier is
	* tripped, or {@code null} if there is no action
	* @throws IllegalArgumentException if {@code parties} is less than 1
	*/
	public CyclicBarrier(int parties, Runnable barrierAction) {
	if (parties <= 0) throw new IllegalArgumentException();
	this.parties = parties;
	this.count = parties;
	this.barrierCommand = barrierAction;
	}

	/**
	* Creates a new {@code CyclicBarrier} that will trip when the
	* given number of parties (threads) are waiting upon it, and
	* does not perform a predefined action when the barrier is tripped.
	*
	* @param parties the number of threads that must invoke {@link #await}
	* before the barrier is tripped
	* @throws IllegalArgumentException if {@code parties} is less than 1
	*/
	public CyclicBarrier(int parties) {
	this(parties, null);
	}

	/**
	* Returns the number of parties required to trip this barrier.
	*
	* @return the number of parties required to trip this barrier
	*/
	public int getParties() {
	return parties;
	}

	/**
	* Waits until all {@linkplain #getParties parties} have invoked
	* {@code await} on this barrier.
	*
	* <p>If the current thread is not the last to arrive then it is
	* disabled for thread scheduling purposes and lies dormant until
	* one of the following things happens:
	* <ul>
	* <li>The last thread arrives; or
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* one of the other waiting threads; or
	* <li>Some other thread times out while waiting for barrier; or
	* <li>Some other thread invokes {@link #reset} on this barrier.
	* </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.
	*
	* <p>If the barrier is {@link #reset} while any thread is waiting,
	* or if the barrier {@linkplain #isBroken is broken} when
	* {@code await} is invoked, or while any thread is waiting, then
	* {@link BrokenBarrierException} is thrown.
	*
	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
	* then all other waiting threads will throw
	* {@link BrokenBarrierException} and the barrier is placed in the broken
	* state.
	*
	* <p>If the current thread is the last thread to arrive, and a
	* non-null barrier action was supplied in the constructor, then the
	* current thread runs the action before allowing the other threads to
	* continue.
	* If an exception occurs during the barrier action then that exception
	* will be propagated in the current thread and the barrier is placed in
	* the broken state.
	*
	* @return the arrival index of the current thread, where index
	* {@code getParties() - 1} indicates the first
	* to arrive and zero indicates the last to arrive
	* @throws InterruptedException if the current thread was interrupted
	* while waiting
	* @throws BrokenBarrierException if <em>another</em> thread was
	* interrupted or timed out while the current thread was
	* waiting, or the barrier was reset, or the barrier was
	* broken when {@code await} was called, or the barrier
	* action (if present) failed due to an exception
	*/
	public int await() throws InterruptedException, BrokenBarrierException {
	try {
	return dowait(false, 0L);
	} catch (TimeoutException toe) {
	throw new Error(toe); // cannot happen
	}
	}

	/**
	* Waits until all {@linkplain #getParties parties} have invoked
	* {@code await} on this barrier, or the specified waiting time elapses.
	*
	* <p>If the current thread is not the last to arrive then it is
	* disabled for thread scheduling purposes and lies dormant until
	* one of the following things happens:
	* <ul>
	* <li>The last thread arrives; or
	* <li>The specified timeout elapses; or
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* one of the other waiting threads; or
	* <li>Some other thread times out while waiting for barrier; or
	* <li>Some other thread invokes {@link #reset} on this barrier.
	* </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.
	*
	* <p>If the specified waiting time elapses then {@link TimeoutException}
	* is thrown. If the time is less than or equal to zero, the
	* method will not wait at all.
	*
	* <p>If the barrier is {@link #reset} while any thread is waiting,
	* or if the barrier {@linkplain #isBroken is broken} when
	* {@code await} is invoked, or while any thread is waiting, then
	* {@link BrokenBarrierException} is thrown.
	*
	* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
	* waiting, then all other waiting threads will throw {@link
	* BrokenBarrierException} and the barrier is placed in the broken
	* state.
	*
	* <p>If the current thread is the last thread to arrive, and a
	* non-null barrier action was supplied in the constructor, then the
	* current thread runs the action before allowing the other threads to
	* continue.
	* If an exception occurs during the barrier action then that exception
	* will be propagated in the current thread and the barrier is placed in
	* the broken state.
	*
	* @param timeout the time to wait for the barrier
	* @param unit the time unit of the timeout parameter
	* @return the arrival index of the current thread, where index
	* {@code getParties() - 1} indicates the first
	* to arrive and zero indicates the last to arrive
	* @throws InterruptedException if the current thread was interrupted
	* while waiting
	* @throws TimeoutException if the specified timeout elapses.
	* In this case the barrier will be broken.
	* @throws BrokenBarrierException if <em>another</em> thread was
	* interrupted or timed out while the current thread was
	* waiting, or the barrier was reset, or the barrier was broken
	* when {@code await} was called, or the barrier action (if
	* present) failed due to an exception
	*/
	public int await(long timeout, TimeUnit unit)
	throws InterruptedException,
	BrokenBarrierException,
	TimeoutException {
	return dowait(true, unit.toNanos(timeout));
	}

	/**
	* Queries if this barrier is in a broken state.
	*
	* @return {@code true} if one or more parties broke out of this
	* barrier due to interruption or timeout since
	* construction or the last reset, or a barrier action
	* failed due to an exception; {@code false} otherwise.
	*/
	public boolean isBroken() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return generation.broken;
	} finally {
	lock.unlock();
	}
	}

	/**
	* Resets the barrier to its initial state. If any parties are
	* currently waiting at the barrier, they will return with a
	* {@link BrokenBarrierException}. Note that resets <em>after</em>
	* a breakage has occurred for other reasons can be complicated to
	* carry out; threads need to re-synchronize in some other way,
	* and choose one to perform the reset. It may be preferable to
	* instead create a new barrier for subsequent use.
	*/
	public void reset() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	breakBarrier(); // break the current generation
	nextGeneration(); // start a new generation
	} finally {
	lock.unlock();
	}
	}

	/**
	* Returns the number of parties currently waiting at the barrier.
	* This method is primarily useful for debugging and assertions.
	*
	* @return the number of parties currently blocked in {@link #await}
	*/
	public int getNumberWaiting() {
	final ReentrantLock lock = this.lock;
	lock.lock();
	try {
	return parties - count;
	} finally {
	lock.unlock();
	}
	}
	}

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