/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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 * 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();
 }
 }
}