/*

  * 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

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  */

 /*

  * 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.*;

 import java.util.concurrent.atomic.*;

 /**

  * 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>

  * 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>

  * 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() + "]";

     }

 }