content: /*
   * 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() + "]";
      }
  }
  