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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/licenses/publicdomain

 */

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: 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();

        }

        public int tryAcquireShared(int acquires) {

            return getState() == 0? 1 : -1;

        }

        public 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() + "]";

    }

}