/*

  * 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.*;

 import java.util.concurrent.locks.*;

 import java.util.concurrent.atomic.*;

 /**

  * A counting semaphore.  Conceptually, a semaphore maintains a set of

  * permits.  Each {@link #acquire} blocks if necessary until a permit is

  * available, and then takes it.  Each {@link #release} adds a permit,

  * potentially releasing a blocking acquirer.

  * However, no actual permit objects are used; the {@code Semaphore} just

  * keeps a count of the number available and acts accordingly.

  *

  * <p>Semaphores are often used to restrict the number of threads than can

  * access some (physical or logical) resource. For example, here is

  * a class that uses a semaphore to control access to a pool of items:

  * <pre>

  * class Pool {

  *   private static final int MAX_AVAILABLE = 100;

  *   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

  *

  *   public Object getItem() throws InterruptedException {

  *     available.acquire();

  *     return getNextAvailableItem();

  *   }

  *

  *   public void putItem(Object x) {

  *     if (markAsUnused(x))

  *       available.release();

  *   }

  *

  *   // Not a particularly efficient data structure; just for demo

  *

  *   protected Object[] items = ... whatever kinds of items being managed

  *   protected boolean[] used = new boolean[MAX_AVAILABLE];

  *

  *   protected synchronized Object getNextAvailableItem() {

  *     for (int i = 0; i < MAX_AVAILABLE; ++i) {

  *       if (!used[i]) {

  *          used[i] = true;

  *          return items[i];

  *       }

  *     }

  *     return null; // not reached

  *   }

  *

  *   protected synchronized boolean markAsUnused(Object item) {

  *     for (int i = 0; i < MAX_AVAILABLE; ++i) {

  *       if (item == items[i]) {

  *          if (used[i]) {

  *            used[i] = false;

  *            return true;

  *          } else

  *            return false;

  *       }

  *     }

  *     return false;

  *   }

  *

  * }

  * </pre>

  *

  * <p>Before obtaining an item each thread must acquire a permit from

  * the semaphore, guaranteeing that an item is available for use. When

  * the thread has finished with the item it is returned back to the

  * pool and a permit is returned to the semaphore, allowing another

  * thread to acquire that item.  Note that no synchronization lock is

  * held when {@link #acquire} is called as that would prevent an item

  * from being returned to the pool.  The semaphore encapsulates the

  * synchronization needed to restrict access to the pool, separately

  * from any synchronization needed to maintain the consistency of the

  * pool itself.

  *

  * <p>A semaphore initialized to one, and which is used such that it

  * only has at most one permit available, can serve as a mutual

  * exclusion lock.  This is more commonly known as a <em>binary

  * semaphore</em>, because it only has two states: one permit

  * available, or zero permits available.  When used in this way, the

  * binary semaphore has the property (unlike many {@link Lock}

  * implementations), that the "lock" can be released by a

  * thread other than the owner (as semaphores have no notion of

  * ownership).  This can be useful in some specialized contexts, such

  * as deadlock recovery.

  *

  * <p> The constructor for this class optionally accepts a

  * <em>fairness</em> parameter. When set false, this class makes no

  * guarantees about the order in which threads acquire permits. In

  * particular, <em>barging</em> is permitted, that is, a thread

  * invoking {@link #acquire} can be allocated a permit ahead of a

  * thread that has been waiting - logically the new thread places itself at

  * the head of the queue of waiting threads. When fairness is set true, the

  * semaphore guarantees that threads invoking any of the {@link

  * #acquire() acquire} methods are selected to obtain permits in the order in

  * which their invocation of those methods was processed

  * (first-in-first-out; FIFO). Note that FIFO ordering necessarily

  * applies to specific internal points of execution within these

  * methods.  So, it is possible for one thread to invoke

  * {@code acquire} before another, but reach the ordering point after

  * the other, and similarly upon return from the method.

  * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not

  * honor the fairness setting, but will take any permits that are

  * available.

  *

  * <p>Generally, semaphores used to control resource access should be

  * initialized as fair, to ensure that no thread is starved out from

  * accessing a resource. When using semaphores for other kinds of

  * synchronization control, the throughput advantages of non-fair

  * ordering often outweigh fairness considerations.

  *

  * <p>This class also provides convenience methods to {@link

  * #acquire(int) acquire} and {@link #release(int) release} multiple

  * permits at a time.  Beware of the increased risk of indefinite

  * postponement when these methods are used without fairness set true.

  *

  * <p>Memory consistency effects: Actions in a thread prior to calling

  * a "release" method such as {@code release()}

  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

  * actions following a successful "acquire" method such as {@code acquire()}

  * in another thread.

  *

  * @since 1.5

  * @author Doug Lea

  *

  */

 public class Semaphore implements java.io.Serializable {

     private static final long serialVersionUID = -3222578661600680210L;

     /** All mechanics via AbstractQueuedSynchronizer subclass */

     private final Sync sync;

     /**

      * Synchronization implementation for semaphore.  Uses AQS state

      * to represent permits. Subclassed into fair and nonfair

      * versions.

      */

     abstract static class Sync extends AbstractQueuedSynchronizer {

         private static final long serialVersionUID = 1192457210091910933L;

         Sync(int permits) {

             setState(permits);

         }

         final int getPermits() {

             return getState();

         }

         final int nonfairTryAcquireShared(int acquires) {

             for (;;) {

                 int available = getState();

                 int remaining = available - acquires;

                 if (remaining < 0 ||

                     compareAndSetState(available, remaining))

                     return remaining;

             }

         }

         protected final boolean tryReleaseShared(int releases) {

             for (;;) {

                 int current = getState();

                 int next = current + releases;

                 if (next < current) // overflow

                     throw new Error("Maximum permit count exceeded");

                 if (compareAndSetState(current, next))

                     return true;

             }

         }

         final void reducePermits(int reductions) {

             for (;;) {

                 int current = getState();

                 int next = current - reductions;

                 if (next > current) // underflow

                     throw new Error("Permit count underflow");

                 if (compareAndSetState(current, next))

                     return;

             }

         }

         final int drainPermits() {

             for (;;) {

                 int current = getState();

                 if (current == 0 || compareAndSetState(current, 0))

                     return current;

             }

         }

     }

     /**

      * NonFair version

      */

     static final class NonfairSync extends Sync {

         private static final long serialVersionUID = -2694183684443567898L;

         NonfairSync(int permits) {

             super(permits);

         }

         protected int tryAcquireShared(int acquires) {

             return nonfairTryAcquireShared(acquires);

         }

     }

     /**

      * Fair version

      */

     static final class FairSync extends Sync {

         private static final long serialVersionUID = 2014338818796000944L;

         FairSync(int permits) {

             super(permits);

         }

         protected int tryAcquireShared(int acquires) {

             for (;;) {

                 if (hasQueuedPredecessors())

                     return -1;

                 int available = getState();

                 int remaining = available - acquires;

                 if (remaining < 0 ||

                     compareAndSetState(available, remaining))

                     return remaining;

             }

         }

     }

     /**

      * Creates a {@code Semaphore} with the given number of

      * permits and nonfair fairness setting.

      *

      * @param permits the initial number of permits available.

      *        This value may be negative, in which case releases

      *        must occur before any acquires will be granted.

      */

     public Semaphore(int permits) {

         sync = new NonfairSync(permits);

     }

     /**

      * Creates a {@code Semaphore} with the given number of

      * permits and the given fairness setting.

      *

      * @param permits the initial number of permits available.

      *        This value may be negative, in which case releases

      *        must occur before any acquires will be granted.

      * @param fair {@code true} if this semaphore will guarantee

      *        first-in first-out granting of permits under contention,

      *        else {@code false}

      */

     public Semaphore(int permits, boolean fair) {

         sync = fair ? new FairSync(permits) : new NonfairSync(permits);

     }

     /**

      * Acquires a permit from this semaphore, blocking until one is

      * available, or the thread is {@linkplain Thread#interrupt interrupted}.

      *

      * <p>Acquires a permit, if one is available and returns immediately,

      * reducing the number of available permits by one.

      *

      * <p>If no permit is available then the current thread becomes

      * disabled for thread scheduling purposes and lies dormant until

      * one of two things happens:

      * <ul>

      * <li>Some other thread invokes the {@link #release} method for this

      * semaphore and the current thread is next to be assigned a permit; 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

      * for a permit,

      * </ul>

      * then {@link InterruptedException} is thrown and the current thread's

      * interrupted status is cleared.

      *

      * @throws InterruptedException if the current thread is interrupted

      */

     public void acquire() throws InterruptedException {

         sync.acquireSharedInterruptibly(1);

     }

     /**

      * Acquires a permit from this semaphore, blocking until one is

      * available.

      *

      * <p>Acquires a permit, if one is available and returns immediately,

      * reducing the number of available permits by one.

      *

      * <p>If no permit is available then the current thread becomes

      * disabled for thread scheduling purposes and lies dormant until

      * some other thread invokes the {@link #release} method for this

      * semaphore and the current thread is next to be assigned a permit.

      *

      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}

      * while waiting for a permit then it will continue to wait, but the

      * time at which the thread is assigned a permit may change compared to

      * the time it would have received the permit had no interruption

      * occurred.  When the thread does return from this method its interrupt

      * status will be set.

      */

     public void acquireUninterruptibly() {

         sync.acquireShared(1);

     }

     /**

      * Acquires a permit from this semaphore, only if one is available at the

      * time of invocation.

      *

      * <p>Acquires a permit, if one is available and returns immediately,

      * with the value {@code true},

      * reducing the number of available permits by one.

      *

      * <p>If no permit is available then this method will return

      * immediately with the value {@code false}.

      *

      * <p>Even when this semaphore has been set to use a

      * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>

      * immediately acquire a permit if one is available, whether or not

      * other threads are currently waiting.

      * This "barging" behavior can be useful in certain

      * circumstances, even though it breaks fairness. If you want to honor

      * the fairness setting, then use

      * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }

      * which is almost equivalent (it also detects interruption).

      *

      * @return {@code true} if a permit was acquired and {@code false}

      *         otherwise

      */

     public boolean tryAcquire() {

         return sync.nonfairTryAcquireShared(1) >= 0;

     }

     /**

      * Acquires a permit from this semaphore, if one becomes available

      * within the given waiting time and the current thread has not

      * been {@linkplain Thread#interrupt interrupted}.

      *

      * <p>Acquires a permit, if one is available and returns immediately,

      * with the value {@code true},

      * reducing the number of available permits by one.

      *

      * <p>If no permit is available then the current thread becomes

      * disabled for thread scheduling purposes and lies dormant until

      * one of three things happens:

      * <ul>

      * <li>Some other thread invokes the {@link #release} method for this

      * semaphore and the current thread is next to be assigned a permit; or

      * <li>Some other thread {@linkplain Thread#interrupt interrupts}

      * the current thread; or

      * <li>The specified waiting time elapses.

      * </ul>

      *

      * <p>If a permit is acquired then the value {@code true} is returned.

      *

      * <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

      * to acquire a permit,

      * </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 for a permit

      * @param unit the time unit of the {@code timeout} argument

      * @return {@code true} if a permit was acquired and {@code false}

      *         if the waiting time elapsed before a permit was acquired

      * @throws InterruptedException if the current thread is interrupted

      */

     public boolean tryAcquire(long timeout, TimeUnit unit)

         throws InterruptedException {

         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));

     }

     /**

      * Releases a permit, returning it to the semaphore.

      *

      * <p>Releases a permit, increasing the number of available permits by

      * one.  If any threads are trying to acquire a permit, then one is

      * selected and given the permit that was just released.  That thread

      * is (re)enabled for thread scheduling purposes.

      *

      * <p>There is no requirement that a thread that releases a permit must

      * have acquired that permit by calling {@link #acquire}.

      * Correct usage of a semaphore is established by programming convention

      * in the application.

      */

     public void release() {

         sync.releaseShared(1);

     }

     /**

      * Acquires the given number of permits from this semaphore,

      * blocking until all are available,

      * or the thread is {@linkplain Thread#interrupt interrupted}.

      *

      * <p>Acquires the given number of permits, if they are available,

      * and returns immediately, reducing the number of available permits

      * by the given amount.

      *

      * <p>If insufficient permits are available then the current thread becomes

      * disabled for thread scheduling purposes and lies dormant until

      * one of two things happens:

      * <ul>

      * <li>Some other thread invokes one of the {@link #release() release}

      * methods for this semaphore, the current thread is next to be assigned

      * permits and the number of available permits satisfies this request; 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

      * for a permit,

      * </ul>

      * then {@link InterruptedException} is thrown and the current thread's

      * interrupted status is cleared.

      * Any permits that were to be assigned to this thread are instead

      * assigned to other threads trying to acquire permits, as if

      * permits had been made available by a call to {@link #release()}.

      *

      * @param permits the number of permits to acquire

      * @throws InterruptedException if the current thread is interrupted

      * @throws IllegalArgumentException if {@code permits} is negative

      */

     public void acquire(int permits) throws InterruptedException {

         if (permits < 0) throw new IllegalArgumentException();

         sync.acquireSharedInterruptibly(permits);

     }

     /**

      * Acquires the given number of permits from this semaphore,

      * blocking until all are available.

      *

      * <p>Acquires the given number of permits, if they are available,

      * and returns immediately, reducing the number of available permits

      * by the given amount.

      *

      * <p>If insufficient permits are available then the current thread becomes

      * disabled for thread scheduling purposes and lies dormant until

      * some other thread invokes one of the {@link #release() release}

      * methods for this semaphore, the current thread is next to be assigned

      * permits and the number of available permits satisfies this request.

      *

      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}

      * while waiting for permits then it will continue to wait and its

      * position in the queue is not affected.  When the thread does return

      * from this method its interrupt status will be set.

      *

      * @param permits the number of permits to acquire

      * @throws IllegalArgumentException if {@code permits} is negative

      *

      */

     public void acquireUninterruptibly(int permits) {

         if (permits < 0) throw new IllegalArgumentException();

         sync.acquireShared(permits);

     }

     /**

      * Acquires the given number of permits from this semaphore, only

      * if all are available at the time of invocation.

      *

      * <p>Acquires the given number of permits, if they are available, and

      * returns immediately, with the value {@code true},

      * reducing the number of available permits by the given amount.

      *

      * <p>If insufficient permits are available then this method will return

      * immediately with the value {@code false} and the number of available

      * permits is unchanged.

      *

      * <p>Even when this semaphore has been set to use a fair ordering

      * policy, a call to {@code tryAcquire} <em>will</em>

      * immediately acquire a permit if one is available, whether or

      * not other threads are currently waiting.  This

      * "barging" behavior can be useful in certain

      * circumstances, even though it breaks fairness. If you want to

      * honor the fairness setting, then use {@link #tryAcquire(int,

      * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }

      * which is almost equivalent (it also detects interruption).

      *

      * @param permits the number of permits to acquire

      * @return {@code true} if the permits were acquired and

      *         {@code false} otherwise

      * @throws IllegalArgumentException if {@code permits} is negative

      */

     public boolean tryAcquire(int permits) {

         if (permits < 0) throw new IllegalArgumentException();

         return sync.nonfairTryAcquireShared(permits) >= 0;

     }

     /**

      * Acquires the given number of permits from this semaphore, if all

      * become available within the given waiting time and the current

      * thread has not been {@linkplain Thread#interrupt interrupted}.

      *

      * <p>Acquires the given number of permits, if they are available and

      * returns immediately, with the value {@code true},

      * reducing the number of available permits by the given amount.

      *

      * <p>If insufficient permits are available then

      * the current thread becomes disabled for thread scheduling

      * purposes and lies dormant until one of three things happens:

      * <ul>

      * <li>Some other thread invokes one of the {@link #release() release}

      * methods for this semaphore, the current thread is next to be assigned

      * permits and the number of available permits satisfies this request; or

      * <li>Some other thread {@linkplain Thread#interrupt interrupts}

      * the current thread; or

      * <li>The specified waiting time elapses.

      * </ul>

      *

      * <p>If the permits are acquired then the value {@code true} is returned.

      *

      * <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

      * to acquire the permits,

      * </ul>

      * then {@link InterruptedException} is thrown and the current thread's

      * interrupted status is cleared.

      * Any permits that were to be assigned to this thread, are instead

      * assigned to other threads trying to acquire permits, as if

      * the permits had been made available by a call to {@link #release()}.

      *

      * <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.  Any permits that were to be assigned to this

      * thread, are instead assigned to other threads trying to acquire

      * permits, as if the permits had been made available by a call to

      * {@link #release()}.

      *

      * @param permits the number of permits to acquire

      * @param timeout the maximum time to wait for the permits

      * @param unit the time unit of the {@code timeout} argument

      * @return {@code true} if all permits were acquired and {@code false}

      *         if the waiting time elapsed before all permits were acquired

      * @throws InterruptedException if the current thread is interrupted

      * @throws IllegalArgumentException if {@code permits} is negative

      */

     public boolean tryAcquire(int permits, long timeout, TimeUnit unit)

         throws InterruptedException {

         if (permits < 0) throw new IllegalArgumentException();

         return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));

     }

     /**

      * Releases the given number of permits, returning them to the semaphore.

      *

      * <p>Releases the given number of permits, increasing the number of

      * available permits by that amount.

      * If any threads are trying to acquire permits, then one

      * is selected and given the permits that were just released.

      * If the number of available permits satisfies that thread's request

      * then that thread is (re)enabled for thread scheduling purposes;

      * otherwise the thread will wait until sufficient permits are available.

      * If there are still permits available

      * after this thread's request has been satisfied, then those permits

      * are assigned in turn to other threads trying to acquire permits.

      *

      * <p>There is no requirement that a thread that releases a permit must

      * have acquired that permit by calling {@link Semaphore#acquire acquire}.

      * Correct usage of a semaphore is established by programming convention

      * in the application.

      *

      * @param permits the number of permits to release

      * @throws IllegalArgumentException if {@code permits} is negative

      */

     public void release(int permits) {

         if (permits < 0) throw new IllegalArgumentException();

         sync.releaseShared(permits);

     }

     /**

      * Returns the current number of permits available in this semaphore.

      *

      * <p>This method is typically used for debugging and testing purposes.

      *

      * @return the number of permits available in this semaphore

      */

     public int availablePermits() {

         return sync.getPermits();

     }

     /**

      * Acquires and returns all permits that are immediately available.

      *

      * @return the number of permits acquired

      */

     public int drainPermits() {

         return sync.drainPermits();

     }

     /**

      * Shrinks the number of available permits by the indicated

      * reduction. This method can be useful in subclasses that use

      * semaphores to track resources that become unavailable. This

      * method differs from {@code acquire} in that it does not block

      * waiting for permits to become available.

      *

      * @param reduction the number of permits to remove

      * @throws IllegalArgumentException if {@code reduction} is negative

      */

     protected void reducePermits(int reduction) {

         if (reduction < 0) throw new IllegalArgumentException();

         sync.reducePermits(reduction);

     }

     /**

      * Returns {@code true} if this semaphore has fairness set true.

      *

      * @return {@code true} if this semaphore has fairness set true

      */

     public boolean isFair() {

         return sync instanceof FairSync;

     }

     /**

      * Queries whether any threads are waiting to acquire. Note that

      * because cancellations may occur at any time, a {@code true}

      * return does not guarantee that any other thread will ever

      * acquire.  This method is designed primarily for use in

      * monitoring of the system state.

      *

      * @return {@code true} if there may be other threads waiting to

      *         acquire the lock

      */

     public final boolean hasQueuedThreads() {

         return sync.hasQueuedThreads();

     }

     /**

      * Returns an estimate of the number of threads waiting to acquire.

      * The value is only an estimate because the number of threads may

      * change dynamically while this method traverses internal data

      * structures.  This method is designed for use in monitoring of the

      * system state, not for synchronization control.

      *

      * @return the estimated number of threads waiting for this lock

      */

     public final int getQueueLength() {

         return sync.getQueueLength();

     }

     /**

      * Returns a collection containing threads that may be waiting to acquire.

      * Because the actual set of threads may change dynamically while

      * constructing this result, the returned collection is only a best-effort

      * estimate.  The elements of the returned collection are in no particular

      * order.  This method is designed to facilitate construction of

      * subclasses that provide more extensive monitoring facilities.

      *

      * @return the collection of threads

      */

     protected Collection<Thread> getQueuedThreads() {

         return sync.getQueuedThreads();

     }

     /**

      * Returns a string identifying this semaphore, as well as its state.

      * The state, in brackets, includes the String {@code "Permits ="}

      * followed by the number of permits.

      *

      * @return a string identifying this semaphore, as well as its state

      */

     public String toString() {

         return super.toString() + "[Permits = " + sync.getPermits() + "]";

     }

 }