/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * 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> {@code
 * 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 java.util.concurrent.locks.Lock}
 * implementations), that the &quot;lock&quot; 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. These methods are generally more efficient and
 * effective than loops. However, they do not establish any preference
 * order. For example, if thread A invokes {@code s.acquire(3}) and
 * thread B invokes {@code s.acquire(2)}, and two permits become
 * available, then there is no guarantee that thread B will obtain
 * them unless its acquire came first and Semaphore {@code s} is in
 * fair mode.
 *
 * <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 &quot;barging&quot; 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. This method has the same effect as the
 * loop {@code for (int i = 0; i < permits; ++i) acquire();} except
 * that it atomically acquires the permits all at once:
 *
 * <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 and 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. This method has the same effect as the
 * loop {@code for (int i = 0; i < permits; ++i) acquireUninterruptibly();}
 * except that it atomically acquires the permits all at once:
 *
 * <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 and 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
 * &quot;barging&quot; 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 and 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 thread
 * 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, or if negative permits are available, releases them.
 * Upon return, zero permits are available.
 *
 * @return the number of permits acquired or, if negative, the
 * number released
 */
 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
 * 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() + "]";
 }
}