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	/*
	* 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.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 "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() + "]";
	}
	}

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