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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;

	/**
	* Interrelated interfaces and static methods for establishing
	* flow-controlled components in which {@link Publisher Publishers}
	* produce items consumed by one or more {@link Subscriber
	* Subscribers}, each managed by a {@link Subscription
	* Subscription}.
	*
	* <p>These interfaces correspond to the <a
	* href="http://www.reactive-streams.org/"> reactive-streams</a>
	* specification. They apply in both concurrent and distributed
	* asynchronous settings: All (seven) methods are defined in {@code
	* void} "one-way" message style. Communication relies on a simple form
	* of flow control (method {@link Subscription#request}) that can be
	* used to avoid resource management problems that may otherwise occur
	* in "push" based systems.
	*
	* <p><b>Examples.</b> A {@link Publisher} usually defines its own
	* {@link Subscription} implementation; constructing one in method
	* {@code subscribe} and issuing it to the calling {@link
	* Subscriber}. It publishes items to the subscriber asynchronously,
	* normally using an {@link Executor}. For example, here is a very
	* simple publisher that only issues (when requested) a single {@code
	* TRUE} item to a single subscriber. Because the subscriber receives
	* only a single item, this class does not use buffering and ordering
	* control required in most implementations (for example {@link
	* SubmissionPublisher}).
	*
	* <pre> {@code
	* class OneShotPublisher implements Publisher<Boolean> {
	* private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
	* private boolean subscribed; // true after first subscribe
	* public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
	* if (subscribed)
	* subscriber.onError(new IllegalStateException()); // only one allowed
	* else {
	* subscribed = true;
	* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
	* }
	* }
	* static class OneShotSubscription implements Subscription {
	* private final Subscriber<? super Boolean> subscriber;
	* private final ExecutorService executor;
	* private Future<?> future; // to allow cancellation
	* private boolean completed;
	* OneShotSubscription(Subscriber<? super Boolean> subscriber,
	* ExecutorService executor) {
	* this.subscriber = subscriber;
	* this.executor = executor;
	* }
	* public synchronized void request(long n) {
	* if (!completed) {
	* completed = true;
	* if (n <= 0) {
	* IllegalArgumentException ex = new IllegalArgumentException();
	* executor.execute(() -> subscriber.onError(ex));
	* } else {
	* future = executor.submit(() -> {
	* subscriber.onNext(Boolean.TRUE);
	* subscriber.onComplete();
	* });
	* }
	* }
	* }
	* public synchronized void cancel() {
	* completed = true;
	* if (future != null) future.cancel(false);
	* }
	* }
	* }}</pre>
	*
	* <p>A {@link Subscriber} arranges that items be requested and
	* processed. Items (invocations of {@link Subscriber#onNext}) are
	* not issued unless requested, but multiple items may be requested.
	* Many Subscriber implementations can arrange this in the style of
	* the following example, where a buffer size of 1 single-steps, and
	* larger sizes usually allow for more efficient overlapped processing
	* with less communication; for example with a value of 64, this keeps
	* total outstanding requests between 32 and 64.
	* Because Subscriber method invocations for a given {@link
	* Subscription} are strictly ordered, there is no need for these
	* methods to use locks or volatiles unless a Subscriber maintains
	* multiple Subscriptions (in which case it is better to instead
	* define multiple Subscribers, each with its own Subscription).
	*
	* <pre> {@code
	* class SampleSubscriber<T> implements Subscriber<T> {
	* final Consumer<? super T> consumer;
	* Subscription subscription;
	* final long bufferSize;
	* long count;
	* SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
	* this.bufferSize = bufferSize;
	* this.consumer = consumer;
	* }
	* public void onSubscribe(Subscription subscription) {
	* long initialRequestSize = bufferSize;
	* count = bufferSize - bufferSize / 2; // re-request when half consumed
	* (this.subscription = subscription).request(initialRequestSize);
	* }
	* public void onNext(T item) {
	* if (--count <= 0)
	* subscription.request(count = bufferSize - bufferSize / 2);
	* consumer.accept(item);
	* }
	* public void onError(Throwable ex) { ex.printStackTrace(); }
	* public void onComplete() {}
	* }}</pre>
	*
	* <p>The default value of {@link #defaultBufferSize} may provide a
	* useful starting point for choosing request sizes and capacities in
	* Flow components based on expected rates, resources, and usages.
	* Or, when flow control is never needed, a subscriber may initially
	* request an effectively unbounded number of items, as in:
	*
	* <pre> {@code
	* class UnboundedSubscriber<T> implements Subscriber<T> {
	* public void onSubscribe(Subscription subscription) {
	* subscription.request(Long.MAX_VALUE); // effectively unbounded
	* }
	* public void onNext(T item) { use(item); }
	* public void onError(Throwable ex) { ex.printStackTrace(); }
	* public void onComplete() {}
	* void use(T item) { ... }
	* }}</pre>
	*
	* @author Doug Lea
	* @since 9
	*/
	public final class Flow {

	private Flow() {} // uninstantiable

	/**
	* A producer of items (and related control messages) received by
	* Subscribers. Each current {@link Subscriber} receives the same
	* items (via method {@code onNext}) in the same order, unless
	* drops or errors are encountered. If a Publisher encounters an
	* error that does not allow items to be issued to a Subscriber,
	* that Subscriber receives {@code onError}, and then receives no
	* further messages. Otherwise, when it is known that no further
	* messages will be issued to it, a subscriber receives {@code
	* onComplete}. Publishers ensure that Subscriber method
	* invocations for each subscription are strictly ordered in <a
	* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
	* order.
	*
	* <p>Publishers may vary in policy about whether drops (failures
	* to issue an item because of resource limitations) are treated
	* as unrecoverable errors. Publishers may also vary about
	* whether Subscribers receive items that were produced or
	* available before they subscribed.
	*
	* @param <T> the published item type
	*/
	@FunctionalInterface
	public static interface Publisher<T> {
	/**
	* Adds the given Subscriber if possible. If already
	* subscribed, or the attempt to subscribe fails due to policy
	* violations or errors, the Subscriber's {@code onError}
	* method is invoked with an {@link IllegalStateException}.
	* Otherwise, the Subscriber's {@code onSubscribe} method is
	* invoked with a new {@link Subscription}. Subscribers may
	* enable receiving items by invoking the {@code request}
	* method of this Subscription, and may unsubscribe by
	* invoking its {@code cancel} method.
	*
	* @param subscriber the subscriber
	* @throws NullPointerException if subscriber is null
	*/
	public void subscribe(Subscriber<? super T> subscriber);
	}

	/**
	* A receiver of messages. The methods in this interface are
	* invoked in strict sequential order for each {@link
	* Subscription}.
	*
	* @param <T> the subscribed item type
	*/
	public static interface Subscriber<T> {
	/**
	* Method invoked prior to invoking any other Subscriber
	* methods for the given Subscription. If this method throws
	* an exception, resulting behavior is not guaranteed, but may
	* cause the Subscription not to be established or to be cancelled.
	*
	* <p>Typically, implementations of this method invoke {@code
	* subscription.request} to enable receiving items.
	*
	* @param subscription a new subscription
	*/
	public void onSubscribe(Subscription subscription);

	/**
	* Method invoked with a Subscription's next item. If this
	* method throws an exception, resulting behavior is not
	* guaranteed, but may cause the Subscription to be cancelled.
	*
	* @param item the item
	*/
	public void onNext(T item);

	/**
	* Method invoked upon an unrecoverable error encountered by a
	* Publisher or Subscription, after which no other Subscriber
	* methods are invoked by the Subscription. If this method
	* itself throws an exception, resulting behavior is
	* undefined.
	*
	* @param throwable the exception
	*/
	public void onError(Throwable throwable);

	/**
	* Method invoked when it is known that no additional
	* Subscriber method invocations will occur for a Subscription
	* that is not already terminated by error, after which no
	* other Subscriber methods are invoked by the Subscription.
	* If this method throws an exception, resulting behavior is
	* undefined.
	*/
	public void onComplete();
	}

	/**
	* Message control linking a {@link Publisher} and {@link
	* Subscriber}. Subscribers receive items only when requested,
	* and may cancel at any time. The methods in this interface are
	* intended to be invoked only by their Subscribers; usages in
	* other contexts have undefined effects.
	*/
	public static interface Subscription {
	/**
	* Adds the given number {@code n} of items to the current
	* unfulfilled demand for this subscription. If {@code n} is
	* less than or equal to zero, the Subscriber will receive an
	* {@code onError} signal with an {@link
	* IllegalArgumentException} argument. Otherwise, the
	* Subscriber will receive up to {@code n} additional {@code
	* onNext} invocations (or fewer if terminated).
	*
	* @param n the increment of demand; a value of {@code
	* Long.MAX_VALUE} may be considered as effectively unbounded
	*/
	public void request(long n);

	/**
	* Causes the Subscriber to (eventually) stop receiving
	* messages. Implementation is best-effort -- additional
	* messages may be received after invoking this method.
	* A cancelled subscription need not ever receive an
	* {@code onComplete} or {@code onError} signal.
	*/
	public void cancel();
	}

	/**
	* A component that acts as both a Subscriber and Publisher.
	*
	* @param <T> the subscribed item type
	* @param <R> the published item type
	*/
	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
	}

	static final int DEFAULT_BUFFER_SIZE = 256;

	/**
	* Returns a default value for Publisher or Subscriber buffering,
	* that may be used in the absence of other constraints.
	*
	* @implNote
	* The current value returned is 256.
	*
	* @return the buffer size value
	*/
	public static int defaultBufferSize() {
	return DEFAULT_BUFFER_SIZE;
	}

	}

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