/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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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;

/**
 * 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;
 }

}