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The core HTTP and composition mechanisms of Ratpack
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/*
 * Copyright 2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package ratpack.stream.internal;

import org.reactivestreams.Publisher;
import org.reactivestreams.Subscriber;
import org.reactivestreams.Subscription;
import ratpack.stream.TransformablePublisher;

import java.util.concurrent.ConcurrentLinkedDeque;
import java.util.concurrent.atomic.AtomicBoolean;

public class MulticastPublisher implements TransformablePublisher {

  private final ConcurrentLinkedDeque> bufferedSubscribers = new ConcurrentLinkedDeque<>();
  private final Publisher upstreamPublisher;
  private final AtomicBoolean requestedUpstream;
  private final AtomicBoolean upstreamFinished;

  public MulticastPublisher(Publisher publisher) {
    this.upstreamPublisher = publisher;
    this.requestedUpstream = new AtomicBoolean();
    this.upstreamFinished = new AtomicBoolean();
  }

  @Override
  public void subscribe(final Subscriber downStreamSubscriber) {
    if (upstreamFinished.get()) {
      downStreamSubscriber.onError(new IllegalStateException("The upstream publisher has completed, either successfully or with error.  No further subscriptions will be accepted"));
    } else {
      ((TransformablePublisher) s -> s.onSubscribe(new Subscription() {
        AtomicBoolean requested = new AtomicBoolean();

        @Override
        public void request(long n) {
          if (requested.compareAndSet(false, true)) {
            bufferedSubscribers.add(s);
            tryUpstreamSubscribe();
          }
        }

        @Override
        public void cancel() {
          // buffer will deal with cancelling this subscription if the downstream subscriber cancels or throws an exception.
          // The downstream subscriber will be "unsubscribed" from this publisher.
          bufferedSubscribers.remove(s);
        }
      })).buffer().subscribe(downStreamSubscriber);
    }
  }

  private void tryUpstreamSubscribe() {
    if (requestedUpstream.compareAndSet(false, true)) {
      this.upstreamPublisher.subscribe(new Subscriber() {
        Subscription subscription;

        @Override
        public void onSubscribe(Subscription s) {
          this.subscription = s;
          this.subscription.request(Long.MAX_VALUE);
        }

        @Override
        public void onNext(T t) {
          for (Subscriber subscriber : bufferedSubscribers) {
            subscriber.onNext(t);
          }
        }

        @Override
        public void onError(Throwable t) {
          upstreamFinished.set(true);
          for (Subscriber subscriber : bufferedSubscribers) {
            subscriber.onError(t);
          }
        }

        @Override
        public void onComplete() {
          upstreamFinished.set(true);
          for (Subscriber subscriber : bufferedSubscribers) {
            subscriber.onComplete();
          }
        }
      });
    }
  }
}