/*
    * ====================================================================
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you 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.
   * ====================================================================
   *
   * This software consists of voluntary contributions made by many
   * individuals on behalf of the Apache Software Foundation.  For more
   * information on the Apache Software Foundation, please see
   * <http://www.apache.org/>.
   *
   */
  package org.apache.hc.core5.reactive;
  
  import java.io.IOException;
  import java.nio.ByteBuffer;
  import java.util.ArrayDeque;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.concurrent.locks.ReentrantLock;
  
  import org.apache.hc.core5.annotation.Contract;
  import org.apache.hc.core5.annotation.ThreadingBehavior;
  import org.apache.hc.core5.http.HttpStreamResetException;
  import org.apache.hc.core5.http.nio.AsyncDataProducer;
  import org.apache.hc.core5.http.nio.DataStreamChannel;
  import org.apache.hc.core5.util.Args;
  import org.reactivestreams.Publisher;
  import org.reactivestreams.Subscriber;
  import org.reactivestreams.Subscription;
  
  /**
   * An asynchronous data producer that supports Reactive Streams.
   *
   * @since 5.0
   */
  @Contract(threading = ThreadingBehavior.SAFE)
  final class ReactiveDataProducer implements AsyncDataProducer, Subscriber<ByteBuffer> {
  
      private static final int BUFFER_WINDOW_SIZE = 5;
  
      private final AtomicReference<DataStreamChannel> requestChannel = new AtomicReference<>();
      private final AtomicReference<Throwable> exception = new AtomicReference<>();
      private final AtomicBoolean complete = new AtomicBoolean(false);
      private final Publisher<ByteBuffer> publisher;
      private final AtomicReference<Subscription> subscription = new AtomicReference<>();
      private final ArrayDeque<ByteBuffer> buffers = new ArrayDeque<>(); // This field requires synchronization
  
      private final ReentrantLock lock;
  
      public ReactiveDataProducer(final Publisher<ByteBuffer> publisher) {
          this.publisher = Args.notNull(publisher, "publisher");
          this.lock = new ReentrantLock();
      }
  
      void setChannel(final DataStreamChannel channel) {
          requestChannel.set(channel);
      }
  
      @Override
      public void onSubscribe(final Subscription subscription) {
          if (this.subscription.getAndSet(subscription) != null) {
              throw new IllegalStateException("Already subscribed");
          }
  
          subscription.request(BUFFER_WINDOW_SIZE);
      }
  
      @Override
      public void onNext(final ByteBuffer byteBuffer) {
          final byte[] copy = new byte[byteBuffer.remaining()];
          byteBuffer.get(copy);
          lock.lock();
          try {
              buffers.add(ByteBuffer.wrap(copy));
          } finally {
              lock.unlock();
          }
          signalReadiness();
      }
  
      @Override
      public void onError(final Throwable throwable) {
          subscription.set(null);
          exception.set(throwable);
         signalReadiness();
     }
 
     @Override
     public void onComplete() {
         subscription.set(null);
         complete.set(true);
         signalReadiness();
     }
 
     private void signalReadiness() {
         final DataStreamChannel channel = requestChannel.get();
         if (channel == null) {
             throw new IllegalStateException("Output channel is not set");
         }
         channel.requestOutput();
     }
 
     @Override
     public int available() {
         if (exception.get() != null || complete.get()) {
             return 1;
         } else {
             lock.lock();
             try {
                 int sum = 0;
                 for (final ByteBuffer buffer : buffers) {
                     sum += buffer.remaining();
                 }
                 return sum;
             } finally {
                 lock.unlock();
             }
         }
     }
 
     @Override
     public void produce(final DataStreamChannel channel) throws IOException {
         if (requestChannel.get() == null) {
             requestChannel.set(channel);
             publisher.subscribe(this);
         }
 
         final Throwable t = exception.get();
         final Subscription s = subscription.get();
         int buffersToReplenish = 0;
         try {
             lock.lock();
             try {
                 if (t != null) {
                     throw new HttpStreamResetException(t.getMessage(), t);
                 } else if (this.complete.get() && buffers.isEmpty()) {
                     channel.endStream();
                 } else {
                     while (!buffers.isEmpty()) {
                         final ByteBuffer nextBuffer = buffers.remove();
                         channel.write(nextBuffer);
                         if (nextBuffer.remaining() > 0) {
                             buffers.push(nextBuffer);
                             break;
                         } else if (s != null) {
                             // We defer the #request call until after we release the buffer lock.
                             buffersToReplenish++;
                         }
                     }
                 }
             } finally {
                 lock.unlock();
             }
         } finally {
             if (s != null && buffersToReplenish > 0) {
                 s.request(buffersToReplenish);
             }
         }
     }
 
     @Override
     public void releaseResources() {
         final Subscription s = subscription.getAndSet(null);
         if (s != null) {
             s.cancel();
         }
     }
 }