/*
    * ====================================================================
    * 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.List;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.LinkedBlockingQueue;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicLong;
  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.Header;
  import org.apache.hc.core5.http.HttpStreamResetException;
  import org.apache.hc.core5.http.nio.AsyncDataConsumer;
  import org.apache.hc.core5.http.nio.CapacityChannel;
  import org.apache.hc.core5.util.Args;
  import org.reactivestreams.Publisher;
  import org.reactivestreams.Subscriber;
  import org.reactivestreams.Subscription;
  
  /**
   * An asynchronous data consumer that supports Reactive Streams.
   *
   * @since 5.0
   */
  @Contract(threading = ThreadingBehavior.SAFE)
  final class ReactiveDataConsumer implements AsyncDataConsumer, Publisher<ByteBuffer> {
  
      private final AtomicLong requests = new AtomicLong(0);
  
      private final BlockingQueue<ByteBuffer> buffers = new LinkedBlockingQueue<>();
      private final AtomicBoolean flushInProgress = new AtomicBoolean(false);
      private final AtomicInteger windowScalingIncrement = new AtomicInteger(0);
      private volatile boolean cancelled;
      private volatile boolean completed;
      private volatile Exception exception;
      private volatile CapacityChannel capacityChannel;
      private volatile Subscriber<? super ByteBuffer> subscriber;
  
      private final ReentrantLock lock = new ReentrantLock();
  
      public void failed(final Exception cause) {
          if (!completed) {
              exception = cause;
              flushToSubscriber();
          }
      }
  
      @Override
      public void updateCapacity(final CapacityChannel capacityChannel) throws IOException {
          throwIfCancelled();
          this.capacityChannel = capacityChannel;
          signalCapacity(capacityChannel);
      }
  
      private void signalCapacity(final CapacityChannel channel) throws IOException {
          final int increment = windowScalingIncrement.getAndSet(0);
          if (increment > 0) {
              channel.update(increment);
          }
      }
  
      private void throwIfCancelled() throws IOException {
          if (cancelled) {
              throw new HttpStreamResetException("Downstream subscriber to ReactiveDataConsumer cancelled");
          }
      }
  
      @Override
      public void consume(final ByteBuffer byteBuffer) throws IOException {
         if (completed) {
             throw new IllegalStateException("Received data past end of stream");
         }
         throwIfCancelled();
 
         final byte[] copy = new byte[byteBuffer.remaining()];
         byteBuffer.get(copy);
         buffers.add(ByteBuffer.wrap(copy));
 
         flushToSubscriber();
     }
 
     @Override
     public void streamEnd(final List<? extends Header> trailers) {
         completed = true;
         flushToSubscriber();
     }
 
     @Override
     public void releaseResources() {
         this.capacityChannel = null;
     }
 
     private void flushToSubscriber() {
         lock.lock();
         try {
             final Subscriber<? super ByteBuffer> s = subscriber;
             if (flushInProgress.getAndSet(true)) {
                 return;
             }
             try {
                 if (s == null) {
                     return;
                 }
                 if (exception != null) {
                     subscriber = null;
                     s.onError(exception);
                     return;
                 }
                 ByteBuffer next;
                 while (requests.get() > 0 && ((next = buffers.poll()) != null)) {
                     final int bytesFreed = next.remaining();
                     s.onNext(next);
                     requests.decrementAndGet();
                     windowScalingIncrement.addAndGet(bytesFreed);
                 }
                 final CapacityChannel localChannel = capacityChannel;
                 if (localChannel != null) {
                     try {
                         signalCapacity(localChannel);
                     } catch (final IOException e) {
                         exception = e;
                         s.onError(e);
                         return;
                     }
                 }
                 if (completed && buffers.isEmpty()) {
                     subscriber = null;
                     s.onComplete();
                 }
             } finally {
                 flushInProgress.set(false);
             }
         } finally {
             lock.unlock();
         }
     }
 
     @Override
     public void subscribe(final Subscriber<? super ByteBuffer> subscriber) {
         this.subscriber = Args.notNull(subscriber, "subscriber");
         subscriber.onSubscribe(new Subscription() {
             @Override
             public void request(final long increment) {
                 if (increment <= 0) {
                     failed(new IllegalArgumentException("The number of elements requested must be strictly positive"));
                     return;
                 }
                 requests.addAndGet(increment);
                 flushToSubscriber();
             }
 
             @Override
             public void cancel() {
                 ReactiveDataConsumer.this.cancelled = true;
                 ReactiveDataConsumer.this.subscriber = null;
             }
         });
     }
 }