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/*
 * Copyright 2021 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.server.internal;

import io.netty.buffer.ByteBuf;
import io.netty.channel.Channel;
import io.netty.channel.ChannelPromise;
import io.netty.handler.codec.http.DefaultHttpContent;
import io.netty.handler.codec.http.LastHttpContent;
import org.reactivestreams.Publisher;
import org.reactivestreams.Subscription;
import ratpack.exec.internal.Continuation;
import ratpack.exec.internal.DefaultExecution;
import ratpack.func.Action;

import javax.annotation.Nullable;

class StreamingResponseBodyWriter implements ResponseBodyWriter, ResponseWritingListener {

  private final Publisher publisher;

  private boolean done;
  private Subscription subscription;
  private ChannelPromise channelPromise;

  @Nullable
  private Continuation continuation;

  public StreamingResponseBodyWriter(Publisher publisher) {
    this.publisher = publisher;
  }

  @Override
  public void dispose() {
    done = true;
  }

  @Override
  public void onClosed() {
    exec(() -> {
      if (!done) {
        done = true;
        if (subscription != null) {
          subscription.cancel();
        }
        if (channelPromise != null) {
          channelPromise.setSuccess();
        }
      }
    });
  }

  @Override
  public void onWritable() {
    // Writability can change while emitting a single item.
    // Only propagate the request if we had previously written without subsequently requesting.
    if (continuation == null) {
      return;
    }

    if (!done && subscription != null) {
      exec(() -> {
        subscription.request(1);
      });
    }
  }

  @Override
  public ChannelPromise write(Channel channel) {
    channelPromise = channel.newPromise();
    publisher.subscribe(new Subscriber(channel));
    return channelPromise;
  }

  private void exec(Runnable rest) {
    if (continuation == null) {
      rest.run();
    } else {
      Continuation continuation = this.continuation;
      this.continuation = null;
      continuation.resume(rest::run);
    }
  }

  private class Subscriber implements org.reactivestreams.Subscriber {

    private final Channel channel;

    public Subscriber(Channel channel) {
      this.channel = channel;
    }

    @Override
    public void onSubscribe(Subscription incomingSubscription) {
      if (incomingSubscription == null) {
        throw new NullPointerException("'subscription' is null");
      }

      if (subscription != null) {
        incomingSubscription.cancel();
        return;
      }

      subscription = incomingSubscription;
      requestOrDelimit();
    }

    @Override
    public void onNext(ByteBuf o) {
      o.touch();

      if (o.readableBytes() == 0) {
        o.release();
        subscription.request(1);
        return;
      }

      if (done) {
        o.release();
        return;
      }

      channel.writeAndFlush(new DefaultHttpContent(o.touch()), channel.voidPromise());
      requestOrDelimit();
    }

    private void requestOrDelimit() {
      if (channel.isWritable()) {
        subscription.request(1);
      } else {
        DefaultExecution.require().delimit(Action.throwException(), continuation ->
          StreamingResponseBodyWriter.this.continuation = continuation
        );
      }
    }

    @Override
    public void onError(Throwable t) {
      exec(() -> {
        if (t == null) {
          throw new NullPointerException("error is null");
        }
        if (!done) {
          done = true;
          channelPromise.setFailure(t);
        }
      });
    }

    @Override
    public void onComplete() {
      exec(() -> {
        if (!done) {
          done = true;
          channel.write(LastHttpContent.EMPTY_LAST_CONTENT, channelPromise);
          channel.flush();
        }
      });
    }
  }
}