/*
 * 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.
 */
package org.apache.beam.sdk.fn.data;

import java.io.IOException;
import org.apache.beam.model.fnexecution.v1.BeamFnApi;
import org.apache.beam.sdk.coders.Coder;
import org.apache.beam.vendor.grpc.v1p21p0.com.google.protobuf.ByteString;
import org.apache.beam.vendor.grpc.v1p21p0.io.grpc.stub.StreamObserver;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.annotations.VisibleForTesting;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * A buffering outbound {@link FnDataReceiver} for the Beam Fn Data API.
 *
 * <p>Encodes individually consumed elements with the provided {@link Coder} producing a single
 * {@link BeamFnApi.Elements} message when the buffer threshold is surpassed.
 *
 * <p>The default buffer threshold can be overridden by specifying the experiment {@code
 * beam_fn_api_data_buffer_limit=<bytes>}
 *
 * <p>TODO: Handle outputting large elements (&gt; 2GiBs). Note that this also applies to the input
 * side as well.
 *
 * <p>TODO: Handle outputting elements that are zero bytes by outputting a single byte as a marker,
 * detect on the input side that no bytes were read and force reading a single byte.
 */
public class BeamFnDataBufferingOutboundObserver<T> implements CloseableFnDataReceiver<T> {
  // TODO: Consider moving this constant out of this class
  public static final String BEAM_FN_API_DATA_BUFFER_LIMIT = "beam_fn_api_data_buffer_limit=";
  @VisibleForTesting static final int DEFAULT_BUFFER_LIMIT_BYTES = 1_000_000;
  private static final Logger LOG =
      LoggerFactory.getLogger(BeamFnDataBufferingOutboundObserver.class);

  public static <T> BeamFnDataBufferingOutboundObserver<T> forLocation(
      LogicalEndpoint endpoint,
      Coder<T> coder,
      StreamObserver<BeamFnApi.Elements> outboundObserver) {
    return forLocationWithBufferLimit(
        DEFAULT_BUFFER_LIMIT_BYTES, endpoint, coder, outboundObserver);
  }

  public static <T> BeamFnDataBufferingOutboundObserver<T> forLocationWithBufferLimit(
      int bufferLimit,
      LogicalEndpoint endpoint,
      Coder<T> coder,
      StreamObserver<BeamFnApi.Elements> outboundObserver) {
    return new BeamFnDataBufferingOutboundObserver<>(
        bufferLimit, endpoint, coder, outboundObserver);
  }

  private long byteCounter;
  private long counter;
  private boolean closed;
  private final int bufferLimit;
  private final Coder<T> coder;
  private final LogicalEndpoint outputLocation;
  private final StreamObserver<BeamFnApi.Elements> outboundObserver;
  private final ByteString.Output bufferedElements;

  private BeamFnDataBufferingOutboundObserver(
      int bufferLimit,
      LogicalEndpoint outputLocation,
      Coder<T> coder,
      StreamObserver<BeamFnApi.Elements> outboundObserver) {
    this.bufferLimit = bufferLimit;
    this.outputLocation = outputLocation;
    this.coder = coder;
    this.outboundObserver = outboundObserver;
    this.bufferedElements = ByteString.newOutput();
    this.closed = false;
  }

  @Override
  public void close() throws Exception {
    if (closed) {
      throw new IllegalStateException("Already closed.");
    }
    closed = true;
    BeamFnApi.Elements.Builder elements = convertBufferForTransmission();
    // This will add an empty data block representing the end of stream.
    elements
        .addDataBuilder()
        .setInstructionId(outputLocation.getInstructionId())
        .setTransformId(outputLocation.getTransformId());

    LOG.debug(
        "Closing stream for instruction {} and "
            + "transform {} having transmitted {} values {} bytes",
        outputLocation.getInstructionId(),
        outputLocation.getTransformId(),
        counter,
        byteCounter);
    outboundObserver.onNext(elements.build());
  }

  @Override
  public void flush() throws IOException {
    if (bufferedElements.size() > 0) {
      outboundObserver.onNext(convertBufferForTransmission().build());
    }
  }

  @Override
  public void accept(T t) throws IOException {
    if (closed) {
      throw new IllegalStateException("Already closed.");
    }
    coder.encode(t, bufferedElements);
    counter += 1;
    if (bufferedElements.size() >= bufferLimit) {
      flush();
    }
  }

  private BeamFnApi.Elements.Builder convertBufferForTransmission() {
    BeamFnApi.Elements.Builder elements = BeamFnApi.Elements.newBuilder();
    if (bufferedElements.size() == 0) {
      return elements;
    }

    elements
        .addDataBuilder()
        .setInstructionId(outputLocation.getInstructionId())
        .setTransformId(outputLocation.getTransformId())
        .setData(bufferedElements.toByteString());

    byteCounter += bufferedElements.size();
    bufferedElements.reset();
    return elements;
  }
}