/*
 * Copyright (C) 2018 Seoul National University
 *
 * 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 org.apache.nemo.runtime.executor.bytetransfer;

import io.netty.buffer.ByteBuf;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.annotation.Nullable;
import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.CompletableFuture;

/**
 * Container for multiple input streams. Represents a transfer context on receiver-side.
 *
 * <h3>Thread safety:</h3>
 * <p>Methods with default access modifier, namely {@link #onNewStream()}, {@link #onByteBuf(ByteBuf)},
 * {@link #onContextClose()}, are not thread-safe, since they are called by a single Netty event loop.</p>
 * <p>Public methods are thread safe,
 * although the execution order may not be linearized if they were called from different threads.</p>
 */
public final class ByteInputContext extends ByteTransferContext {

  private static final Logger LOG = LoggerFactory.getLogger(ByteInputContext.class.getName());

  private final CompletableFuture<Iterator<InputStream>> completedFuture = new CompletableFuture<>();
  private final ClosableBlockingQueue<ByteBufInputStream> byteBufInputStreams = new ClosableBlockingQueue<>();
  private volatile ByteBufInputStream currentByteBufInputStream = null;

  private final Iterator<InputStream> inputStreams = new Iterator<InputStream>() {
    @Override
    public boolean hasNext() {
      try {
        return byteBufInputStreams.peek() != null;
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new RuntimeException(e);
      }
    }

    @Override
    public InputStream next() {
      try {
        return byteBufInputStreams.take();
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
        LOG.error("Interrupted while taking byte buf.", e);
        throw new NoSuchElementException();
      }
    }
  };

  /**
   * Creates an input context.
   * @param remoteExecutorId    id of the remote executor
   * @param contextId           identifier for this context
   * @param contextDescriptor   user-provided context descriptor
   * @param contextManager      {@link ContextManager} for the channel
   */
  ByteInputContext(final String remoteExecutorId,
                   final ContextId contextId,
                   final byte[] contextDescriptor,
                   final ContextManager contextManager) {
    super(remoteExecutorId, contextId, contextDescriptor, contextManager);
  }

  /**
   * Returns {@link Iterator} of {@link InputStream}s.
   * This method always returns the same {@link Iterator} instance.
   * @return {@link Iterator} of {@link InputStream}s.
   */
  public Iterator<InputStream> getInputStreams() {
    return inputStreams;
  }

  /**
   * Returns a future, which is completed when the corresponding transfer for this context gets done.
   * @return a {@link CompletableFuture} for the same value that {@link #getInputStreams()} returns
   */
  public CompletableFuture<Iterator<InputStream>> getCompletedFuture() {
    return completedFuture;
  }

  /**
   * Called when a punctuation for sub-stream incarnation is detected.
   */
  void onNewStream() {
    if (currentByteBufInputStream != null) {
      currentByteBufInputStream.byteBufQueue.close();
    }
    currentByteBufInputStream = new ByteBufInputStream();
    byteBufInputStreams.put(currentByteBufInputStream);
  }

  /**
   * Called when {@link ByteBuf} is supplied to this context.
   * @param byteBuf the {@link ByteBuf} to supply
   */
  void onByteBuf(final ByteBuf byteBuf) {
    if (currentByteBufInputStream == null) {
      throw new RuntimeException("Cannot accept ByteBuf: No sub-stream is opened.");
    }
    if (byteBuf.readableBytes() > 0) {
      currentByteBufInputStream.byteBufQueue.put(byteBuf);
    } else {
      // ignore empty data frames
      byteBuf.release();
    }
  }

  /**
   * Called when {@link #onByteBuf(ByteBuf)} event is no longer expected.
   */
  void onContextClose() {
    if (currentByteBufInputStream != null) {
      currentByteBufInputStream.byteBufQueue.close();
    }
    byteBufInputStreams.close();
    completedFuture.complete(inputStreams);
    deregister();
  }

  @Override
  public void onChannelError(@Nullable final Throwable cause) {
    setChannelError(cause);

    if (currentByteBufInputStream != null) {
      currentByteBufInputStream.byteBufQueue.closeExceptionally(cause);
    }
    byteBufInputStreams.closeExceptionally(cause);
    completedFuture.completeExceptionally(cause);
    deregister();
  }

  /**
   * An {@link InputStream} implementation that reads data from a composition of {@link ByteBuf}s.
   */
  private static final class ByteBufInputStream extends InputStream {

    private final ClosableBlockingQueue<ByteBuf> byteBufQueue = new ClosableBlockingQueue<>();

    @Override
    public int read() throws IOException {
      try {
        final ByteBuf head = byteBufQueue.peek();
        if (head == null) {
          // end of stream event
          return -1;
        }
        final int b = head.readUnsignedByte();
        if (head.readableBytes() == 0) {
          // remove and release header if no longer required
          byteBufQueue.take();
          head.release();
        }
        return b;
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new IOException(e);
      }
    }

    @Override
    public int read(final byte[] bytes, final int baseOffset, final int maxLength) throws IOException {
      if (bytes == null) {
        throw new NullPointerException();
      }
      if (baseOffset < 0 || maxLength < 0 || maxLength > bytes.length - baseOffset) {
        throw new IndexOutOfBoundsException();
      }
      try {
        // the number of bytes that has been read so far
        int readBytes = 0;
        // the number of bytes to read
        int capacity = maxLength;
        while (capacity > 0) {
          final ByteBuf head = byteBufQueue.peek();
          if (head == null) {
            // end of stream event
            return readBytes == 0 ? -1 : readBytes;
          }
          final int toRead = Math.min(head.readableBytes(), capacity);
          head.readBytes(bytes, baseOffset + readBytes, toRead);
          if (head.readableBytes() == 0) {
            byteBufQueue.take();
            head.release();
          }
          readBytes += toRead;
          capacity -= toRead;
        }
        return readBytes;
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new IOException(e);
      }
    }

    @Override
    public long skip(final long n) throws IOException {
      if (n <= 0) {
        return 0;
      }
      try {
        // the number of bytes that has been skipped so far
        long skippedBytes = 0;
        // the number of bytes to skip
        long toSkip = n;
        while (toSkip > 0) {
          final ByteBuf head = byteBufQueue.peek();
          if (head == null) {
            // end of stream event
            return skippedBytes;
          }
          if (head.readableBytes() > toSkip) {
            head.skipBytes((int) toSkip);
            skippedBytes += toSkip;
            return skippedBytes;
          } else {
            // discard the whole ByteBuf
            skippedBytes += head.readableBytes();
            toSkip -= head.readableBytes();
            byteBufQueue.take();
            head.release();
          }
        }
        return skippedBytes;
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new IOException(e);
      }
    }

    @Override
    public int available() throws IOException {
      try {
        final ByteBuf head = byteBufQueue.peek();
        if (head == null) {
          return 0;
        } else {
          return head.readableBytes();
        }
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new IOException(e);
      }
    }
  }
}