/*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project 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 io.netty.handler.codec;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufHolder;
  import io.netty.buffer.CompositeByteBuf;
  import io.netty.buffer.Unpooled;
  import io.netty.channel.ChannelFuture;
  import io.netty.channel.ChannelFutureListener;
  import io.netty.channel.ChannelHandler;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.util.ReferenceCountUtil;
  
  import java.util.List;
  
  /**
   * An abstract {@link ChannelHandler} that aggregates a series of message objects into a single aggregated message.
   * <p>
   * 'A series of messages' is composed of the following:
   * <ul>
   * <li>a single start message which optionally contains the first part of the content, and</li>
   * <li>1 or more content messages.</li>
   * </ul>
   * The content of the aggregated message will be the merged content of the start message and its following content
   * messages. If this aggregator encounters a content message where {@link #isLastContentMessage(ByteBufHolder)}
   * return {@code true} for, the aggregator will finish the aggregation and produce the aggregated message and expect
   * another start message.
   * </p>
   *
   * @param <I> the type that covers both start message and content message
   * @param <S> the type of the start message
   * @param <C> the type of the content message (must be a subtype of {@link ByteBufHolder})
   * @param <O> the type of the aggregated message (must be a subtype of {@code S} and {@link ByteBufHolder})
   */
  public abstract class MessageAggregator<I, S, C extends ByteBufHolder, O extends ByteBufHolder>
          extends MessageToMessageDecoder<I> {
  
      private static final int DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS = 1024;
  
      private final int maxContentLength;
      private O currentMessage;
      private boolean handlingOversizedMessage;
  
      private int maxCumulationBufferComponents = DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS;
      private ChannelHandlerContext ctx;
      private ChannelFutureListener continueResponseWriteListener;
  
      /**
       * Creates a new instance.
       *
       * @param maxContentLength
       *        the maximum length of the aggregated content.
       *        If the length of the aggregated content exceeds this value,
       *        {@link #handleOversizedMessage(ChannelHandlerContext, Object)} will be called.
       */
      protected MessageAggregator(int maxContentLength) {
          validateMaxContentLength(maxContentLength);
          this.maxContentLength = maxContentLength;
      }
  
      protected MessageAggregator(int maxContentLength, Class<? extends I> inboundMessageType) {
          super(inboundMessageType);
          validateMaxContentLength(maxContentLength);
          this.maxContentLength = maxContentLength;
      }
  
      private static void validateMaxContentLength(int maxContentLength) {
          if (maxContentLength <= 0) {
              throw new IllegalArgumentException("maxContentLength must be a positive integer: " + maxContentLength);
          }
      }
  
      @Override
      public boolean acceptInboundMessage(Object msg) throws Exception {
          // No need to match last and full types because they are subset of first and middle types.
          if (!super.acceptInboundMessage(msg)) {
              return false;
          }
  
          @SuppressWarnings("unchecked")
          I in = (I) msg;
  
          return (isContentMessage(in) || isStartMessage(in)) && !isAggregated(in);
      }
  
     /**
      * Returns {@code true} if and only if the specified message is a start message. Typically, this method is
      * implemented as a single {@code return} statement with {@code instanceof}:
      * <pre>
      * return msg instanceof MyStartMessage;
      * </pre>
      */
     protected abstract boolean isStartMessage(I msg) throws Exception;
 
     /**
      * Returns {@code true} if and only if the specified message is a content message. Typically, this method is
      * implemented as a single {@code return} statement with {@code instanceof}:
      * <pre>
      * return msg instanceof MyContentMessage;
      * </pre>
      */
     protected abstract boolean isContentMessage(I msg) throws Exception;
 
     /**
      * Returns {@code true} if and only if the specified message is the last content message. Typically, this method is
      * implemented as a single {@code return} statement with {@code instanceof}:
      * <pre>
      * return msg instanceof MyLastContentMessage;
      * </pre>
      * or with {@code instanceof} and boolean field check:
      * <pre>
      * return msg instanceof MyContentMessage && msg.isLastFragment();
      * </pre>
      */
     protected abstract boolean isLastContentMessage(C msg) throws Exception;
 
     /**
      * Returns {@code true} if and only if the specified message is already aggregated.  If this method returns
      * {@code true}, this handler will simply forward the message to the next handler as-is.
      */
     protected abstract boolean isAggregated(I msg) throws Exception;
 
     /**
      * Returns the maximum allowed length of the aggregated message.
      */
     public final int maxContentLength() {
         return maxContentLength;
     }
 
     /**
      * Returns the maximum number of components in the cumulation buffer.  If the number of
      * the components in the cumulation buffer exceeds this value, the components of the
      * cumulation buffer are consolidated into a single component, involving memory copies.
      * The default value of this property is {@value #DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS}.
      */
     public final int maxCumulationBufferComponents() {
         return maxCumulationBufferComponents;
     }
 
     /**
      * Sets the maximum number of components in the cumulation buffer.  If the number of
      * the components in the cumulation buffer exceeds this value, the components of the
      * cumulation buffer are consolidated into a single component, involving memory copies.
      * The default value of this property is {@value #DEFAULT_MAX_COMPOSITEBUFFER_COMPONENTS}
      * and its minimum allowed value is {@code 2}.
      */
     public final void setMaxCumulationBufferComponents(int maxCumulationBufferComponents) {
         if (maxCumulationBufferComponents < 2) {
             throw new IllegalArgumentException(
                     "maxCumulationBufferComponents: " + maxCumulationBufferComponents +
                     " (expected: >= 2)");
         }
 
         if (ctx == null) {
             this.maxCumulationBufferComponents = maxCumulationBufferComponents;
         } else {
             throw new IllegalStateException(
                     "decoder properties cannot be changed once the decoder is added to a pipeline.");
         }
     }
 
     public final boolean isHandlingOversizedMessage() {
         return handlingOversizedMessage;
     }
 
     protected final ChannelHandlerContext ctx() {
         if (ctx == null) {
             throw new IllegalStateException("not added to a pipeline yet");
         }
         return ctx;
     }
 
     @Override
     protected void decode(final ChannelHandlerContext ctx, I msg, List<Object> out) throws Exception {
         O currentMessage = this.currentMessage;
 
         if (isStartMessage(msg)) {
             handlingOversizedMessage = false;
             if (currentMessage != null) {
                 throw new MessageAggregationException();
             }
 
             @SuppressWarnings("unchecked")
             S m = (S) msg;
 
             // if content length is set, preemptively close if it's too large
             if (hasContentLength(m)) {
                 if (contentLength(m) > maxContentLength) {
                     // handle oversized message
                     invokeHandleOversizedMessage(ctx, m);
                     return;
                 }
             }
 
             // Send the continue response if necessary (e.g. 'Expect: 100-continue' header)
             Object continueResponse = newContinueResponse(m);
             if (continueResponse != null) {
                 // Cache the write listener for reuse.
                 ChannelFutureListener listener = continueResponseWriteListener;
                 if (listener == null) {
                     continueResponseWriteListener = listener = new ChannelFutureListener() {
                         @Override
                         public void operationComplete(ChannelFuture future) throws Exception {
                             if (!future.isSuccess()) {
                                 ctx.fireExceptionCaught(future.cause());
                             }
                         }
                     };
                 }
                 ctx.writeAndFlush(continueResponse).addListener(listener);
             }
 
             if (m instanceof DecoderResultProvider && !((DecoderResultProvider) m).decoderResult().isSuccess()) {
                 O aggregated;
                 if (m instanceof ByteBufHolder && ((ByteBufHolder) m).content().isReadable()) {
                     aggregated = beginAggregation(m, ((ByteBufHolder) m).content().retain());
                 } else {
                     aggregated = beginAggregation(m, Unpooled.EMPTY_BUFFER);
                 }
                 finishAggregation(aggregated);
                 out.add(aggregated);
                 this.currentMessage = null;
                 return;
             }
 
             // A streamed message - initialize the cumulative buffer, and wait for incoming chunks.
             CompositeByteBuf content = ctx.alloc().compositeBuffer(maxCumulationBufferComponents);
             if (m instanceof ByteBufHolder) {
                 appendPartialContent(content, ((ByteBufHolder) m).content());
             }
             this.currentMessage = beginAggregation(m, content);
 
         } else if (isContentMessage(msg)) {
             @SuppressWarnings("unchecked")
             final C m = (C) msg;
             final ByteBuf partialContent = ((ByteBufHolder) msg).content();
             final boolean isLastContentMessage = isLastContentMessage(m);
             if (handlingOversizedMessage) {
                 if (isLastContentMessage) {
                     this.currentMessage = null;
                 }
                 // already detect the too long frame so just discard the content
                 return;
             }
 
             if (currentMessage == null) {
                 throw new MessageAggregationException();
             }
 
             // Merge the received chunk into the content of the current message.
             CompositeByteBuf content = (CompositeByteBuf) currentMessage.content();
 
             // Handle oversized message.
             if (content.readableBytes() > maxContentLength - partialContent.readableBytes()) {
                 // By convention, full message type extends first message type.
                 @SuppressWarnings("unchecked")
                 S s = (S) currentMessage;
                 invokeHandleOversizedMessage(ctx, s);
                 return;
             }
 
             // Append the content of the chunk.
             appendPartialContent(content, partialContent);
 
             // Give the subtypes a chance to merge additional information such as trailing headers.
             aggregate(currentMessage, m);
 
             final boolean last;
             if (m instanceof DecoderResultProvider) {
                 DecoderResult decoderResult = ((DecoderResultProvider) m).decoderResult();
                 if (!decoderResult.isSuccess()) {
                     if (currentMessage instanceof DecoderResultProvider) {
                         ((DecoderResultProvider) currentMessage).setDecoderResult(
                                 DecoderResult.failure(decoderResult.cause()));
                     }
                     last = true;
                 } else {
                     last = isLastContentMessage;
                 }
             } else {
                 last = isLastContentMessage;
             }
 
             if (last) {
                 finishAggregation(currentMessage);
 
                 // All done
                 out.add(currentMessage);
                 this.currentMessage = null;
             }
         } else {
             throw new MessageAggregationException();
         }
     }
 
     private static void appendPartialContent(CompositeByteBuf content, ByteBuf partialContent) {
         if (partialContent.isReadable()) {
             partialContent.retain();
             content.addComponent(partialContent);
             content.writerIndex(content.writerIndex() + partialContent.readableBytes());
         }
     }
 
     /**
      * Returns {@code true} if and only if the specified start message already contains the information about the
      * length of the whole content.
      */
     protected abstract boolean hasContentLength(S start) throws Exception;
 
     /**
      * Retrieves the length of the whole content from the specified start message. This method is invoked only when
      * {@link #hasContentLength(Object)} returned {@code true}.
      */
     protected abstract long contentLength(S start) throws Exception;
 
     /**
      * Returns the 'continue response' for the specified start message if necessary. For example, this method is
      * useful to handle an HTTP 100-continue header.
      *
      * @return the 'continue response', or {@code null} if there's no message to send
      */
     protected abstract Object newContinueResponse(S start) throws Exception;
 
     /**
      * Creates a new aggregated message from the specified start message and the specified content.  If the start
      * message implements {@link ByteBufHolder}, its content is appended to the specified {@code content}.
      * This aggregator will continue to append the received content to the specified {@code content}.
      */
     protected abstract O beginAggregation(S start, ByteBuf content) throws Exception;
 
     /**
      * Transfers the information provided by the specified content message to the specified aggregated message.
      * Note that the content of the specified content message has been appended to the content of the specified
      * aggregated message already, so that you don't need to.  Use this method to transfer the additional information
      * that the content message provides to {@code aggregated}.
      */
     protected void aggregate(O aggregated, C content) throws Exception { }
 
     /**
      * Invoked when the specified {@code aggregated} message is about to be passed to the next handler in the pipeline.
      */
     protected void finishAggregation(O aggregated) throws Exception { }
 
     private void invokeHandleOversizedMessage(ChannelHandlerContext ctx, S oversized) throws Exception {
         handlingOversizedMessage = true;
         currentMessage = null;
         try {
             handleOversizedMessage(ctx, oversized);
         } finally {
             // Release the message in case it is a full one.
             ReferenceCountUtil.release(oversized);
         }
     }
 
     /**
      * Invoked when an incoming request exceeds the maximum content length.  The default behvaior is to trigger an
      * {@code exceptionCaught()} event with a {@link TooLongFrameException}.
      *
      * @param ctx the {@link ChannelHandlerContext}
      * @param oversized the accumulated message up to this point, whose type is {@code S} or {@code O}
      */
     protected void handleOversizedMessage(ChannelHandlerContext ctx, S oversized) throws Exception {
         ctx.fireExceptionCaught(
                 new TooLongFrameException("content length exceeded " + maxContentLength() + " bytes."));
     }
 
     @Override
     public void channelInactive(ChannelHandlerContext ctx) throws Exception {
         // release current message if it is not null as it may be a left-over
         if (currentMessage != null) {
             currentMessage.release();
             currentMessage = null;
         }
 
         super.channelInactive(ctx);
     }
 
     @Override
     public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
         this.ctx = ctx;
     }
 
     @Override
     public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
         super.handlerRemoved(ctx);
 
         // release current message if it is not null as it may be a left-over as there is not much more we can do in
         // this case
         if (currentMessage != null) {
             currentMessage.release();
             currentMessage = null;
         }
     }
 }