/*
    * 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 org.jboss.netty.handler.codec.http;
  
  import org.jboss.netty.buffer.ChannelBuffer;
  import org.jboss.netty.buffer.ChannelBuffers;
  import org.jboss.netty.channel.ChannelHandlerContext;
  import org.jboss.netty.channel.ChannelStateEvent;
  import org.jboss.netty.channel.Channels;
  import org.jboss.netty.channel.LifeCycleAwareChannelHandler;
  import org.jboss.netty.channel.MessageEvent;
  import org.jboss.netty.channel.SimpleChannelUpstreamHandler;
  import org.jboss.netty.handler.codec.embedder.DecoderEmbedder;
  
  /**
   * Decodes the content of the received {@link HttpRequest} and {@link HttpChunk}.
   * The original content is replaced with the new content decoded by the
   * {@link DecoderEmbedder}, which is created by {@link #newContentDecoder(String)}.
   * Once decoding is finished, the value of the <tt>'Content-Encoding'</tt>
   * header is set to the target content encoding, as returned by {@link #getTargetContentEncoding(String)}.
   * Also, the <tt>'Content-Length'</tt> header is updated to the length of the
   * decoded content.  If the content encoding of the original is not supported
   * by the decoder, {@link #newContentDecoder(String)} should return {@code null}
   * so that no decoding occurs (i.e. pass-through).
   * <p>
   * Please note that this is an abstract class.  You have to extend this class
   * and implement {@link #newContentDecoder(String)} properly to make this class
   * functional.  For example, refer to the source code of {@link HttpContentDecompressor}.
   * <p>
   * This handler must be placed after {@link HttpMessageDecoder} in the pipeline
   * so that this handler can intercept HTTP requests after {@link HttpMessageDecoder}
   * converts {@link ChannelBuffer}s into HTTP requests.
   */
  public abstract class HttpContentDecoder extends SimpleChannelUpstreamHandler
                                           implements LifeCycleAwareChannelHandler {
  
      private DecoderEmbedder<ChannelBuffer> decoder;
  
      /**
       * Creates a new instance.
       */
      protected HttpContentDecoder() {
      }
  
      @Override
      public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
          Object msg = e.getMessage();
          if (msg instanceof HttpResponse && ((HttpResponse) msg).getStatus().getCode() == 100) {
              // 100-continue response must be passed through.
              ctx.sendUpstream(e);
          } else if (msg instanceof HttpMessage) {
              HttpMessage m = (HttpMessage) msg;
  
              // Clean-up the previous decoder if not cleaned up correctly.
              finishDecode();
  
              // Determine the content encoding.
              String contentEncoding = m.headers().get(HttpHeaders.Names.CONTENT_ENCODING);
              if (contentEncoding != null) {
                  contentEncoding = contentEncoding.trim();
              } else {
                  contentEncoding = HttpHeaders.Values.IDENTITY;
              }
  
              boolean hasContent = m.isChunked() || m.getContent().readable();
              if (hasContent && (decoder = newContentDecoder(contentEncoding)) != null) {
                  // Decode the content and remove or replace the existing headers
                  // so that the message looks like a decoded message.
                  String targetContentEncoding = getTargetContentEncoding(contentEncoding);
                  if (HttpHeaders.Values.IDENTITY.equals(targetContentEncoding)) {
                      // Do NOT set the 'Content-Encoding' header if the target encoding is 'identity'
                      // as per: http://tools.ietf.org/html/rfc2616#section-14.11
                      m.headers().remove(HttpHeaders.Names.CONTENT_ENCODING);
                  } else {
                      m.headers().set(HttpHeaders.Names.CONTENT_ENCODING, targetContentEncoding);
                  }
  
                  if (!m.isChunked()) {
                      ChannelBuffer content = m.getContent();
                      // Decode the content
                      content = ChannelBuffers.wrappedBuffer(
                              decode(content), finishDecode());
  
                      // Replace the content.
                      m.setContent(content);
                      if (m.headers().contains(HttpHeaders.Names.CONTENT_LENGTH)) {
                         m.headers().set(
                                 HttpHeaders.Names.CONTENT_LENGTH,
                                 Integer.toString(content.readableBytes()));
                     }
                 }
             }
 
             // Because HttpMessage is a mutable object, we can simply forward the received event.
             ctx.sendUpstream(e);
         } else if (msg instanceof HttpChunk) {
             HttpChunk c = (HttpChunk) msg;
             ChannelBuffer content = c.getContent();
 
             // Decode the chunk if necessary.
             if (decoder != null) {
                 if (!c.isLast()) {
                     content = decode(content);
                     if (content.readable()) {
                         c.setContent(content);
                         ctx.sendUpstream(e);
                     }
                 } else {
                     ChannelBuffer lastProduct = finishDecode();
 
                     // Generate an additional chunk if the decoder produced
                     // the last product on closure,
                     if (lastProduct.readable()) {
                         Channels.fireMessageReceived(
                                 ctx, new DefaultHttpChunk(lastProduct), e.getRemoteAddress());
                     }
 
                     // Emit the last chunk.
                     ctx.sendUpstream(e);
                 }
             } else {
                 ctx.sendUpstream(e);
             }
         } else {
             ctx.sendUpstream(e);
         }
     }
 
     @Override
     public void channelClosed(ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception {
         // Clean-up the previous decoder if not cleaned up correctly.
         finishDecode();
 
         super.channelClosed(ctx, e);
     }
 
     /**
      * Returns a new {@link DecoderEmbedder} that decodes the HTTP message
      * content encoded in the specified <tt>contentEncoding</tt>.
      *
      * @param contentEncoding the value of the {@code "Content-Encoding"} header
      * @return a new {@link DecoderEmbedder} if the specified encoding is supported.
      *         {@code null} otherwise (alternatively, you can throw an exception
      *         to block unknown encoding).
      */
     protected abstract DecoderEmbedder<ChannelBuffer> newContentDecoder(String contentEncoding) throws Exception;
 
     /**
      * Returns the expected content encoding of the decoded content.
      * This method returns {@code "identity"} by default, which is the case for
      * most decoders.
      *
      * @param contentEncoding the value of the {@code "Content-Encoding"} header
      * @return the expected content encoding of the new content
      */
     protected String getTargetContentEncoding(String contentEncoding) throws Exception {
         return HttpHeaders.Values.IDENTITY;
     }
 
     private ChannelBuffer decode(ChannelBuffer buf) {
         decoder.offer(buf);
         return ChannelBuffers.wrappedBuffer(decoder.pollAll(new ChannelBuffer[decoder.size()]));
     }
 
     private ChannelBuffer finishDecode() {
         if (decoder == null) {
             return ChannelBuffers.EMPTY_BUFFER;
         }
 
         ChannelBuffer result;
         if (decoder.finish()) {
             result = ChannelBuffers.wrappedBuffer(decoder.pollAll(new ChannelBuffer[decoder.size()]));
         } else {
             result = ChannelBuffers.EMPTY_BUFFER;
         }
         decoder = null;
         return result;
     }
 
     public void beforeAdd(ChannelHandlerContext ctx) throws Exception {
         // NOOP
     }
 
     public void afterAdd(ChannelHandlerContext ctx) throws Exception {
         // NOOP
     }
 
     public void beforeRemove(ChannelHandlerContext ctx) throws Exception {
         // NOOP
     }
 
     public void afterRemove(ChannelHandlerContext ctx) throws Exception {
         finishDecode();
     }
 }