/*
    * 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:
    *
    *   https://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.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelInboundHandler;
  import io.netty.channel.ChannelInboundHandlerAdapter;
  import io.netty.channel.ChannelPipeline;
  import io.netty.util.ReferenceCountUtil;
  import io.netty.util.ReferenceCounted;
  import io.netty.util.internal.TypeParameterMatcher;
  
  import java.util.List;
  
  /**
   * {@link ChannelInboundHandlerAdapter} which decodes from one message to an other message.
   *
   *
   * For example here is an implementation which decodes a {@link String} to an {@link Integer} which represent
   * the length of the {@link String}.
   *
   * <pre>
   *     public class StringToIntegerDecoder extends
   *             {@link MessageToMessageDecoder}&lt;{@link String}&gt; {
   *
   *         {@code @Override}
   *         public void decode({@link ChannelHandlerContext} ctx, {@link String} message,
   *                            List&lt;Object&gt; out) throws {@link Exception} {
   *             out.add(message.length());
   *         }
   *     }
   * </pre>
   *
   * Be aware that you need to call {@link ReferenceCounted#retain()} on messages that are just passed through if they
   * are of type {@link ReferenceCounted}. This is needed as the {@link MessageToMessageDecoder} will call
   * {@link ReferenceCounted#release()} on decoded messages.
   *
   */
  public abstract class MessageToMessageDecoder<I> extends ChannelInboundHandlerAdapter {
  
      private final TypeParameterMatcher matcher;
      private boolean decodeCalled;
      private boolean messageProduced;
  
      /**
       * Create a new instance which will try to detect the types to match out of the type parameter of the class.
       */
      protected MessageToMessageDecoder() {
          matcher = TypeParameterMatcher.find(this, MessageToMessageDecoder.class, "I");
      }
  
      /**
       * Create a new instance
       *
       * @param inboundMessageType    The type of messages to match and so decode
       */
      protected MessageToMessageDecoder(Class<? extends I> inboundMessageType) {
          matcher = TypeParameterMatcher.get(inboundMessageType);
      }
  
      /**
       * Returns {@code true} if the given message should be handled. If {@code false} it will be passed to the next
       * {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
       */
      public boolean acceptInboundMessage(Object msg) throws Exception {
          return matcher.match(msg);
      }
  
      @Override
      public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
          decodeCalled = true;
          CodecOutputList out = CodecOutputList.newInstance();
          try {
              if (acceptInboundMessage(msg)) {
                  @SuppressWarnings("unchecked")
                  I cast = (I) msg;
                  try {
                      decode(ctx, cast, out);
                  } finally {
                      ReferenceCountUtil.release(cast);
                  }
              } else {
                  out.add(msg);
              }
          } catch (DecoderException e) {
              throw e;
         } catch (Exception e) {
             throw new DecoderException(e);
         } finally {
             try {
                 int size = out.size();
                 messageProduced |= size > 0;
                 for (int i = 0; i < size; i++) {
                     ctx.fireChannelRead(out.getUnsafe(i));
                 }
             } finally {
                 out.recycle();
             }
         }
     }
 
     @Override
     public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
         if (!isSharable()) {
             // Only use local vars if this decoder is not sharable as otherwise this is not safe to do.
             if (decodeCalled && !messageProduced && !ctx.channel().config().isAutoRead()) {
                 ctx.read();
             }
             decodeCalled = false;
             messageProduced = false;
         }
         ctx.fireChannelReadComplete();
     }
 
     /**
      * Decode from one message to an other. This method will be called for each written message that can be handled
      * by this decoder.
      *
      * @param ctx           the {@link ChannelHandlerContext} which this {@link MessageToMessageDecoder} belongs to
      * @param msg           the message to decode to an other one
      * @param out           the {@link List} to which decoded messages should be added
      * @throws Exception    is thrown if an error occurs
      */
     protected abstract void decode(ChannelHandlerContext ctx, I msg, List<Object> out) throws Exception;
 }