/*
    * 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.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelOutboundHandler;
  import io.netty.channel.ChannelOutboundHandlerAdapter;
  import io.netty.channel.ChannelPipeline;
  import io.netty.channel.ChannelPromise;
  import io.netty.util.ReferenceCountUtil;
  import io.netty.util.ReferenceCounted;
  import io.netty.util.internal.StringUtil;
  import io.netty.util.internal.TypeParameterMatcher;
  
  import java.util.List;
  
  /**
   * {@link ChannelOutboundHandlerAdapter} which encodes from one message to an other message
   *
   * For example here is an implementation which decodes an {@link Integer} to an {@link String}.
   *
   * <pre>
   *     public class IntegerToStringEncoder extends
   *             {@link MessageToMessageEncoder}&lt;{@link Integer}&gt; {
   *
   *         {@code @Override}
   *         public void encode({@link ChannelHandlerContext} ctx, {@link Integer} message, List&lt;Object&gt; out)
   *                 throws {@link Exception} {
   *             out.add(message.toString());
   *         }
   *     }
   * </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 MessageToMessageEncoder} will call
   * {@link ReferenceCounted#release()} on encoded messages.
   */
  public abstract class MessageToMessageEncoder<I> extends ChannelOutboundHandlerAdapter {
  
      private final TypeParameterMatcher matcher;
  
      /**
       * Create a new instance which will try to detect the types to match out of the type parameter of the class.
       */
      protected MessageToMessageEncoder() {
          matcher = TypeParameterMatcher.find(this, MessageToMessageEncoder.class, "I");
      }
  
      /**
       * Create a new instance
       *
       * @param outboundMessageType   The type of messages to match and so encode
       */
      protected MessageToMessageEncoder(Class<? extends I> outboundMessageType) {
          matcher = TypeParameterMatcher.get(outboundMessageType);
      }
  
      /**
       * Returns {@code true} if the given message should be handled. If {@code false} it will be passed to the next
       * {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
       */
      public boolean acceptOutboundMessage(Object msg) throws Exception {
          return matcher.match(msg);
      }
  
      @Override
      public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
          CodecOutputList out = null;
          try {
              if (acceptOutboundMessage(msg)) {
                  out = CodecOutputList.newInstance();
                  @SuppressWarnings("unchecked")
                  I cast = (I) msg;
                  try {
                      encode(ctx, cast, out);
                  } finally {
                      ReferenceCountUtil.release(cast);
                  }
  
                  if (out.isEmpty()) {
                      out.recycle();
                      out = null;
  
                      throw new EncoderException(
                              StringUtil.simpleClassName(this) + " must produce at least one message.");
                  }
             } else {
                 ctx.write(msg, promise);
             }
         } catch (EncoderException e) {
             throw e;
         } catch (Throwable t) {
             throw new EncoderException(t);
         } finally {
             if (out != null) {
                 final int sizeMinusOne = out.size() - 1;
                 if (sizeMinusOne == 0) {
                     ctx.write(out.get(0), promise);
                 } else if (sizeMinusOne > 0) {
                     // Check if we can use a voidPromise for our extra writes to reduce GC-Pressure
                     // See https://github.com/netty/netty/issues/2525
                     ChannelPromise voidPromise = ctx.voidPromise();
                     boolean isVoidPromise = promise == voidPromise;
                     for (int i = 0; i < sizeMinusOne; i ++) {
                         ChannelPromise p;
                         if (isVoidPromise) {
                             p = voidPromise;
                         } else {
                             p = ctx.newPromise();
                         }
                         ctx.write(out.getUnsafe(i), p);
                     }
                     ctx.write(out.getUnsafe(sizeMinusOne), promise);
                 }
                 out.recycle();
             }
         }
     }
 
     /**
      * Encode from one message to an other. This method will be called for each written message that can be handled
      * by this encoder.
      *
      * @param ctx           the {@link ChannelHandlerContext} which this {@link MessageToMessageEncoder} belongs to
      * @param msg           the message to encode to an other one
      * @param out           the {@link List} into which the encoded msg should be added
      *                      needs to do some kind of aggregation
      * @throws Exception    is thrown if an error occurs
      */
     protected abstract void encode(ChannelHandlerContext ctx, I msg, List<Object> out) throws Exception;
 }