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   /*
    * 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.spdy;
  
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.MessageToMessageCodec;
  import io.netty.handler.codec.http.HttpMessage;
  import io.netty.handler.codec.spdy.SpdyHttpHeaders.Names;
  import io.netty.util.ReferenceCountUtil;
  
  import java.util.ArrayDeque;
  import java.util.List;
  import java.util.Queue;
  
  /**
   * {@link MessageToMessageCodec} that takes care of adding the right {@link SpdyHttpHeaders.Names#STREAM_ID} to the
   * {@link HttpMessage} if one is not present. This makes it possible to just re-use plan handlers current used
   * for HTTP.
   */
  public class SpdyHttpResponseStreamIdHandler extends
          MessageToMessageCodec<Object, HttpMessage> {
      private static final Integer NO_ID = -1;
      private final Queue<Integer> ids = new ArrayDeque<Integer>();
  
      @Override
      public boolean acceptInboundMessage(Object msg) throws Exception {
          return msg instanceof HttpMessage || msg instanceof SpdyRstStreamFrame;
      }
  
      @Override
      protected void encode(ChannelHandlerContext ctx, HttpMessage msg, List<Object> out) throws Exception {
          Integer id = ids.poll();
          if (id != null && id.intValue() != NO_ID && !msg.headers().contains(SpdyHttpHeaders.Names.STREAM_ID)) {
              msg.headers().setInt(Names.STREAM_ID, id);
          }
  
          out.add(ReferenceCountUtil.retain(msg));
      }
  
      @Override
      protected void decode(ChannelHandlerContext ctx, Object msg, List<Object> out) throws Exception {
          if (msg instanceof HttpMessage) {
              boolean contains = ((HttpMessage) msg).headers().contains(SpdyHttpHeaders.Names.STREAM_ID);
              if (!contains) {
                  ids.add(NO_ID);
              } else {
                  ids.add(((HttpMessage) msg).headers().getInt(Names.STREAM_ID));
              }
          } else if (msg instanceof SpdyRstStreamFrame) {
              ids.remove(((SpdyRstStreamFrame) msg).streamId());
          }
  
          out.add(ReferenceCountUtil.retain(msg));
      }
  }