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   /*
    * Copyright 2014 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.http.websocketx.extensions;
  
  import static io.netty.util.internal.ObjectUtil.checkNonEmpty;
  
  import io.netty.channel.ChannelDuplexHandler;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelPromise;
  import io.netty.handler.codec.CodecException;
  import io.netty.handler.codec.http.HttpHeaderNames;
  import io.netty.handler.codec.http.HttpRequest;
  import io.netty.handler.codec.http.HttpResponse;
  
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Iterator;
  import java.util.List;
  
  /**
   * This handler negotiates and initializes the WebSocket Extensions.
   *
   * This implementation negotiates the extension with the server in a defined order,
   * ensures that the successfully negotiated extensions are consistent between them,
   * and initializes the channel pipeline with the extension decoder and encoder.
   *
   * Find a basic implementation for compression extensions at
   * <tt>io.netty.handler.codec.http.websocketx.extensions.compression.WebSocketClientCompressionHandler</tt>.
   */
  public class WebSocketClientExtensionHandler extends ChannelDuplexHandler {
  
      private final List<WebSocketClientExtensionHandshaker> extensionHandshakers;
  
      /**
       * Constructor
       *
       * @param extensionHandshakers
       *      The extension handshaker in priority order. A handshaker could be repeated many times
       *      with fallback configuration.
       */
      public WebSocketClientExtensionHandler(WebSocketClientExtensionHandshaker... extensionHandshakers) {
          this.extensionHandshakers = Arrays.asList(checkNonEmpty(extensionHandshakers, "extensionHandshakers"));
      }
  
      @Override
      public void write(final ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
          if (msg instanceof HttpRequest && WebSocketExtensionUtil.isWebsocketUpgrade(((HttpRequest) msg).headers())) {
              HttpRequest request = (HttpRequest) msg;
              String headerValue = request.headers().getAsString(HttpHeaderNames.SEC_WEBSOCKET_EXTENSIONS);
              List<WebSocketExtensionData> extraExtensions =
                new ArrayList<WebSocketExtensionData>(extensionHandshakers.size());
              for (WebSocketClientExtensionHandshaker extensionHandshaker : extensionHandshakers) {
                  extraExtensions.add(extensionHandshaker.newRequestData());
              }
              String newHeaderValue = WebSocketExtensionUtil
                .computeMergeExtensionsHeaderValue(headerValue, extraExtensions);
  
              request.headers().set(HttpHeaderNames.SEC_WEBSOCKET_EXTENSIONS, newHeaderValue);
          }
  
          super.write(ctx, msg, promise);
      }
  
      @Override
      public void channelRead(ChannelHandlerContext ctx, Object msg)
              throws Exception {
          if (msg instanceof HttpResponse) {
              HttpResponse response = (HttpResponse) msg;
  
              if (WebSocketExtensionUtil.isWebsocketUpgrade(response.headers())) {
                  String extensionsHeader = response.headers().getAsString(HttpHeaderNames.SEC_WEBSOCKET_EXTENSIONS);
  
                  if (extensionsHeader != null) {
                      List<WebSocketExtensionData> extensions =
                              WebSocketExtensionUtil.extractExtensions(extensionsHeader);
                      List<WebSocketClientExtension> validExtensions =
                              new ArrayList<WebSocketClientExtension>(extensions.size());
                      int rsv = 0;
  
                      for (WebSocketExtensionData extensionData : extensions) {
                          Iterator<WebSocketClientExtensionHandshaker> extensionHandshakersIterator =
                                  extensionHandshakers.iterator();
                          WebSocketClientExtension validExtension = null;
  
                          while (validExtension == null && extensionHandshakersIterator.hasNext()) {
                              WebSocketClientExtensionHandshaker extensionHandshaker =
                                     extensionHandshakersIterator.next();
                             validExtension = extensionHandshaker.handshakeExtension(extensionData);
                         }
 
                         if (validExtension != null && ((validExtension.rsv() & rsv) == 0)) {
                             rsv = rsv | validExtension.rsv();
                             validExtensions.add(validExtension);
                         } else {
                             throw new CodecException(
                                     "invalid WebSocket Extension handshake for \"" + extensionsHeader + '"');
                         }
                     }
 
                     for (WebSocketClientExtension validExtension : validExtensions) {
                         WebSocketExtensionDecoder decoder = validExtension.newExtensionDecoder();
                         WebSocketExtensionEncoder encoder = validExtension.newExtensionEncoder();
                         ctx.pipeline().addAfter(ctx.name(), decoder.getClass().getName(), decoder);
                         ctx.pipeline().addAfter(ctx.name(), encoder.getClass().getName(), encoder);
                     }
                 }
 
                 ctx.pipeline().remove(ctx.name());
             }
         }
 
         super.channelRead(ctx, msg);
     }
 }