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   /*
    * Copyright 2015 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.protobuf;
  
  import com.google.protobuf.CodedInputStream;
  import com.google.protobuf.nano.CodedInputByteBufferNano;
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.ByteToMessageDecoder;
  import io.netty.handler.codec.CorruptedFrameException;
  
  import java.util.List;
  
  /**
   * A decoder that splits the received {@link ByteBuf}s dynamically by the
   * value of the Google Protocol Buffers
   * <a href="https://developers.google.com/protocol-buffers/docs/encoding#varints">Base
   * 128 Varints</a> integer length field in the message. For example:
   * <pre>
   * BEFORE DECODE (302 bytes)       AFTER DECODE (300 bytes)
   * +--------+---------------+      +---------------+
   * | Length | Protobuf Data |----->| Protobuf Data |
   * | 0xAC02 |  (300 bytes)  |      |  (300 bytes)  |
   * +--------+---------------+      +---------------+
   * </pre>
   *
   * @see CodedInputStream
   * @see CodedInputByteBufferNano
   */
  public class ProtobufVarint32FrameDecoder extends ByteToMessageDecoder {
  
      // TODO maxFrameLength + safe skip + fail-fast option
      //      (just like LengthFieldBasedFrameDecoder)
  
      @Override
      protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out)
              throws Exception {
          in.markReaderIndex();
          int preIndex = in.readerIndex();
          int length = readRawVarint32(in);
          if (preIndex == in.readerIndex()) {
              return;
          }
          if (length < 0) {
              throw new CorruptedFrameException("negative length: " + length);
          }
  
          if (in.readableBytes() < length) {
              in.resetReaderIndex();
          } else {
              out.add(in.readRetainedSlice(length));
          }
      }
  
      /**
       * Reads variable length 32bit int from buffer
       *
       * @return decoded int if buffers readerIndex has been forwarded else nonsense value
       */
      private static int readRawVarint32(ByteBuf buffer) {
          if (!buffer.isReadable()) {
              return 0;
          }
          buffer.markReaderIndex();
          byte tmp = buffer.readByte();
          if (tmp >= 0) {
              return tmp;
          } else {
              int result = tmp & 127;
              if (!buffer.isReadable()) {
                  buffer.resetReaderIndex();
                  return 0;
              }
              if ((tmp = buffer.readByte()) >= 0) {
                  result |= tmp << 7;
              } else {
                  result |= (tmp & 127) << 7;
                  if (!buffer.isReadable()) {
                      buffer.resetReaderIndex();
                      return 0;
                  }
                  if ((tmp = buffer.readByte()) >= 0) {
                      result |= tmp << 14;
                  } else {
                      result |= (tmp & 127) << 14;
                      if (!buffer.isReadable()) {
                         buffer.resetReaderIndex();
                         return 0;
                     }
                     if ((tmp = buffer.readByte()) >= 0) {
                         result |= tmp << 21;
                     } else {
                         result |= (tmp & 127) << 21;
                         if (!buffer.isReadable()) {
                             buffer.resetReaderIndex();
                             return 0;
                         }
                         result |= (tmp = buffer.readByte()) << 28;
                         if (tmp < 0) {
                             throw new CorruptedFrameException("malformed varint.");
                         }
                     }
                 }
             }
             return result;
         }
     }
 }