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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.example.factorial;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.ByteToMessageDecoder;
  import io.netty.handler.codec.CorruptedFrameException;
  
  import java.math.BigInteger;
  import java.util.List;
  
  /**
   * Decodes the binary representation of a {@link BigInteger} prepended
   * with a magic number ('F' or 0x46) and a 32-bit integer length prefix into a
   * {@link BigInteger} instance.  For example, { 'F', 0, 0, 0, 1, 42 } will be
   * decoded into new BigInteger("42").
   */
  public class BigIntegerDecoder extends ByteToMessageDecoder {
  
      @Override
      protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
          // Wait until the length prefix is available.
          if (in.readableBytes() < 5) {
              return;
          }
  
          in.markReaderIndex();
  
          // Check the magic number.
          int magicNumber = in.readUnsignedByte();
          if (magicNumber != 'F') {
              in.resetReaderIndex();
              throw new CorruptedFrameException("Invalid magic number: " + magicNumber);
          }
  
          // Wait until the whole data is available.
          int dataLength = in.readInt();
          if (in.readableBytes() < dataLength) {
              in.resetReaderIndex();
              return;
          }
  
          // Convert the received data into a new BigInteger.
          byte[] decoded = new byte[dataLength];
          in.readBytes(decoded);
  
          out.add(new BigInteger(decoded));
      }
  }