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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.MessageToByteEncoder;
  
  import java.math.BigInteger;
  
  /**
   * Encodes a {@link Number} into the binary representation prepended with
   * a magic number ('F' or 0x46) and a 32-bit length prefix.  For example, 42
   * will be encoded to { 'F', 0, 0, 0, 1, 42 }.
   */
  public class NumberEncoder extends MessageToByteEncoder<Number> {
  
      @Override
      protected void encode(ChannelHandlerContext ctx, Number msg, ByteBuf out) {
          // Convert to a BigInteger first for easier implementation.
          BigInteger v;
          if (msg instanceof BigInteger) {
              v = (BigInteger) msg;
          } else {
              v = new BigInteger(String.valueOf(msg));
          }
  
          // Convert the number into a byte array.
          byte[] data = v.toByteArray();
          int dataLength = data.length;
  
          // Write a message.
          out.writeByte((byte) 'F'); // magic number
          out.writeInt(dataLength);  // data length
          out.writeBytes(data);      // data
      }
  }