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   /*
    * Copyright 2021 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.dns;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandler;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.MessageToMessageEncoder;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.UnstableApi;
  
  import java.util.List;
  
  @UnstableApi
  @ChannelHandler.Sharable
  public final class TcpDnsResponseEncoder extends MessageToMessageEncoder<DnsResponse> {
      private final DnsRecordEncoder encoder;
  
      /**
       * Creates a new encoder with {@linkplain DnsRecordEncoder#DEFAULT the default record encoder}.
       */
      public TcpDnsResponseEncoder() {
          this(DnsRecordEncoder.DEFAULT);
      }
  
      /**
       * Creates a new encoder with the specified {@code encoder}.
       */
      public TcpDnsResponseEncoder(DnsRecordEncoder encoder) {
          this.encoder = ObjectUtil.checkNotNull(encoder, "encoder");
      }
  
      @Override
      protected void encode(ChannelHandlerContext ctx, DnsResponse response, List<Object> out) throws Exception {
          ByteBuf buf = ctx.alloc().ioBuffer(1024);
  
          buf.writerIndex(buf.writerIndex() + 2);
          DnsMessageUtil.encodeDnsResponse(encoder, response, buf);
          buf.setShort(0, buf.readableBytes() - 2);
  
          out.add(buf);
      }
  }