/*
    * 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.ExtensionRegistry;
  import com.google.protobuf.ExtensionRegistryLite;
  import com.google.protobuf.Message;
  import com.google.protobuf.MessageLite;
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufUtil;
  import io.netty.channel.ChannelHandler.Sharable;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelPipeline;
  import io.netty.handler.codec.ByteToMessageDecoder;
  import io.netty.handler.codec.LengthFieldBasedFrameDecoder;
  import io.netty.handler.codec.LengthFieldPrepender;
  import io.netty.handler.codec.MessageToMessageDecoder;
  import io.netty.util.internal.ObjectUtil;
  
  import java.util.List;
  
  /**
   * Decodes a received {@link ByteBuf} into a
   * <a href="https://github.com/google/protobuf">Google Protocol Buffers</a>
   * {@link Message} and {@link MessageLite}. Please note that this decoder must
   * be used with a proper {@link ByteToMessageDecoder} such as {@link ProtobufVarint32FrameDecoder}
   * or {@link LengthFieldBasedFrameDecoder} if you are using a stream-based
   * transport such as TCP/IP. A typical setup for TCP/IP would be:
   * <pre>
   * {@link ChannelPipeline} pipeline = ...;
   *
   * // Decoders
   * pipeline.addLast("frameDecoder",
   *                  new {@link LengthFieldBasedFrameDecoder}(1048576, 0, 4, 0, 4));
   * pipeline.addLast("protobufDecoder",
   *                  new {@link ProtobufDecoder}(MyMessage.getDefaultInstance()));
   *
   * // Encoder
   * pipeline.addLast("frameEncoder", new {@link LengthFieldPrepender}(4));
   * pipeline.addLast("protobufEncoder", new {@link ProtobufEncoder}());
   * </pre>
   * and then you can use a {@code MyMessage} instead of a {@link ByteBuf}
   * as a message:
   * <pre>
   * void channelRead({@link ChannelHandlerContext} ctx, Object msg) {
   *     MyMessage req = (MyMessage) msg;
   *     MyMessage res = MyMessage.newBuilder().setText(
   *                               "Did you say '" + req.getText() + "'?").build();
   *     ch.write(res);
   * }
   * </pre>
   */
  @Sharable
  public class ProtobufDecoder extends MessageToMessageDecoder<ByteBuf> {
  
      private static final boolean HAS_PARSER;
  
      static {
          boolean hasParser = false;
          try {
              // MessageLite.getParserForType() is not available until protobuf 2.5.0.
              MessageLite.class.getDeclaredMethod("getParserForType");
              hasParser = true;
          } catch (Throwable t) {
              // Ignore
          }
  
          HAS_PARSER = hasParser;
      }
  
      private final MessageLite prototype;
      private final ExtensionRegistryLite extensionRegistry;
  
      /**
       * Creates a new instance.
       */
      public ProtobufDecoder(MessageLite prototype) {
          this(prototype, null);
      }
  
      public ProtobufDecoder(MessageLite prototype, ExtensionRegistry extensionRegistry) {
          this(prototype, (ExtensionRegistryLite) extensionRegistry);
      }
  
      public ProtobufDecoder(MessageLite prototype, ExtensionRegistryLite extensionRegistry) {
          this.prototype = ObjectUtil.checkNotNull(prototype, "prototype").getDefaultInstanceForType();
         this.extensionRegistry = extensionRegistry;
     }
 
     @Override
     protected void decode(ChannelHandlerContext ctx, ByteBuf msg, List<Object> out)
             throws Exception {
         final byte[] array;
         final int offset;
         final int length = msg.readableBytes();
         if (msg.hasArray()) {
             array = msg.array();
             offset = msg.arrayOffset() + msg.readerIndex();
         } else {
             array = ByteBufUtil.getBytes(msg, msg.readerIndex(), length, false);
             offset = 0;
         }
 
         if (extensionRegistry == null) {
             if (HAS_PARSER) {
                 out.add(prototype.getParserForType().parseFrom(array, offset, length));
             } else {
                 out.add(prototype.newBuilderForType().mergeFrom(array, offset, length).build());
             }
         } else {
             if (HAS_PARSER) {
                 out.add(prototype.getParserForType().parseFrom(
                         array, offset, length, extensionRegistry));
             } else {
                 out.add(prototype.newBuilderForType().mergeFrom(
                         array, offset, length, extensionRegistry).build());
             }
         }
     }
 }