jsowell-charger-web/jsowell-netty/src/main/java/com/jsowell/netty/decoder/StartAndLengthFieldFrameDecoder.java

package com.jsowell.netty.decoder;

import com.jsowell.common.constant.Constants;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;
import lombok.extern.slf4j.Slf4j;

import java.nio.charset.StandardCharsets;
import java.util.List;

@Slf4j
public class StartAndLengthFieldFrameDecoder extends ByteToMessageDecoder {
	private static final int HEADER_LENGTH_DNY = 3; // "DNY" 包头的长度
	private static final int HEADER_LENGTH_68 = 1; // 68 包头的长度

	// 构造函数，初始化起始标志
	public StartAndLengthFieldFrameDecoder() {}

	protected void decode(ChannelHandlerContext ctx, ByteBuf buffer, List<Object> out) throws Exception {
		// log.info("StartAndLengthFieldFrameDecoder.decode");
		// 记录包头开始的index
		int beginReader;

		// 循环查找包头
		while (true) {
			if (buffer.readableBytes() < Math.min(HEADER_LENGTH_DNY, HEADER_LENGTH_68)) {
				return; // 数据长度不足，等待更多数据
			}

			// 获取包头开始的index
			beginReader = buffer.readerIndex();
			buffer.markReaderIndex();

			// 判断是否为DNY包头或68包头
			if (isStartOfDnyHeader(buffer, beginReader) || isStartOf68Header(buffer, beginReader)) {
				break; // 读到了协议的开始标志，结束while循环
			}

			// 未读到包头，略过一个字节
			buffer.resetReaderIndex();
			buffer.readByte();
		}

		// 检查包头是否是 "DNY"
		if (buffer.readableBytes() >= HEADER_LENGTH_DNY) {
			byte[] headerBytes = new byte[HEADER_LENGTH_DNY];
			buffer.getBytes(beginReader, headerBytes, 0, HEADER_LENGTH_DNY);
			String header = new String(headerBytes, StandardCharsets.UTF_8);
			// log.info("检查包头是否是DNY, header:{}", header);
			if (Constants.EBIKE_HEADER.equals(header)) {
				// 处理 DNY 协议
				decodeDnyMessage(buffer, out, beginReader);
				return;
			}
		}

		// 检查包头是否是 68 协议
		if (buffer.readableBytes() >= HEADER_LENGTH_68) {
			if (buffer.getUnsignedByte(beginReader) == 0x68) {
				// 处理 68 协议
				decode68Message(buffer, out, beginReader);
				return;
			}
		}

		// 未知协议，还原读指针
		buffer.resetReaderIndex();
	}

	// 判断是否为DNY包头
	private boolean isStartOfDnyHeader(ByteBuf buffer, int beginReader) {
		if (buffer.readableBytes() >= HEADER_LENGTH_DNY) {
			byte[] headerBytes = new byte[HEADER_LENGTH_DNY];
			buffer.getBytes(beginReader, headerBytes, 0, HEADER_LENGTH_DNY);
			String header = new String(headerBytes, StandardCharsets.UTF_8);
			return Constants.EBIKE_HEADER.equals(header);
		}
		return false;
	}

	// 判断是否为68包头
	private boolean isStartOf68Header(ByteBuf buffer, int beginReader) {
		if (buffer.readableBytes() >= HEADER_LENGTH_68) {
			return buffer.getUnsignedByte(beginReader) == 0x68;
		}
		return false;
	}

	// 处理68协议消息
	private void decode68Message(ByteBuf buffer, List<Object> out, int beginReader) {
		// 检查剩余数据是否足够
		if (buffer.readableBytes() < HEADER_LENGTH_68 + 1 + 2) {
			buffer.readerIndex(beginReader);
			return;
		}

		// 获取消息长度
		int length = buffer.getUnsignedByte(beginReader + HEADER_LENGTH_68);
		// 检查剩余数据是否足够
		if (buffer.readableBytes() < HEADER_LENGTH_68 + 1 + length + 2) {
			buffer.readerIndex(beginReader);
			return;
		}

		// 读取 data 数据 最后+2是帧校验域长度
		ByteBuf frame = buffer.retainedSlice(beginReader, HEADER_LENGTH_68 + 1 + length + 2);
		buffer.readerIndex(beginReader + HEADER_LENGTH_68 + 1 + length + 2);
		out.add(frame);
	}

	// 处理DNY协议消息
	private void decodeDnyMessage(ByteBuf buffer, List<Object> out, int beginReader) {
		// 检查剩余数据是否足够
		if (buffer.readableBytes() < HEADER_LENGTH_DNY + 1) {
			buffer.readerIndex(beginReader);
			return;
		}

		// 获取消息长度
		int length = buffer.getUnsignedByte(beginReader + HEADER_LENGTH_DNY);
		// log.info("获取消息长度, length:{}", length);
		// 检查剩余数据是否足够
		if (buffer.readableBytes() < HEADER_LENGTH_DNY + 1 + length) {
			buffer.readerIndex(beginReader);
			return;
		}

		// 读取 data 数据
		ByteBuf frame = buffer.retainedSlice(beginReader, HEADER_LENGTH_DNY + length + 2);
		buffer.readerIndex(beginReader + HEADER_LENGTH_DNY + length + 2);


		out.add(frame);
	}

}