TCP 粘包与基于长度前缀的拆包

// 运行环境：Node.js 14+
// 保存为 aj-l4.js，执行：node aj-l4.js
const net = require('net')

// 编码：加 4 字节长度前缀
function encode(msg) {
  const payload = Buffer.from(msg)
  const head = Buffer.alloc(4)
  head.writeUInt32BE(payload.length, 0) // 大端 32 位长度
  return Buffer.concat([head, payload])
}

// 解码：维护缓冲区，按长度前缀拆出完整消息
class Decoder {
  constructor() { this.buf = Buffer.alloc(0) }
  push(chunk) {
    this.buf = Buffer.concat([this.buf, chunk])
    const out = []
    while (this.buf.length >= 4) {
      const len = this.buf.readUInt32BE(0)
      if (this.buf.length < 4 + len) break // 负载还没收齐，等更多
      out.push(this.buf.slice(4, 4 + len).toString())
      this.buf = this.buf.slice(4 + len)
    }
    return out
  }
}

// 调用示例：模拟粘包（两条拼一起发）与半包（分两段发）
const d = new Decoder()
console.log(d.push(Buffer.concat([encode('hello'), encode('world')]))) // ['hello','world']（粘包被拆开）
const part1 = encode('longmessage').slice(0, 6)
const part2 = encode('longmessage').slice(6)
console.log(d.push(part1), d.push(part2)) // [] 然后 ['longmessage']（半包被拼合）