parsing tcp data

tcp.js

const net = require('net')
const DELIMT = '----AxEB04'

// create a server, when the data arrives, push it to the existing buffer
// convert to string to check wether the delimiter is present
// if not, message is still not completed
// if yes, split the data

function parseDelimitedMessage() {
 let buff = new Buffer('');
 let server = net.createServer(c => {
    c.on('data', function(d) {
      buff = Buffer.concat([buff, d])
      let data = buff.toString('utf8')
      let index = data.indexOf(DELIMT)

      if(index > -1) {
        buff = buff.slice(0, index)
        console.log("Previous Message received", buff.toString('utf8'))
        buff = new Buffer(data.slice(index + DELIMT.length))
        console.log("New Message started")
      } else {
        console.log("Reciving Previous Message")
      }
    })
  }).listen(5000, () => {
    console.log('listening')
  })
}

//here when we have a message we send the length of the incomming message,
// in the next messages have a copy of the total length and recieve the streams upto that length
// rest will be a new message, so create a new Buffer

function parseLengthMessage() {
  let len = 0;
  let buff;

  let server = net.createServer(c => {
    c.on('data', data => {
      if(!len)  {
        len = data.readUIntBE(0, 4)
        buff = Buffer.from(data.slice(4))
      }

      if(buff.length + data.length >= len) {
        let diff = len - buff.length
        // hopefully slice slices bytes/octets
        buff = Buffer.concat([buff, data.slice(0, diff)])
        console.log(buff.toString('utf8'), "message")
        // not set new length
        len = data.slice(diff).readUIntBE(0, 4);
        buff = Buffer.from(data.slice(diff + 4))
      } else {
        buff = Buffer.concat([buff, data])
      }
    })
  }).listen(5000, () => {})
}

function generateRandomSentence(totLen, sentLength) {
  totLen = totLen || 500
  const alphabets = 'abcdefghijklmnopqrstuvwxyz'

  let wordSplit = 5
  let sentence = '';

  for(let i = 0; i < totLen; i++) {
    let start = Math.floor((Math.random() * 100) % 21)
    let word =  alphabets.slice(start, start + wordSplit)

    sentence += (word + ( sentence.length % sentLength == 0 ? "\n" : " "))
  }

  return sentence
}

function sendMessageWithLength(client, message) {
  console.log('sending', message, '---')
  let buff = Buffer.alloc(4)
  let hex = message.length.toString(16)

  if(hex.length % 2 !== 0) hex = '0' + hex
  buff.writeUIntBE(`0x${hex}`, 0, 4)

  client.write(buff)
  client.write(Buffer.from(message, 'utf8'))
}

function sendMessageWithDelimit(client, message) {
  console.log(message + DELIMT)

  let buffer = new Buffer(message + DELIMT)
  client.write(buffer)
}

//parseDelimitedMessage()
parseLengthMessage()

let client = net.connect({port: 5000, address: '192.168.1.131'}, () => {
  console.log('connected')

  for(let i = 1; i <= 5; i++) {
    setTimeout(() => {
      sendMessageWithLength(client, generateRandomSentence(5, 4))
      //sendMessageWithDelimit(client, generateRandomSentence(5, 4))
    }, 1000 * i)
  }
})