const shortId = require('shortid');
const {CRC} = require('crc-full');

const MTU = 22;
const HEAD_LENGTH = 21;
const PAYLOAD_LENGTH = MTU - HEAD_LENGTH;
const FLAG = 255;
const FLAG_BIT = new Uint8Array([255]);

const crc = new CRC('CRC8', 8, 0x07, 0x00, 0x00, false, false);

function sendData(data) {
  let chunkSize = Math.ceil(data.length / PAYLOAD_LENGTH)
  let cs = chunkSize.toString(32).padStart(2);
  let id = shortId().padStart(13);
  let chunks = [];

  for (let offset = 0; offset < chunkSize; offset++) {
    let co = (offset + 1).toString(32).padStart(2);
    let chunk = Buffer.concat([
      Buffer.from(`${co}${cs}${id} `),
      data.slice(offset, PAYLOAD_LENGTH + (PAYLOAD_LENGTH * offset))
    ]);
    let crcCode = crc.compute(chunk).toString(32).padStart(2);

    chunk = Buffer.concat([
      FLAG_BIT,
      Buffer.from(crcCode),
      chunk
    ]);

    chunks.push(chunk);
  }

  chunks.map(chunk => {
    console.log(chunk.toString())
  });

  return chunks;
}

let map = new Map();

function recvData(data) {
  let x = data[0];

  if (x !== FLAG) {
    return;
  }

  let crcCode = parseInt(data.slice(1, 3), 32);

  if (isNaN(crcCode)) {
    return;
  }

  let content = data.slice(3);

  if (crc.compute(content) !== crcCode) {
    return;
  }

  let offset = parseInt(content.slice(0, 2), 32);
  let chunkSize = parseInt(content.slice(2, 4), 32);
  let id =content.slice(4, 17).toString().trim();
  let body = content.slice(18);

  let cache = map.get(id) || {length: 0};

  let index = offset - 1;

  if (!cache[index]) {
    cache.length++;
  }

  cache[offset - 1] = body;

  if (cache.length === chunkSize) {
    console.log(Buffer.concat(Array.from(cache)).toString());
    map.delete(id);
  } else {
    map.set(id, cache)
  }
}

let chunks = sendData(Buffer.from('hello!'));

console.log('----')

for (let chunk of chunks) {
  recvData(chunk);
}

console.log('----');

for (let chunk of chunks.reverse()) {
  recvData(chunk);
}