const fs = require("fs");
const path = require("path");
const file = path.join(process.cwd(), "path_to_save.d2s");

function calculateSum(data) {
  let sum = 0;
  for (let i = 0; i < data.length; i++) {
    let ch = data[i];
    if (i >= 12 && i < 16) {
      ch = 0;
    }
    ch += sum < 0;
    sum = (sum << 1) + ch;
  }

  return sum;
}

function littleToBigEndian(number) {
  return new DataView(
    Int32Array.of(
      new DataView(Int32Array.of(number).buffer).getUint32(0, true)
    ).buffer
  );
}

function ashex(buffer) {
  return buffer.getUint32(0, false).toString(16);
}

async function readSafeFile() {
  return await new Promise((resolve, reject) => {
    fs.readFile(file, (err, data) => {
      if (err) return reject(err);
      return resolve(data);
    });
  });
}

async function writeCheckSumToSafeFile(data) {
  return await new Promise((resolve, reject) => {
    fs.writeFile(file, data, err => {
      if (err) reject(err);
      resolve();
    });
  });
}

readSafeFile().then(data => {
  const sum = calculateSum(data);
  const bufferSum = littleToBigEndian(sum);
  const hex = ashex(bufferSum);
  const newData = data;
  for (let i = 0; i < 4; i++) {
    newData[12 + i] = bufferSum.getInt8(i);
  }
  writeCheckSumToSafeFile(newData).then(() => console.log(hex));
});