Node.js Solution to Project Euler #13

euler13.js

const fs = require('fs');

const readInt8 = () =>
  new Promise((resolve, reject) => {
    fs.readFile('input.txt', 'utf8', (err, data) => {
      let newData = data
        .split(/\n/g)
        .map(e => new Int8Array(e.match(/./g).map(e => parseInt(e))));
      err ? reject(err) : resolve(newData);
    });
  });

const split = carry => {
  if (carry > 9) {
    // Greater than 2 digits so split
    carry += '';
    return [parseInt(carry.slice(0, -1)), parseInt(carry.slice(-1))];
  } else {
    // Less than 2 digits, don't split
    return [0, carry];
  }
};

const summation = arr => {
  const max = Math.max(...arr.map(e => e.length)) + 1;
  let offset = 1,
    carry = 0,
    res = '';

  // Loop until last digit of largest num
  while (offset < max) {
    // Loop over each number
    for (num of arr) {
      // Length is fixed when Int8Array is constructed
      carry += num[num.length - offset] || 0;
    }
    // Separate Digits [rest, rightmost]
    const sep = split(carry);
    carry = sep[0]; // Set carry to leftover
    res += sep[1]; // Append result
    ++offset;
  }
  // Flip result
  res = res
    .split('')
    .reverse()
    .join('');

  // If there is leftover carry, add it
  if (carry > 0) res = carry + res;

  return res;
};

const run = async () => {
  const arr = await readInt8(),
    sum = summation(arr);
  console.log(`Full Sum: ${sum}`);
  console.log(`First 10 digits: ${sum.slice(0, 10)}`);
};

run().catch(err => err);