Advent2017 - 1

1.js

const cal = input =>
  input
    .split('')
    .map(Number)
    .reduce((mem, x, i, arr) => {
      if (i === (arr.length - 1)) return x === arr[0] ? (mem + x) : mem;
      if (x === arr[i + 1]) return mem + x;

      return mem;
    }, 0);

const cal2 = input =>
  input
    .split('')
    .map(Number)
    .reduce((mem, x, i, arr) => {
      const wi = ((arr.length / 2) + i) % arr.length;
      if (x === arr[wi]) return mem + x;

      return mem;
    }, 0);