Advent of code 2017 — Day 1

Advent of code 2017 — Day 1.js

// http://adventofcode.com/2017/day/1

// Sums the digits matching the next digit.
// The list is circular: is the first and last digits are the same, the last digit matches.
const sumMatchesNext = input =>
  Array.from(String(input)) // Turn the input into an array of characters
    .map(Number) // Turn the caracters into number
    .reduce((sum, digit, i, digits) => {
      const index = (i + 1) % digits.length; // The list is circular
      // If the digit is the same as the next digit, add it to the sum
      if (digit === digits[index]) {
        return sum + digit;
      } else {
        return sum;
      }
    }, 0);

// Solution to the second part
const sumMatchesHalwfayAround = input =>
  Array.from(String(input)) // Turn the input into an array of characters
    .map(Number) // Turn the caracters into number
    .reduce((sum, digit, i, digits) => {
      const index = i + digits.length / 2;
      if (digit === digits[index % digits.length]) {
        return sum + digit;
      } else {
        return sum;
      }
    }, 0);