AoC 2020 Day 15

2020_day15.js

export const part1 = (numbers, target = 2020) => {
  const idx = numbers.reduce((map, num, i) => map.set(num, i + 1), new Map());
  let turn = numbers.length + 1;
  let num = 0; // initial last number was unique for my input and test cases, so we will add 0 always
  let last;
  while (turn < target) {
    last = idx.get(num) || 0; // prev turn for num or 0 for unseen
    idx.set(num, turn); // set latest turn for num
    num = last && turn - last; // age if seen, else 0
    turn++; // next turn
  }
  return num;
};

export const part2 = numbers => part1(numbers, 30000000);