advent-code-2021-day-14.js

const input = `NNCB

CH -> B
HH -> N
CB -> H
NH -> C
HB -> C
HC -> B
HN -> C
NN -> C
BH -> H
NC -> B
NB -> B
BN -> B
BB -> N
BC -> B
CC -> N
CN -> C`;

function run() {
  let [original, rest] = input.split("\n\n");
  let mapping = rest.split("\n").reduce((acc, line) => {
    let [from, to] = line.split(" -> ");
    acc[from] = to;
    return acc;
  }, {});

  let pairs = new Map();

  let lastTwo = original.slice(-2)

  for (let i = 0; i < original.length - 1; i++) {
    const pair = original[i] + original[i + 1];
    pairs.set(pair, pairs.has(pair) ? pairs.get(pair) + 1 : 1);
  }

  function step(pairs) {
    const newPairs = new Map();
    for (let [pair, count] of pairs) {
      const [l, r] = pair;
      const c = mapping[pair];
      if (c) {
        [l + c, c + r].forEach((newPair) => {
          newPairs.set(
            newPair,
            newPairs.has(newPair) ? newPairs.get(newPair) + count : count
          );
        });
      } else {
        newPairs.set(pair, count);
      }
    }
    return newPairs;
  }

  for (let i = 0; i < 40; i++) {
    pairs = step(pairs);
  }

  const score = Object.entries(
    Array.from(pairs).reduce((acc, [pair, count]) => {
      acc[pair[0]] = (acc[pair[0]] ?? 0) + count;
      if (pair === lastTwo) {
        acc[pair[1]] = (acc[pair[1]] ?? 0) + 1;
      }
      return acc;
    }, {})
  );

  score.sort((a, b) => b[1] - a[1]);
  console.log(score, score[0][1] - score[score.length - 1][1]);
}

run();