day11.ts

import { divmod, type MainArgs, parseFile } from './lib/utils.ts';

type Parsed = number[];

function mapAdd(m: Map<number, number>, k: number, v = 1): void {
  m.set(k, (m.get(k) ?? 0) + v);
}

function blink(inp: Parsed, num: number): number {
  // Each stone k occurs v times.  On a new iteration, there are a multiple
  // ways that a given numbered stone might be created.  Add all of those ways.
  // However, no matter how a stone got created, it will have the same set of
  // descendants.
  let prev = new Map<number, number>();
  for (const s of inp) {
    mapAdd(prev, s);
  }
  for (let i = 0; i < num; i++) {
    const next = new Map<number, number>();
    for (const [k, v] of prev) {
      if (k === 0) {
        mapAdd(next, 1, v);
      } else {
        const ks = String(k);
        const [div, mod] = divmod(ks.length, 2);
        if (mod === 0) {
          mapAdd(next, Number(ks.slice(0, div)), v);
          mapAdd(next, Number(ks.slice(div)), v);
        } else {
          mapAdd(next, k * 2024, v);
        }
      }
    }
    prev = next;
  }
  return prev.values().reduce((t, v) => t + v, 0);
}

function part1(inp: Parsed): number {
  return blink(inp, 25);
}

function part2(inp: Parsed): number {
  return blink(inp, 75);
}

export default async function main(args: MainArgs): Promise<[number, number]> {
  const inp = await parseFile<Parsed>(args);
  return [part1(inp), part2(inp)];
}