Advent of Code 2025 - Day 11 - Iterative "solution"

part-1-2-alt.js

// It actually doesn't work for part 2
const devices = new Map(
  input.trim().split('\n').concat('out').map(line => {
    const [device, ...outputs] = line.split(/\W+/);
    return [device, outputs];
  })
);

const countPaths = (start, end = 'out') => {
  let frontier = new Set([start]);
  const visited = new Map([[start, 1]]);
  while (frontier.size) {
    const newFrontier = new Set();
    for (const device of frontier) {
      const count = visited.get(device);
      for (const output of devices.get(device)) {
        newFrontier.add(output);
        visited.set(output, (visited.get(output) ?? 0) + count);
      }
    }
    frontier = newFrontier;
  }
  return visited.get(end) ?? 0;
};

// Part one: it works
console.log(countPaths('you', 'out'));

// Part two: it doesn't. Why?!
console.log(
  countPaths('svr', 'fft') * countPaths('fft', 'dac') * countPaths('dac', 'out') +
  countPaths('svr', 'dac') * countPaths('dac', 'fft') * countPaths('fft', 'out')
);