Matteo Pettenò

Advent of Code 2025

Day 10

Day 11 - Reactor

Day 12
Solution Code

Part 1

The problem models a network of connected electrical devices as a directed graph, where each device points to its downstream output devices. We need to find the total number of distinct paths leading from the starting device "you" to the target output device "out". To solve Part 1:

  1. Parse the input lines into an adjacency list mapping each device ID to its list of connected output devices.
  2. Implement depth-first search (DFS) with memoization to count paths from any given device to the destination "out".
  3. Evaluate the DFS from "you": if the target "out" is reached, return 1; otherwise, sum the path counts of all neighbor outputs and memoize the result.
  4. Return the total path count from "you" to "out".

Part 2

In Part 2, we need to find the total number of paths from the server rack ("svr") to "out" that visit both key intermediate devices ("dac" and "fft") in any order. Because the graph is directed and acyclic, visiting both devices implies one of two mutually exclusive orderings: svr -> dac -> fft -> out or svr -> fft -> dac -> out. To solve Part 2:

  1. Use the memoized path-counting helper from Part 1 to compute path counts between any two specific nodes (countPathsBetween(start, target)).
  2. Calculate the total paths for the first sequence (dac then fft) by multiplying independent path segments: countPathsBetween("svr", "dac") * countPathsBetween("dac", "fft") * countPathsBetween("fft", "out").
  3. Calculate the total paths for the second sequence (fft then dac) similarly: countPathsBetween("svr", "fft") * countPathsBetween("fft", "dac") * countPathsBetween("dac", "out").
  4. Sum the results of both sequence orderings to get the total number of valid paths visiting both devices.

Solver