Year 2025 Day 11

Author: maneatingape

README.md

@@ -87,6 +87,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 8 | [Playground](https://adventofcode.com/2025/day/8) | [Source](src/year2025/day08.rs) | 527 |
 | 9 | [Movie Theater](https://adventofcode.com/2025/day/9) | [Source](src/year2025/day09.rs) | 668 |
 | 10 | [Factory](https://adventofcode.com/2025/day/10) | [Source](src/year2025/day10.rs) | 117* |
+| 11 | [Reactor](https://adventofcode.com/2025/day/11) | [Source](src/year2025/day11.rs) | 75 |
 
 ## 2024
 

benches/benchmark.rs

@@ -95,5 +95,5 @@ benchmark!(year2024
 );
 
 benchmark!(year2025
- day01, day02, day03, day04, day05, day06, day07, day08, day09, day10
+ day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11
 );

src/lib.rs

@@ -74,5 +74,5 @@ library!(year2024 "Locate the Chief Historian in time for the big Christmas slei
 );
 
 library!(year2025 "Finish the North Pole decorations in time for Christmas."
- day01, day02, day03, day04, day05, day06, day07, day08, day09, day10
+ day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11
 );

src/main.rs

@@ -141,5 +141,5 @@ run!(year2024
 );
 
 run!(year2025
- day01, day02, day03, day04, day05, day06, day07, day08, day09, day10
+ day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11
 );

src/year2025/day11.rs

@@ -0,0 +1,45 @@
+//! # Reactor
+type Input = Vec<Vec<usize>>;
+
+pub fn parse(input: &str) -> Input {
+ let mut graph = vec![vec![]; 26 * 26 * 26];
+
+ for line in input.lines() {
+ let mut edges = line.split_ascii_whitespace();
+ let from = edges.next().unwrap();
+ graph[to_index(from)].extend(edges.map(to_index));
+ }
+
+ graph
+}
+
+pub fn part1(input: &Input) -> u64 {
+ paths(input, "you", "out")
+}
+
+pub fn part2(input: &Input) -> u64 {
+ let one = paths(input, "svr", "fft") * paths(input, "fft", "dac") * paths(input, "dac", "out");
+ let two = paths(input, "svr", "dac") * paths(input, "dac", "fft") * paths(input, "fft", "out");
+ one + two
+}
+
+fn paths(input: &Input, from: &str, to: &str) -> u64 {
+ let mut cache = vec![u64::MAX; input.len()];
+ dfs(input, &mut cache, to_index(from), to_index(to))
+}
+
+fn dfs(input: &Input, cache: &mut [u64], node: usize, end: usize) -> u64 {
+ if node == end {
+ 1
+ } else if cache[node] == u64::MAX {
+ let result = input[node].iter().map(|&next| dfs(input, cache, next, end)).sum();
+ cache[node] = result;
+ result
+ } else {
+ cache[node]
+ }
+}
+
+fn to_index(s: &str) -> usize {
+ s.bytes().take(3).fold(0, |acc, b| 26 * acc + usize::from(b - b'a'))
+}

tests/test.rs

@@ -87,5 +87,5 @@ test!(year2024
 );
 
 test!(year2025
- day01, day02, day03, day04, day05, day06, day07, day08, day09, day10
+ day01, day02, day03, day04, day05, day06, day07, day08, day09, day10, day11
 );

tests/year2025/day11.rs

@@ -0,0 +1,40 @@
+use aoc::year2025::day11::*;
+
+const EXAMPLE_ONE: &str = "\
+aaa: you hhh
+you: bbb ccc
+bbb: ddd eee
+ccc: ddd eee fff
+ddd: ggg
+eee: out
+fff: out
+ggg: out
+hhh: ccc fff iii
+iii: out";
+
+const EXAMPLE_TWO: &str = "\
+svr: aaa bbb
+aaa: fft
+fft: ccc
+bbb: tty
+tty: ccc
+ccc: ddd eee
+ddd: hub
+hub: fff
+eee: dac
+dac: fff
+fff: ggg hhh
+ggg: out
+hhh: out";
+
+#[test]
+fn part1_test() {
+ let input = parse(EXAMPLE_ONE);
+ assert_eq!(part1(&input), 5);
+}
+
+#[test]
+fn part2_test() {
+ let input = parse(EXAMPLE_TWO);
+ assert_eq!(part2(&input), 2);
+}