#![feature(test)] extern crate test; use aoc2023::{boilerplate, common::*}; use fnv::FnvHashMap as HashMap; const DAY: usize = 8; type Parsed<'a> = (Vec, HashMap<&'a str, [&'a str; 2]>); #[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)] enum Direction { Left = 0, Right = 1, } fn parse_input(raw: &str) -> Parsed { let (directions, map) = raw.split_once("\n\n").unwrap(); let directions = directions .bytes() .map(|i| match i { b'L' => Direction::Left, b'R' => Direction::Right, _ => unreachable!(), }) .collect(); let map = map.lines().map(|l| (&l[0..=2], [&l[7..=9], &l[12..=14]])).collect(); (directions, map) } fn steps_until((directions, map): &Parsed, start: &str, target: &str) -> usize { directions .iter() .cycle() .scan(start, |pos, dir| { let next = map.get(pos)?[*dir as usize]; (!next.ends_with(target)).then(|| *pos = next) }) .count() + 1 } fn part1(parsed: &Parsed) -> usize { steps_until(parsed, "AAA", "ZZZ") } // I’m honestly not sure why this works. It seems each path only has a single ghost node, and that // node occurs right before looping, so we can just compute the least common multiple of their step counts. // I assume this holds true for other inputs (it can’t have been random), // but I don’t see it anywhere in the task and only found out by experimentation. fn part2(parsed: &Parsed) -> usize { parsed.1.keys().filter(|start| start.ends_with("A")).fold(1, |acc, n| lcm(acc, steps_until(parsed, n, "Z"))) } #[cfg(test)] const TEST_INPUT_P2: &str = "LR 11A = (11B, XXX) 11B = (XXX, 11Z) 11Z = (11B, XXX) 22A = (22B, XXX) 22B = (22C, 22C) 22C = (22Z, 22Z) 22Z = (22B, 22B) XXX = (XXX, XXX)"; boilerplate! { TEST_INPUT == "LLR AAA = (BBB, BBB) BBB = (AAA, ZZZ) ZZZ = (ZZZ, ZZZ)", tests: { part1: { TEST_INPUT => 6 }, part2: { TEST_INPUT_P2 => 6 }, }, bench1 == 12083, bench2 == 13385272668829, bench_parse: |(v, m): &Parsed| { assert_eq!(m["AAA"], ["MJJ", "QBJ"]); (v.len(), v[0]) } => (281, Direction::Left), }