use std::fmt::Write; use aoc::err; const INPUT: &str = include_str!("../input/day13.txt"); pub fn run() -> aoc::Result { let mut res = String::with_capacity(128); writeln!(res, "part 1: {}", part1(INPUT)?)?; writeln!(res, "part 2: {}", part2(INPUT)?)?; Ok(res) } fn part1(input: &str) -> aoc::Result { let mut lines = input.lines(); let earliest_timestamp = lines .next() .ok_or_else(|| err!("input was empty"))? .parse::() .map_err(|e| err!("couldn't parse first line: {}", e))?; let bus_ids = lines .next() .ok_or_else(|| err!("no second line"))? .split(',') .filter_map(|num| { if num == "x" { None } else { Some(num.parse::().map_err(|e| err!("{}", e))) } }) .collect::>>()?; let (bus_id, earliest_departure) = bus_ids .iter() .map(|id| { let next_departure = ((earliest_timestamp / id) * id) + id; (id, next_departure) }) .min_by_key(|(_, next_departure)| *next_departure) .unwrap(); Ok(bus_id * (earliest_departure - earliest_timestamp)) } fn part2(input: &str) -> aoc::Result { let mut lines = input.lines(); // we don't need the first line anymore, skip it lines.next().ok_or_else(|| err!("input was empty"))?; find_timestamp(lines.next().ok_or_else(|| err!("no second line"))?) } fn find_timestamp(input: &str) -> aoc::Result { let bus_ids: Vec<(u64, u64)> = input .split(',') .enumerate() .filter_map(|(idx, num)| { if num == "x" { None } else { Some((idx as u64, num.parse::().map_err(|e| err!("{}", e)))) } }) .map(|(idx, res)| match res { Ok(num) => Ok((idx, num)), Err(e) => Err(e), }) .collect::>()?; // previous constraints is empty for now let mut current_solution = 0; let mut step = 1; for constraint in bus_ids { while !satisfies_constraint(current_solution, constraint) { current_solution += step; } let (_, divisor) = constraint; step *= divisor; } Ok(current_solution) } fn satisfies_constraint(solution: u64, (remainder, divisor): (u64, u64)) -> bool { ((solution + remainder) % divisor) == 0 } #[cfg(test)] mod tests { use super::*; const PROVIDED: &str = include_str!("../input/day13_provided.txt"); #[test] fn part1_provided() { assert_eq!(part1(PROVIDED).unwrap(), 295); } #[test] fn part1_real() { assert_eq!(part1(INPUT).unwrap(), 3269); } #[test] fn part2_small_samples() { let tests = &[ ("17,x,13,19", 3417), ("67,7,59,61", 754018), ("67,x,7,59,61", 779210), ("67,7,x,59,61", 1261476), ("1789,37,47,1889", 1202161486), ]; for &(input, expected) in tests { assert_eq!(find_timestamp(input).unwrap(), expected); } } #[test] fn part2_provided() { assert_eq!(part2(PROVIDED).unwrap(), 1068781); } #[test] fn part2_real() { assert_eq!(part2(INPUT).unwrap(), 672754131923874); } }