2020: day13: part 2
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@ -3,11 +3,13 @@ use std::fmt::Write;
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use aoc::err;
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const INPUT: &str = include_str!("../input/day13.txt");
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const PROVIDED: &str = include_str!("../input/day13_provided.txt");
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pub fn run() -> aoc::Result<String> {
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let mut res = String::with_capacity(128);
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writeln!(res, "part 1: {}", part1(INPUT)?)?;
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writeln!(res, "part 2: {}", part2(INPUT)?)?;
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Ok(res)
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}
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@ -46,12 +48,55 @@ fn part1(input: &str) -> aoc::Result<u64> {
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Ok(bus_id * (earliest_departure - earliest_timestamp))
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}
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fn part2(input: &str) -> aoc::Result<u64> {
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let mut lines = input.lines();
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// we don't need the first line anymore, skip it
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lines.next().ok_or_else(|| err!("input was empty"))?;
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let bus_ids: Vec<(u64, u64)> = lines
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.next()
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.ok_or_else(|| err!("no second line"))?
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.split(',')
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.enumerate()
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.filter_map(|(idx, num)| {
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if num == "x" {
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None
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} else {
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Some((idx as u64, num.parse::<u64>().map_err(|e| err!("{}", e))))
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}
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})
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.map(|(idx, res)| match res {
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Ok(num) => Ok((idx, num)),
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Err(e) => Err(e),
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})
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.collect::<aoc::Result<_>>()?;
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// previous constraints is empty for now
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let mut current_solution = 0;
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let mut step = 1;
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for constraint in bus_ids {
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while !satisfies_constraint(current_solution, constraint) {
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current_solution += step;
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}
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let (_, divisor) = constraint;
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step *= divisor;
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}
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Ok(current_solution)
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}
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fn satisfies_constraint(solution: u64, (remainder, divisor): (u64, u64)) -> bool {
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((solution + remainder) % divisor) == 0
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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const PROVIDED: &str = include_str!("../input/day13_provided.txt");
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#[test]
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fn part1_provided() {
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assert_eq!(part1(PROVIDED).unwrap(), 295);
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@ -61,4 +106,14 @@ mod tests {
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fn part1_real() {
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assert_eq!(part1(INPUT).unwrap(), 3269);
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}
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#[test]
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fn part2_provided() {
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assert_eq!(part2(PROVIDED).unwrap(), 1068781);
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}
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#[test]
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fn part2_real() {
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assert_eq!(part2(INPUT).unwrap(), 672754131923874);
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}
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}
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