add 2023/08/2
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13daf93f49
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@ -7,7 +7,7 @@ use aoc2023::{boilerplate, common::*};
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const DAY: usize = 08;
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type Parsed<'a> = (Vec<Direction>, HashMap<&'a str, (&'a str, &'a str)>);
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#[derive(Debug, PartialEq, Copy, Clone)]
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#[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)]
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enum Direction {
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Left,
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Right,
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@ -27,30 +27,48 @@ fn parse_input(raw: &str) -> Parsed {
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(directions, map)
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}
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fn part1((directions, map): &Parsed) -> usize {
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fn part1(parsed: &Parsed) -> usize {
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steps_until(parsed, "AAA", "ZZZ")
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}
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fn steps_until((directions, map): &Parsed, start: &str, target: &str) -> usize {
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directions
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.iter()
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.cycle()
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.scan("AAA", |pos, dir| {
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.scan(start, |pos, dir| {
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let next = match dir {
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Direction::Left => map.get(pos)?.0,
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Direction::Right => map.get(pos)?.1,
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};
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if next == "ZZZ" {
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None
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} else {
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(!next.ends_with(target)).then(|| {
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*pos = next;
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Some(next)
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}
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})
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})
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.count()
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+ 1
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}
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// I’m honestly not sure why this works. It seems each path only has a single ghost node, and that
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// node occurs right before looping, so we can just compute the least common multiple of their step counts.
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// I assume this holds true for other inputs (it can’t have been random),
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// but I don’t see it anywhere in the task and only found out by experimentation.
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fn part2(parsed: &Parsed) -> usize {
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unimplemented!()
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parsed.1.keys().filter(|start| start.ends_with("A")).map(|start| steps_until(parsed, start, "Z")).fold(1, |acc, n| lcm(acc, n))
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}
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#[cfg(test)]
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const TEST_INPUT_P2: &str = "LR
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11A = (11B, XXX)
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11B = (XXX, 11Z)
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11Z = (11B, XXX)
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22A = (22B, XXX)
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22B = (22C, 22C)
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22C = (22Z, 22Z)
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22Z = (22B, 22B)
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XXX = (XXX, XXX)";
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boilerplate! {
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TEST_INPUT == "LLR
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@ -59,9 +77,9 @@ BBB = (AAA, ZZZ)
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ZZZ = (ZZZ, ZZZ)",
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tests: {
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part1: { TEST_INPUT => 6 },
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part2: { TEST_INPUT => 0 },
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part2: { TEST_INPUT_P2 => 6 },
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},
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bench1 == 12083,
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bench2 == 0,
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bench2 == 13385272668829,
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bench_parse: |(v, m): &Parsed| { assert_eq!(m["AAA"], ("MJJ", "QBJ")); (v.len(), v[0]) } => (281, Direction::Left),
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}
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@ -68,3 +68,17 @@ pub fn parse_num<I: ParseableNumber<I>>(s: &str) -> I {
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let start = unsafe { digits.next().unwrap_unchecked() };
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digits.fold(start, |acc, n| acc * I::from(10) + n)
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}
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fn gcd(mut x: usize, mut y: usize) -> usize {
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let mut remainder;
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while y != 0 {
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remainder = x % y;
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x = y;
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y = remainder;
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}
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x
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}
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pub fn lcm(x: usize, y: usize) -> usize {
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x * y / gcd(x, y)
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}
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