104 lines
2.8 KiB
Rust
104 lines
2.8 KiB
Rust
#![feature(test)]
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extern crate test;
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use std::hint::unreachable_unchecked;
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use aoc2023::{boilerplate, common::*};
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const DAY: usize = 8;
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type Parsed<'a> = (Vec<Direction>, Vec<u16>, [[u16; 2]; LUT_SIZE]);
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#[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)]
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enum Direction {
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Left = 0,
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Right = 1,
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}
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const LUT_SIZE: usize = 27483; // pack("ZZZ") + 1
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fn pack(s: &[u8; 3]) -> u16 {
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s.iter().cloned().fold(0, |acc, n| (acc << 5) + (n as u16 & 0b11111))
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}
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fn parse_input(raw: &str) -> Parsed {
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let (directions, map) = raw.split_once("\n\n").unwrap();
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let directions = directions
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.bytes()
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.map(|i| match i {
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b'L' => Direction::Left,
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b'R' => Direction::Right,
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_ => unsafe { unreachable_unchecked() },
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})
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.collect();
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let mut lut = [[0u16, 0]; LUT_SIZE];
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let mut indices = Vec::new();
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for x in map.lines().map(|l| l.as_bytes()) {
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unsafe {
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let idx = pack(&*x[0..=2].as_ptr().cast());
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let left = pack(&*x[7..=9].as_ptr().cast());
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let right = pack(&*x[12..=14].as_ptr().cast());
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lut[idx as usize] = [left, right];
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if x[2] == b'A' {
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indices.push(idx);
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}
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}
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}
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(directions, indices, lut)
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}
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#[inline]
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fn ends_with(packed: u16, suffix: u8) -> bool {
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packed & 0b11111 == suffix as u16 & 0b11111
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}
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fn steps_until((directions, _, map): &Parsed, start: u16) -> usize {
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directions
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.iter()
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.cycle()
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.scan(start, |pos, dir| {
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let next = map[*pos as usize][*dir as usize];
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(!ends_with(next, b'Z')).then(|| *pos = next)
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})
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.count()
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+ 1
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}
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fn part1(parsed: &Parsed) -> usize {
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steps_until(parsed, pack(&[b'A'; 3]))
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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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parsed.1.iter().filter(|&&n| ends_with(n, b'A')).fold(1, |acc, n| lcm(acc, steps_until(parsed, *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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AAA = (BBB, BBB)
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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_P2 => 6 },
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},
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bench1 == 12083,
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bench2 == 13385272668829,
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bench_parse: |(v, v2, m): &Parsed| { assert_eq!(m[0], [0, 0]); (v.len(), v2.len(), v[0]) } => (281, 6, Direction::Left),
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}
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