126 lines
3.4 KiB
Rust
126 lines
3.4 KiB
Rust
advent_of_code::solution!(21);
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use std::collections::{HashMap, VecDeque};
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#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
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struct Point(usize, usize);
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type Map = [[char; 131]; 131];
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fn parse(input: &str) -> (Point, Map) {
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let mut m = [['#'; 131]; 131];
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let mut start: Point = Point(0, 0);
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for (x, line) in input.lines().enumerate() {
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if line.is_empty() {
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continue;
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}
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for (y, t) in line.chars().enumerate() {
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match t {
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'.' => m[x][y] = '.',
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'#' => {}
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'S' => {
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start.0 = x;
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start.1 = y;
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m[x][y] = '.';
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}
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n @ _ => panic!("bad map char {}", n),
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}
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}
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}
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(start, m)
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}
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fn infinite_step(start: &Point, map: &Map) -> HashMap<Point, usize> {
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let mut visited: HashMap<Point, usize> = HashMap::new();
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let mut frontier: VecDeque<(Point, usize)> = VecDeque::new();
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frontier.push_back((*start, 0));
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while let Some((cur, dist)) = frontier.pop_front() {
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if visited.contains_key(&cur) {
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continue;
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}
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visited.insert(cur, dist);
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let x = cur.0;
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let y = cur.1;
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if x > 0 && map[x - 1][y] == '.' {
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let p = Point(x - 1, y);
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if !visited.contains_key(&p) {
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frontier.push_back((p, dist + 1));
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}
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}
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if x < map.len() - 1 && map[x + 1][y as usize] == '.' {
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let p = Point(x + 1, y);
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if !visited.contains_key(&p) {
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frontier.push_back((p, dist + 1));
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}
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}
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if y > 0 && map[x][y - 1] == '.' {
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let p = Point(x, y - 1);
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if !visited.contains_key(&p) {
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frontier.push_back((p, dist + 1));
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}
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}
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if y < map[x].len() - 1 && map[x][y + 1] == '.' {
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let p = Point(x, y + 1);
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if !visited.contains_key(&p) {
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frontier.push_back((p, dist + 1));
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}
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}
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}
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visited
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}
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pub fn part_one(input_str: &str) -> Option<usize> {
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let input = parse(input_str);
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let visited = infinite_step(&input.0, &input.1);
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let r = visited
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.values()
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.filter(|t| **t <= 64 && **t % 2 == 0)
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.count();
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Some(r)
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}
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pub fn part_two(input_str: &str) -> Option<usize> {
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let input = parse(input_str);
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let visited = infinite_step(&input.0, &input.1);
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let even_corners = visited
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.values()
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.filter(|v| **v % 2 == 0 && **v > 65)
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.count();
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let odd_corners = visited
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.values()
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.filter(|v| **v % 2 == 1 && **v > 65)
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.count();
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let even_full = visited.values().filter(|v| **v % 2 == 0).count();
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let odd_full = visited.values().filter(|v| **v % 2 == 1).count();
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let n = ((26501365 - (input.1.len() / 2)) / input.1.len()) as usize;
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assert_eq!(n, 202300);
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let r = ((n + 1) * (n + 1)) * odd_full + (n * n) * even_full - (n + 1) * odd_corners
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+ n * even_corners;
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Some(r)
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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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#[test]
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fn test_part_one() {
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let result = part_one(&advent_of_code::template::read_file("examples", DAY));
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assert_eq!(result, Some(42));
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}
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#[test]
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fn test_part_two() {
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let result = part_two(&advent_of_code::template::read_file("examples", DAY));
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assert_eq!(result, Some(3314964269439));
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}
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}
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