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| 1 | +#![feature(test)] |
| 2 | + |
| 3 | +use std::collections::VecDeque; |
| 4 | + |
| 5 | +use aoc::grid::Direction; |
| 6 | + |
| 7 | +#[derive(Debug)] |
| 8 | +struct Input { |
| 9 | + end: (usize, usize), |
| 10 | + grid: Vec<Vec<bool>>, |
| 11 | +} |
| 12 | + |
| 13 | +fn setup(input: &str) -> Input { |
| 14 | + let mut end = None; |
| 15 | + let grid = input |
| 16 | + .trim() |
| 17 | + .lines() |
| 18 | + .enumerate() |
| 19 | + .map(|(y, l)| { |
| 20 | + l.bytes() |
| 21 | + .enumerate() |
| 22 | + .map(|(x, b)| match b { |
| 23 | + b'.' => true, |
| 24 | + b'#' => false, |
| 25 | + b'S' => true, |
| 26 | + b'E' => { |
| 27 | + end = Some((x, y)); |
| 28 | + true |
| 29 | + } |
| 30 | + _ => panic!(), |
| 31 | + }) |
| 32 | + .collect() |
| 33 | + }) |
| 34 | + .collect(); |
| 35 | + |
| 36 | + Input { |
| 37 | + end: end.unwrap(), |
| 38 | + grid, |
| 39 | + } |
| 40 | +} |
| 41 | + |
| 42 | +fn solve<const N: usize>(input: &Input) -> usize { |
| 43 | + let n = N as isize; |
| 44 | + let width = input.grid[0].len(); |
| 45 | + let height = input.grid.len(); |
| 46 | + |
| 47 | + let mut shortest = vec![usize::MAX; width * height]; |
| 48 | + let mut queue = VecDeque::from([(0, input.end)]); |
| 49 | + let mut visited = vec![false; width * height]; |
| 50 | + while let Some((d, (x, y))) = queue.pop_front() { |
| 51 | + let idx = y * width + x; |
| 52 | + if std::mem::replace(&mut visited[idx], true) { |
| 53 | + continue; |
| 54 | + } |
| 55 | + |
| 56 | + shortest[idx] = d; |
| 57 | + |
| 58 | + queue.extend( |
| 59 | + Direction::iter() |
| 60 | + .flat_map(|d| d.step(x, y, width, height)) |
| 61 | + .filter(|&(nx, ny)| input.grid[ny][nx]) |
| 62 | + .map(|q| (d + 1, q)), |
| 63 | + ); |
| 64 | + } |
| 65 | + |
| 66 | + let mut out = 0; |
| 67 | + for csy in 0..height { |
| 68 | + for csx in 0..width { |
| 69 | + if !input.grid[csy][csx] { |
| 70 | + continue; |
| 71 | + } |
| 72 | + |
| 73 | + for dy in -n..=n { |
| 74 | + let n = n - dy.abs(); |
| 75 | + for dx in -n..=n { |
| 76 | + let cex = csx as isize + dx; |
| 77 | + let cey = csy as isize + dy; |
| 78 | + if !(0..width as isize).contains(&cex) || !(0..height as isize).contains(&cey) { |
| 79 | + continue; |
| 80 | + } |
| 81 | + |
| 82 | + let cex = cex as usize; |
| 83 | + let cey = cey as usize; |
| 84 | + if !input.grid[cey][cex] { |
| 85 | + continue; |
| 86 | + } |
| 87 | + |
| 88 | + let d = (dx.abs() + dy.abs()) as usize; |
| 89 | + if shortest[cey * width + cex] + d + 100 <= shortest[csy * width + csx] { |
| 90 | + out += 1; |
| 91 | + } |
| 92 | + } |
| 93 | + } |
| 94 | + } |
| 95 | + } |
| 96 | + |
| 97 | + out |
| 98 | +} |
| 99 | + |
| 100 | +fn part1(input: &Input) -> usize { |
| 101 | + solve::<2>(input) |
| 102 | +} |
| 103 | + |
| 104 | +fn part2(input: &Input) -> usize { |
| 105 | + solve::<20>(input) |
| 106 | +} |
| 107 | + |
| 108 | +aoc::main!(2024, 20); |
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