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+use std::{ + cmp::Reverse, + collections::BinaryHeap, +}; +use ahash::AHashSet; use glam::IVec2; enum Direction { @@ -24,42 +28,6 @@ impl Direction { } } -#[derive(Debug, Clone, Eq, PartialEq, Hash)] -enum Tile { - Corrupted, - Safe, -} -use Tile::*; - -fn parse(input: &str) -> Vec { - input - .lines() - .map(|l| { - let (y, x) = l.split_once(',').unwrap(); - IVec2 { - x: x.parse().unwrap(), - y: y.parse().unwrap(), - } - }) - .collect() -} - -fn build_grid(dimensions: &IVec2, bytes_amount: usize, positions: &[IVec2]) -> Vec> { - let mut grid = vec![]; - for px in 0..=dimensions.x { - let mut line = vec![]; - for py in 0..=dimensions.y { - if positions[..bytes_amount].contains(&IVec2::new(px, py)) { - line.push(Corrupted); - } else { - line.push(Safe); - } - } - grid.push(line); - } - grid -} - #[derive(Debug, Clone, Eq, PartialEq, Hash)] struct Node { score: u32, @@ -78,67 +46,102 @@ impl PartialOrd for Node { } } -fn find_path(grid: &[Vec], start_pos: &IVec2, end_pos: &IVec2) -> u32 { - let mut distances = vec![vec![u32::MAX; grid[0].len()]; grid.len()]; +type Distance = (u32, Vec); + +fn parse(input: &str) -> Vec { + input + .lines() + .map(|l| { + let (y, x) = l.split_once(',').unwrap(); + IVec2 { + x: x.parse().unwrap(), + y: y.parse().unwrap(), + } + }) + .collect() +} + +fn get_score<'a>(scores: &'a mut [Vec], pos: &IVec2) -> &'a mut Distance { + &mut scores[pos.x as usize][pos.y as usize] +} + +fn find_path(corrupt_pos: &AHashSet, dimensions: &IVec2) -> (Vec, u32) { + let mut scores = + vec![vec![(u32::MAX, vec![]); dimensions.y as usize + 1]; dimensions.x as usize + 1]; let mut to_visit = BinaryHeap::new(); - distances[start_pos.x as usize][start_pos.y as usize] = 0; + let start_pos = IVec2::new(0, 0); + + get_score(&mut scores, &start_pos).0 = 0; to_visit.push(Reverse(Node { score: 0, - position: *start_pos, + position: start_pos, })); while let Some(Reverse(curr)) = to_visit.pop() { - if curr.position == *end_pos { - return curr.score; + if curr.position == *dimensions { + return (get_score(&mut scores, &curr.position).1.clone(), curr.score); } - if curr.score > distances[curr.position.x as usize][curr.position.y as usize] { + if curr.score > get_score(&mut scores, &curr.position).0 { continue; } for direction in Direction::all() { let new_pos = curr.position + direction.to_vec(); - let Some(Some(t)) = grid - .get(new_pos.x as usize) - .map(|l| l.get(new_pos.y as usize)) - else { + if new_pos.cmplt(start_pos).any() + || new_pos.cmpgt(*dimensions).any() + || corrupt_pos.contains(&new_pos) + { continue; - }; - if *t != Corrupted { - let new_score = curr.score + 1; - if new_score < distances[new_pos.x as usize][new_pos.y as usize] { - distances[new_pos.x as usize][new_pos.y as usize] = new_score; - to_visit.push(Reverse(Node { - score: new_score, - position: new_pos, - })); - } + } + + let new_score = curr.score + 1; + if new_score < get_score(&mut scores, &new_pos).0 { + get_score(&mut scores, &new_pos).0 = new_score; + + let mut prev_positions = get_score(&mut scores, &curr.position).1.clone(); + prev_positions.push(curr.position); + get_score(&mut scores, &new_pos).1.extend(prev_positions); + + to_visit.push(Reverse(Node { + score: new_score, + position: new_pos, + })); } } } - u32::MAX + (vec![], u32::MAX) } fn part1(positions: &[IVec2], dimensions: &IVec2) -> u32 { - let grid = build_grid(dimensions, 1024, positions); - find_path(&grid, &IVec2::new(0, 0), dimensions) + find_path( + &AHashSet::from_iter(positions[..1024].iter().map(|i| i.to_owned())), + dimensions, + ) + .1 } fn part2(positions: &[IVec2], dimensions: &IVec2) -> String { - let start_pos = IVec2::new(0, 0); - // todo: use binary search - for i in 1024.. { - let grid = build_grid(dimensions, i, positions); - if find_path(&grid, &start_pos, dimensions) == u32::MAX { + let mut prev_path = AHashSet::new(); + for i in 1024..positions.len() { + if !prev_path.is_empty() && !prev_path.contains(&positions[i - 1]) { + continue; + } + let (path, len) = find_path( + &AHashSet::from_iter(positions[..i].iter().map(|i| i.to_owned())), + dimensions, + ); + prev_path = AHashSet::from_iter(path); + if len == u32::MAX { return positions .get(i - 1) .map(|p| format!("{},{}", p.y, p.x)) .unwrap(); } } - unreachable!() + panic!("not found") } fn main() {