Day 20 down

Author: BakerNet

README.md

@@ -10,7 +10,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 
 | Year | Completed |
 | :---: | :---: |
-| [2024](aoc2024) | 38/50 |
+| [2024](aoc2024) | 40/50 |
 | [2023](aoc2023) | 50/50 |
 
 ---

aoc2024/README.md

@@ -25,7 +25,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 17](https://adventofcode.com/2024/day/17) | [code](src/bin/17.rs) | ⭐ | ⭐ |
 | [Day 18](https://adventofcode.com/2024/day/18) | [code](src/bin/18.rs) | ⭐ | ⭐ |
 | [Day 19](https://adventofcode.com/2024/day/19) | [code](src/bin/19.rs) | ⭐ | ⭐ |
-| [Day 20](https://adventofcode.com/2024/day/20) | [code](src/bin/20.rs) | _ | _ |
+| [Day 20](https://adventofcode.com/2024/day/20) | [code](src/bin/20.rs) | ⭐ | ⭐ |
 | [Day 21](https://adventofcode.com/2024/day/21) | [code](src/bin/21.rs) | _ | _ |
 | [Day 22](https://adventofcode.com/2024/day/22) | [code](src/bin/22.rs) | _ | _ |
 | [Day 23](https://adventofcode.com/2024/day/23) | [code](src/bin/23.rs) | _ | _ |

aoc2024/data/examples/20.txt

@@ -0,0 +1,15 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############

aoc2024/src/bin/20.rs

@@ -0,0 +1,86 @@
+use std::collections::VecDeque;
+
+use aoc_utils::*;
+
+advent_of_code::solution!(20);
+
+#[cfg(not(test))]
+const SAVE_CUTOFF: usize = 100;
+#[cfg(test)]
+const SAVE_CUTOFF: usize = 50;
+
+fn find_path(start: Point, end: Point, map: &[Vec<char>]) -> Vec<Point> {
+    let bounds = Bounds(map.len() - 1, map[0].len() - 1);
+    let seen = &mut vec![vec![false; bounds.1 + 1]; bounds.0 + 1];
+    seen[start.0][start.1] = true;
+    let mut queue = VecDeque::new();
+    let mut path = Vec::new();
+    queue.push_back(start);
+    while !queue.is_empty() {
+        let curr = queue.pop_front().unwrap();
+        path.push(curr);
+        if curr == end {
+            break;
+        }
+        Dir::neighbors(curr, bounds).into_iter().for_each(|p| {
+            if !seen[p.0][p.1] {
+                seen[p.0][p.1] = true;
+                if map[p.0][p.1] != '#' {
+                    queue.push_back(p);
+                }
+            }
+        });
+    }
+    path
+}
+
+fn find_cheats(path: &[Point], max_dist: usize) -> u64 {
+    path.iter()
+        .enumerate()
+        .take(path.len() - SAVE_CUTOFF - 1)
+        .fold(0, |mut acc, (i, p)| {
+            path.iter()
+                .enumerate()
+                .skip(i + SAVE_CUTOFF + 2)
+                .for_each(|(j, p2)| {
+                    let d = dist(*p, *p2);
+                    if j - i - d >= SAVE_CUTOFF && d <= max_dist {
+                        acc += 1;
+                    }
+                });
+            acc
+        })
+}
+
+pub fn part_one(input: &str) -> Option<u64> {
+    let map = input.c_map();
+    let start = find_point(&map, 'S');
+    let end = find_point(&map, 'E');
+    let path = find_path(start, end, &map);
+    Some(find_cheats(&path, 2))
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    let map = input.c_map();
+    let start = find_point(&map, 'S');
+    let end = find_point(&map, 'E');
+    let path = find_path(start, end, &map);
+    Some(find_cheats(&path, 20))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(1));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(285));
+    }
+}

aoc_utils/src/lib.rs

@@ -39,6 +39,12 @@ pub enum Dir {
     Right,
 }
 
+pub fn dist(p: Point, p2: Point) -> usize {
+    let x = if p.0 > p2.0 { p.0 - p2.0 } else { p2.0 - p.0 };
+    let y = if p.1 > p2.1 { p.1 - p2.1 } else { p2.1 - p.1 };
+    x + y
+}
+
 impl Dir {
     pub fn neighbors(p: Point, b: Bounds) -> ArrayVec<[Point; 4]> {
         let mut ns = array_vec!([Point; 4]);