20th day

Author: majcn

data/examples/20.txt

@@ -0,0 +1,17 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############
+
+EXAMPLE: LIMIT=50

src/bin/20.rs

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+advent_of_code::solution!(20);
+
+use advent_of_code::maneatingape::grid::*;
+use advent_of_code::maneatingape::parse::ParseOps;
+use advent_of_code::maneatingape::point::*;
+
+fn parse_data(input: &str) -> (Grid<u8>, usize) {
+    const DEFAULT_LIMIT: usize = 100;
+
+    let default_right = format!("{DEFAULT_LIMIT}",);
+    let (left, right) = input.split_once("\n\n").unwrap_or((input, &default_right));
+
+    let grid = Grid::parse(left);
+    let limit = right.unsigned();
+
+    (grid, limit)
+}
+
+fn find_path(grid: Grid<u8>, start_position: Point) -> Vec<Point> {
+    let mut result = vec![];
+    let mut next_position = Some(start_position);
+
+    while let Some(position) = next_position {
+        result.push(position);
+
+        next_position = ORTHOGONAL.into_iter().map(|d| position + d).find(|&p| {
+            (result.len() < 2 || result[result.len() - 2] != p)
+                && grid.contains(p)
+                && grid[p] != b'#'
+        });
+    }
+
+    result
+}
+
+fn part_x<const N: usize>(grid: Grid<u8>, limit: usize) -> u32 {
+    let start_location = grid.find(b'S').unwrap();
+
+    let path = find_path(grid, start_location);
+    let normal_cost = path.len();
+
+    let mut result = 0;
+
+    for (i1, &p1) in path.iter().enumerate() {
+        for (i2, &p2) in path.iter().skip(i1 + limit).rev().enumerate() {
+            let point_distance = p1.manhattan(p2) as usize;
+            if point_distance <= N {
+                let cheated_cost = i1 + point_distance + i2;
+                if normal_cost - cheated_cost >= limit {
+                    result += 1;
+                }
+            }
+        }
+    }
+
+    result
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let (grid, limit) = parse_data(input);
+
+    let result = part_x::<2>(grid, limit);
+
+    Some(result)
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let (grid, limit) = parse_data(input);
+
+    let result = part_x::<20>(grid, limit);
+
+    Some(result)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let input = advent_of_code::template::read_file("examples", DAY);
+        let result = part_one(&input);
+        assert_eq!(result, Some(1));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let input = advent_of_code::template::read_file("examples", DAY);
+        let result = part_two(&input);
+        assert_eq!(result, Some(285));
+    }
+}