solution: day 10

data/examples/10.txt

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+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732

src/bin/10.rs

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+use hashbrown::HashMap;
+use itertools::Itertools;
+
+fn score_trailhead(
+    acc: &mut HashMap<(usize, usize), usize>,
+    grid: &[Vec<usize>],
+    start: (usize, usize),
+) {
+    let (i, j) = start;
+    let n = grid[i][j];
+
+    if n == 9 {
+        *acc.entry((i, j)).or_default() += 1;
+    }
+
+    let dirs = [(-1, 0), (0, -1), (1, 0), (0, 1)];
+
+    for (di, dj) in dirs {
+        let (ni, nj) = (i.wrapping_add(di as usize), j.wrapping_add(dj as usize));
+        if let Some(nn) = grid.get(ni).and_then(|row| row.get(nj)) {
+            if *nn == n + 1 {
+                score_trailhead(acc, grid, (ni, nj));
+            }
+        }
+    }
+}
+
+fn parse_grid(input: &str) -> Vec<Vec<usize>> {
+    input
+        .lines()
+        .map(|x| {
+            x.chars()
+                .map(|y| y.to_digit(10).unwrap_or(11) as usize)
+                .collect_vec()
+        })
+        .collect_vec()
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    let grid = parse_grid(input);
+
+    let mut score = 0;
+    let mut acc = HashMap::new();
+
+    for (i, row) in grid.iter().enumerate() {
+        for (j, _) in row.iter().copied().enumerate().filter(|x| x.1 == 0) {
+            acc.clear();
+            score_trailhead(&mut acc, &grid, (i, j));
+            score += acc.len();
+        }
+    }
+
+    score.into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let grid = parse_grid(input);
+
+    let mut acc = HashMap::new();
+
+    for (i, row) in grid.iter().enumerate() {
+        for (j, _) in row.iter().copied().enumerate().filter(|x| x.1 == 0) {
+            score_trailhead(&mut acc, &grid, (i, j));
+        }
+    }
+
+    acc.values().sum::<usize>().into()
+}
+
+aoc::solution!(10);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(
+            part_one(&aoc::template::read_file("examples", 10)),
+            Some(36)
+        );
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(
+            part_two(&aoc::template::read_file("examples", 10)),
+            Some(81)
+        );
+    }
+}