Day 10

Author: UnderscoreTud

README.md

@@ -34,8 +34,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 7](./src/bin/07.rs) | `80.3µs` | `113.7µs` |
 | [Day 8](./src/bin/08.rs) | `10.2µs` | `12.4µs` |
 | [Day 9](./src/bin/09.rs) | `2.9ms` | `49.1ms` |
+| [Day 10](./src/bin/10.rs) | `95.6µs` | `96.8µs` |
 
-**Total: 175.92ms**
+**Total: 176.11ms**
 <!--- benchmarking table --->
 
 ---

data/examples/10.txt

@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732

src/bin/10.rs

@@ -0,0 +1,128 @@
+use std::collections::HashSet;
+use itertools::Itertools;
+
+advent_of_code::solution!(10);
+
+const DIRECTIONS: [(isize, isize); 4] = [
+    (0, -1),
+    (1, 0),
+    (0, 1),
+    (-1, 0),
+];
+
+#[derive(Debug)]
+struct Map {
+    width: usize,
+    height: usize,
+    heightmap: Vec<u8>,
+    trailheads: Vec<usize>,
+}
+
+impl Map {
+    fn parse(input: &str) -> Map {
+        let width = input.lines().next().unwrap_or_default().len();
+        let mut trailheads = Vec::new();
+        let heightmap = input.lines()
+            .flat_map(|line| line.chars())
+            .map(|char| char.to_digit(10).unwrap() as u8)
+            .enumerate()
+            .map(|(index, height)| {
+                if height == 0 { trailheads.push(index) }
+                height
+            })
+            .collect_vec();
+        let height = heightmap.len() / width;
+        Map {
+            height,
+            width,
+            heightmap,
+            trailheads,
+        }
+    }
+
+    fn xy_to_index(&self, pos: (usize, usize)) -> Option<usize> {
+        if (0..self.width).contains(&pos.0) && (0..self.height).contains(&pos.1) {
+            Some(pos.1 * self.width + pos.0)
+        } else {
+            None
+        }
+    }
+
+    fn index_to_xy(&self, index: &usize) -> Option<(usize, usize)> {
+        if index < &(self.width * self.height) {
+            Some((index % self.width, index / self.width))
+        } else {
+            None
+        }
+    }
+
+    fn score(&self, pos: (usize, usize), current: u8, visited: &mut HashSet<(usize, usize)>) {
+        if self.xy_to_index(pos).map(|index| self.heightmap[index]).is_none_or(|height| current != height) {
+            return;
+        }
+        if current == 9 {
+            visited.insert(pos);
+            return;
+        }
+        for direction in DIRECTIONS {
+            let Some(pos) = offset(&pos, &direction) else { continue };
+            self.score(pos, current + 1, visited);
+        }
+    }
+
+    fn rating(&self, pos: (usize, usize), current: u8) -> u32 {
+        if self.xy_to_index(pos).map(|index| self.heightmap[index]).is_none_or(|height| current != height) {
+            return 0;
+        }
+        if current == 9 {
+            return 1;
+        }
+        let mut sum = 0;
+        for direction in DIRECTIONS {
+            let Some(pos) = offset(&pos, &direction) else { continue };
+            sum += self.rating(pos, current + 1);
+        }
+        sum
+    }
+}
+
+fn offset(pos: &(usize, usize), direction: &(isize, isize)) -> Option<(usize, usize)> {
+    let x = pos.0.checked_add_signed(direction.0)?;
+    let y = pos.1.checked_add_signed(direction.1)?;
+    Some((x, y))
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let map = Map::parse(input);
+    Some(map.trailheads.iter()
+        .map(|trailhead| {
+            let mut visited = HashSet::new();
+            map.score(map.index_to_xy(trailhead).unwrap(), 0, &mut visited);
+            visited.len() as u32
+        })
+        .sum())
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let map = Map::parse(input);
+    Some(map.trailheads.iter()
+        .map(|trailhead| map.rating(map.index_to_xy(trailhead).unwrap(), 0))
+        .sum())
+}
+
+#[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(36));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(81));
+    }
+}