Day 21... tapped out on part 2

README.md

@@ -28,7 +28,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 18](https://adventofcode.com/2023/day/18) | [code](src/bin/18.rs) | ⭐ | ⭐ |
 | [Day 19](https://adventofcode.com/2023/day/19) | [code](src/bin/19.rs) | ⭐ | ⭐ |
 | [Day 20](https://adventofcode.com/2023/day/20) | [code](src/bin/20.rs) | ⭐ | ⭐ |
-| [Day 21](https://adventofcode.com/2023/day/21) | [code](src/bin/21.rs) |  |  |
+| [Day 21](https://adventofcode.com/2023/day/21) | [code](src/bin/21.rs) | ⭐ | - |
 | [Day 22](https://adventofcode.com/2023/day/22) | [code](src/bin/22.rs) |  |  |
 | [Day 23](https://adventofcode.com/2023/day/23) | [code](src/bin/23.rs) |  |  |
 | [Day 24](https://adventofcode.com/2023/day/24) | [code](src/bin/24.rs) |  |  |

data/examples/21.txt

@@ -0,0 +1,11 @@
+...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........

src/bin/21.rs

@@ -0,0 +1,73 @@
+use std::collections::HashSet;
+
+advent_of_code::solution!(21);
+
+pub fn valid_neighbors(point: (usize, usize), map: &Vec<Vec<char>>) -> Vec<(usize, usize)> {
+    let mut ret = Vec::with_capacity(4);
+    if point.0 > 0 && map[point.0 - 1][point.1] != '#' {
+        ret.push((point.0 - 1, point.1));
+    }
+    if point.0 < map.len() - 1 && map[point.0 + 1][point.1] != '#' {
+        ret.push((point.0 + 1, point.1));
+    }
+    if point.1 > 0 && map[point.0][point.1 - 1] != '#' {
+        ret.push((point.0, point.1 - 1));
+    }
+    if point.1 < map[0].len() - 1 && map[point.0][point.1 + 1] != '#' {
+        ret.push((point.0, point.1 + 1));
+    }
+    ret
+}
+
+pub fn part_one(input: &str) -> Option<u64> {
+    let map = input
+        .lines()
+        .map(|line| line.chars().collect())
+        .collect::<Vec<Vec<_>>>();
+    let start = map
+        .iter()
+        .enumerate()
+        .find_map(|(row, v)| {
+            v.iter()
+                .enumerate()
+                .find_map(|(col, c)| if *c == 'S' { Some(col) } else { None })
+                .map(|col| (row, col))
+        })
+        .expect("There should be a starting tile.");
+    let final_plots = (0..64).fold(
+        vec![start].iter().copied().collect::<HashSet<_>>(),
+        |acc, _| {
+            let mut new_acc = HashSet::new();
+            acc.iter().for_each(|point| {
+                let mut neighbors = valid_neighbors(*point, &map);
+                neighbors.drain(0..).for_each(|p| {
+                    new_acc.insert(p);
+                });
+            });
+            new_acc
+        },
+    );
+    Some(final_plots.len() as u64)
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    // I actually have no idea how to do part 2
+    input.find(|_| false)
+}
+
+#[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(42));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, None);
+    }
+}