diff --git a/2023/src/day21.rs b/2023/src/day21.rs
new file mode 100644
index 0000000..c2b07c4
--- /dev/null
+++ b/2023/src/day21.rs
@@ -0,0 +1,128 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+use std::collections::{HashMap, VecDeque};
+
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+struct Point(usize, usize);
+
+type Map = [[char; 131]; 131];
+
+#[aoc_generator(day21)]
+fn parse(input: &str) -> (Point, Map) {
+    let mut m = [['#'; 131]; 131];
+    let mut start: Point = Point(0, 0);
+    for (x, line) in input.lines().enumerate() {
+        if line.is_empty() {
+            continue;
+        }
+        for (y, t) in line.chars().enumerate() {
+            match t {
+                '.' => m[x][y] = '.',
+                '#' => {}
+                'S' => {
+                    start.0 = x;
+                    start.1 = y;
+                    m[x][y] = '.';
+                }
+                n @ _ => panic!("bad map char {}", n),
+            }
+        }
+    }
+    (start, m)
+}
+
+fn infinite_step(start: &Point, map: &Map) -> HashMap<Point, usize> {
+    let mut visited: HashMap<Point, usize> = HashMap::new();
+    let mut frontier: VecDeque<(Point, usize)> = VecDeque::new();
+    frontier.push_back((*start, 0));
+
+    while let Some((cur, dist)) = frontier.pop_front() {
+        if visited.contains_key(&cur) {
+            continue;
+        }
+
+        visited.insert(cur, dist);
+
+        let x = cur.0;
+        let y = cur.1;
+        if x > 0 && map[x - 1][y] == '.' {
+            let p = Point(x - 1, y);
+            if !visited.contains_key(&p) {
+                frontier.push_back((p, dist + 1));
+            }
+        }
+        if x < map.len() - 1 && map[x + 1][y as usize] == '.' {
+            let p = Point(x + 1, y);
+            if !visited.contains_key(&p) {
+                frontier.push_back((p, dist + 1));
+            }
+        }
+        if y > 0 && map[x][y - 1] == '.' {
+            let p = Point(x, y - 1);
+            if !visited.contains_key(&p) {
+                frontier.push_back((p, dist + 1));
+            }
+        }
+        if y < map[x].len() - 1 && map[x][y + 1] == '.' {
+            let p = Point(x, y + 1);
+            if !visited.contains_key(&p) {
+                frontier.push_back((p, dist + 1));
+            }
+        }
+    }
+
+    visited
+}
+
+#[aoc(day21, part1)]
+fn part1(input: &(Point, Map)) -> usize {
+    let visited = infinite_step(&input.0, &input.1);
+    visited
+        .values()
+        .filter(|t| **t <= 64 && **t % 2 == 0)
+        .count()
+}
+
+#[aoc(day21, part2)]
+fn part2(input: &(Point, Map)) -> usize {
+    let visited = infinite_step(&input.0, &input.1);
+
+    let even_corners = visited
+        .values()
+        .filter(|v| **v % 2 == 0 && **v > 65)
+        .count();
+    let odd_corners = visited
+        .values()
+        .filter(|v| **v % 2 == 1 && **v > 65)
+        .count();
+
+    let even_full = visited.values().filter(|v| **v % 2 == 0).count();
+    let odd_full = visited.values().filter(|v| **v % 2 == 1).count();
+
+    let n = ((26501365 - (input.1.len() / 2)) / input.1.len()) as usize;
+    assert_eq!(n, 202300);
+
+    ((n + 1) * (n + 1)) * odd_full + (n * n) * even_full - (n + 1) * odd_corners + n * even_corners
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE_DATA: &'static str = r#"...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+..........."#;
+
+    #[test]
+    fn sample_data() {
+        let input = parse(&SAMPLE_DATA);
+        assert_eq!(part1(&input), 42);
+    }
+}
diff --git a/2023/src/lib.rs b/2023/src/lib.rs
index 1add4d4..492c1a0 100644
--- a/2023/src/lib.rs
+++ b/2023/src/lib.rs
@@ -20,5 +20,6 @@ mod day17;
 mod day18;
 mod day19;
 mod day20;
+mod day21;
 
 aoc_lib! { year = 2023 }