diff --git a/src/day20.rs b/src/day20.rs
index 20dd3df..95db47b 100644
--- a/src/day20.rs
+++ b/src/day20.rs
@@ -1,5 +1,4 @@
 use std::collections::{HashMap, VecDeque};
-
 use aoc_runner_derive::{aoc, aoc_generator};
 use num::Integer;
 
diff --git a/src/day21.rs b/src/day21.rs
new file mode 100644
index 0000000..0f9e5fb
--- /dev/null
+++ b/src/day21.rs
@@ -0,0 +1,186 @@
+use std::collections::HashSet;
+use aoc_runner_derive::{aoc, aoc_generator};
+
+const STEPS: i32 = 64;
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+struct Position(i32, i32);
+
+#[derive(Debug, PartialEq)]
+struct Garden {
+    rocks: HashSet<Position>,
+    start: Position,
+    extents: (Position, Position),
+}
+
+#[aoc_generator(day21, part1)]
+fn parse(input: &str) -> Garden {
+    let mut rocks: HashSet<Position> = HashSet::new();
+    let mut start: Position = Position(0, 0);
+    for (row, line) in input.lines().enumerate() {
+        for (col, ch) in line.chars().enumerate() {
+            match ch {
+                '#' => {
+                    rocks.insert(Position(row as i32, col as i32));
+                }
+                'S' => {
+                    start = Position(row as i32, col as i32);
+                }
+                _ => {}
+            };
+        }
+    }
+    let mut trans_rocks: HashSet<Position> = HashSet::new();
+    for rock in rocks.iter() {
+        let trans_rock = Position(rock.0 - start.0, start.1 - rock.1);
+        trans_rocks.insert(trans_rock);
+    }
+    rocks = trans_rocks;
+    let num_rows = input.lines().count() as i32;
+    let num_cols = input.lines().next().unwrap().chars().count() as i32;
+    let extents = (
+        Position(-num_rows / 2, -num_cols / 2),
+        Position(num_rows / 2, num_cols / 2),
+    );
+    Garden {
+        rocks,
+        start: Position(0, 0),
+        extents,
+    }
+}
+
+#[aoc(day21, part1)]
+fn part1(map: &Garden) -> u32 {
+    let mut visited: HashSet<Position> = HashSet::new();
+    visited.insert(map.start);
+    for _ in 0..STEPS {
+        let mut new_visited = HashSet::new();
+        for pos in visited.iter().clone() {
+            let neighbors = [
+                Position(pos.0 - 1, pos.1),
+                Position(pos.0 + 1, pos.1),
+                Position(pos.0, pos.1 - 1),
+                Position(pos.0, pos.1 + 1),
+            ]
+            .into_iter()
+            .filter(|p| in_bounds(*p, map.extents) && !map.rocks.contains(p));
+            for neighbor in neighbors {
+                new_visited.insert(neighbor);
+            }
+        }
+        visited = new_visited;
+    }
+    visited.len() as u32
+}
+
+fn in_bounds(pos: Position, extents: (Position, Position)) -> bool {
+    pos.0 >= extents.0 .0 && pos.0 <= extents.1 .0 && pos.1 >= extents.0 .1 && pos.1 < extents.1 .1
+}
+
+fn in_bounds_part2(pos: Position, extents: Position) -> bool {
+    pos.0 >= 0 && pos.0 <= extents.0 && pos.1 >= 0 && pos.1 < extents.1
+}
+
+struct GardenPart2 {
+    rocks: HashSet<Position>,
+    start: Position,
+    extents: Position,
+}
+
+#[aoc_generator(day21, part2)]
+fn parse_part2(input: &str) -> GardenPart2 {
+    let mut rocks: HashSet<Position> = HashSet::new();
+    for (row, line) in input.lines().enumerate() {
+        for (col, ch) in line.chars().enumerate() {
+            match ch {
+                '#' => {
+                    rocks.insert(Position(row as i32, col as i32));
+                }
+                _ => {}
+            };
+        }
+    }
+    let num_rows = input.lines().count() as i32;
+    let num_cols = input.lines().next().unwrap().chars().count() as i32;
+    let mut expanded_rocks: HashSet<Position> = HashSet::new();
+    for rock in rocks.iter() {
+        for row_mul in 0..5 {
+            for col_mul in 0..5 {
+                let expanded_rock = Position(
+                    rock.0 + (num_rows * row_mul as i32),
+                    rock.1 + (num_cols * col_mul as i32),
+                );
+                expanded_rocks.insert(expanded_rock);
+            }
+        }
+    }
+    GardenPart2 {
+        rocks: expanded_rocks,
+        start: Position(num_rows * 5 / 2, num_cols * 5 / 2),
+        extents: Position(num_rows * 5, num_cols * 5),
+    }
+}
+
+#[aoc(day21, part2)]
+fn part2(map: &GardenPart2) -> i64 {
+    let b0: i64 = walk(&map, 65) as i64;
+    let b1: i64 = walk(&map, 65 + 131) as i64;
+    let b2: i64 = walk(&map, 65 + 2 * 131) as i64;
+    let n: i64 = 202300;
+    // below uses Cramer's Rule to solve for x0, x1, x2
+    let det_a: f64 = -2.0;
+    let det_a0: f64 = -b0 as f64 + 2.0 * b1 as f64 - b2 as f64;
+    let det_a1: f64 = 3.0 * b0 as f64 - 4.0 * b1 as f64 + b2 as f64;
+    let det_a2: f64 = -2.0 * b0 as f64;
+    let x0: i64 = (det_a0 / det_a) as i64;
+    let x1: i64 = (det_a1 / det_a) as i64;
+    let x2: i64 = (det_a2 / det_a) as i64;
+
+    x0 * n * n + x1 * n + x2
+}
+
+fn walk(garden: &GardenPart2, steps: u32) -> u32 {
+    let mut visited: HashSet<Position> = HashSet::new();
+    visited.insert(garden.start);
+    for _ in 0..steps {
+        let mut new_visited = HashSet::new();
+        for pos in visited.iter().clone() {
+            let neighbors = [
+                Position(pos.0 - 1, pos.1),
+                Position(pos.0 + 1, pos.1),
+                Position(pos.0, pos.1 - 1),
+                Position(pos.0, pos.1 + 1),
+            ]
+            .into_iter()
+            .filter(|p| in_bounds_part2(*p, garden.extents) && !garden.rocks.contains(p));
+            for neighbor in neighbors {
+                new_visited.insert(neighbor);
+            }
+        }
+        visited = new_visited;
+    }
+    visited.len() as u32
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const EX: &str = r"...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........";
+
+    #[test]
+    fn part1_example() {
+        assert_eq!(part1(&parse(EX)), 16);
+    }
+}
\ No newline at end of file
diff --git a/src/lib.rs b/src/lib.rs
index 507f881..14025a9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,4 @@
+mod day21;
 mod day20;
 mod day19;
 mod day18;