diff --git a/day21.py b/day21.py
new file mode 100644
index 0000000..936a28b
--- /dev/null
+++ b/day21.py
@@ -0,0 +1,87 @@
+from collections import deque, defaultdict
+from tools.aoc import AOCDay
+from tools.coordinate import Coordinate
+from tools.grid import Grid
+from typing import Any
+
+
+def walk(grid: Grid, start: Coordinate, steps: int) -> int:
+    q = deque()
+    q.append((start, 0))
+    s_mod = steps % 2
+    v = set()
+    v2 = set()
+    while q:
+        coord, dist = q.popleft()
+        #print("check ", coord, dist)
+        if coord in v or coord in v2:
+            #print(" -> in v")
+            continue
+        if dist % 2 == s_mod:
+            #print(f" -> add to v because {dist} even or == {steps}")
+            v.add(coord)
+        else:
+            v2.add(coord)
+        if dist >= steps:
+            #print(f" -> >= {steps}")
+            continue
+        for n in coord.getNeighbours(includeDiagonal=False):
+            if grid.get(n % (grid.maxX + 1)):
+                q.append((n, dist + 1))
+
+    #print([x for x in v])
+    return len(v)
+
+
+class Day(AOCDay):
+    inputs = [
+        [
+            (16, "input21_test"),
+            (3562, "input21"),
+        ],
+        [
+            #(16733044, "input21_test"),
+            (None, "input21"),
+        ]
+    ]
+
+    def parse_input(self) -> (Grid, Coordinate):
+        grid = Grid().from_data(self.getInput(), translate={'.': True, '#': False})
+        start = None
+        for x in grid.getActiveCells():
+            if grid.get(x) == 'S':
+                start = x
+                grid.set(x, True)
+
+        return grid, start
+
+    def part1(self) -> Any:
+        needed_steps = 6 if 'test' in self._current_test_file else 64
+        grid, start = self.parse_input()
+        return walk(grid, start, needed_steps)
+
+    def part2(self) -> Any:
+        grid, start = self.parse_input()
+        needed_steps = 5000 if 'test' in self._current_test_file else 26501365
+        k = walk(grid, start, len(self.getInput()))
+        g = walk(grid, start, 2 * len(self.getInput()))
+        v = walk(grid, start, 3 * len(self.getInput()))
+        print(k, g, v, g-k, v-g)
+        return ""
+        j = 0
+        mem = defaultdict(list)
+        for i in range(1, 250, 2):
+            k = walk(grid, start, i)
+            diff = k - j
+            j = k
+            mem[diff].append(i)
+            print(i, "=>", k, "diff", diff, "mod", k % 81)
+        for k, v in mem.items():
+            if len(v) > 1:
+                print(k, "=>", v)
+        return ""
+
+
+if __name__ == '__main__':
+    day = Day(2023, 21)
+    day.run(verbose=True)
\ No newline at end of file
diff --git a/inputs/input21 b/inputs/input21
new file mode 100644
index 0000000..6bfb54b
--- /dev/null
+++ b/inputs/input21
@@ -0,0 +1,131 @@
+...................................................................................................................................
+.......#.......#...#....#.........##.#.#.............#....#.#.............#.#...............#.....#......#.#.............#.........
+....#...........#..##......#.#.....................#....................#.....#............#..#......#...............#.#.........#.
+......................##......#...###.#.#.....#.....#...................#......#.......#...#.......#...............#.............#.
+.......#.#.#.....##..##..................#.#...........#..........................#......#..##.#.......#..........#..#.....#..#..#.
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+....#.#.......#..#......#.......#......#.###.....##......#.............#.....#.............#..##.###.......#....#..............#...
+...................................................................................................................................
diff --git a/inputs/input21_test b/inputs/input21_test
new file mode 100644
index 0000000..c66328c
--- /dev/null
+++ b/inputs/input21_test
@@ -0,0 +1,11 @@
+...........
+.....###.#.
+.###.##..#.
+..#.#...#..
+....#.#....
+.##..S####.
+.##..#...#.
+.......##..
+.##.#.####.
+.##..##.##.
+...........
\ No newline at end of file