diff --git a/day21.py b/day21.py
new file mode 100644
index 0000000..162c226
--- /dev/null
+++ b/day21.py
@@ -0,0 +1,94 @@
+from tools.aoc import AOCDay
+from tools.coordinate import Coordinate
+from tools.grid import Grid, GridTransformation
+from typing import Any
+
+
+FLIPS = [
+    GridTransformation.ROTATE_RIGHT,
+    GridTransformation.ROTATE_RIGHT,
+    GridTransformation.ROTATE_RIGHT,
+    GridTransformation.ROTATE_RIGHT,
+    GridTransformation.FLIP_HORIZONTALLY,
+    GridTransformation.ROTATE_RIGHT,
+    GridTransformation.ROTATE_RIGHT,
+    GridTransformation.ROTATE_RIGHT,
+]
+
+
+def init_grid() -> Grid:
+    grid = Grid()
+    grid.set(Coordinate(1, 0))
+    grid.set(Coordinate(2, 1))
+    grid.set(Coordinate(0, 2))
+    grid.set(Coordinate(1, 2))
+    grid.set(Coordinate(2, 2))
+    return grid
+
+
+class Day(AOCDay):
+    inputs = [
+        [
+            (12, "input21_test"),
+            (176, "input21")
+        ],
+        [
+            (2368161, "input21")
+        ]
+    ]
+
+    def load_replacement_grids(self) -> dict:
+        ret = {}
+        for line in self.getInput():
+            rep, grid_string = line.split(" => ")
+            ret[rep] = Grid.from_str(grid_string, true_char='#')
+            rep_grid_size = len(rep.split("/")[0]) + 1
+            ret[rep].minX, ret[rep].maxX, ret[rep].minY, ret[rep].maxY = 0, rep_grid_size, 0, rep_grid_size
+
+        return ret
+
+    def interact(self, interact_count: int) -> int:
+        grid = Grid.from_str('.#./..#/###')
+        grid_size = 3
+        replacements = self.load_replacement_grids()
+
+        for i in range(2 if self._current_test_file.endswith('test') else interact_count):
+            new_grid = Grid()
+            block_size = 3
+            replace_size = 4
+            if grid_size % 2 == 0:
+                block_size = 2
+                replace_size = 3
+
+            for x in range(0, grid_size, block_size):
+                for y in range(0, grid_size, block_size):
+                    sub_grid = grid.sub_grid(x, y, x + block_size - 1, y + block_size - 1)
+                    sub_grid.minX, sub_grid.minY, sub_grid.maxX, sub_grid.maxY = 0, 0, block_size - 1, block_size - 1
+                    for f in FLIPS:
+                        sub_grid.transform(f)
+                        sub_grid.maxX, sub_grid.maxY = block_size - 1 + sub_grid.minX, block_size - 1 + sub_grid.minY
+                        if str(sub_grid) in replacements:
+                            new_grid.update(
+                                x // block_size * replace_size,
+                                y // block_size * replace_size,
+                                replacements[str(sub_grid)]
+                            )
+                            break
+                    else:
+                        raise ValueError("No sub_grid matches found.")
+
+            grid = new_grid
+            grid_size = grid_size // block_size * replace_size
+
+        return grid.getOnCount()
+
+    def part1(self) -> Any:
+        return self.interact(5)
+
+    def part2(self) -> Any:
+        return self.interact(18)
+
+
+if __name__ == '__main__':
+    day = Day(2017, 21)
+    day.run(verbose=True)
diff --git a/inputs/input21 b/inputs/input21
new file mode 100644
index 0000000..6bb054b
--- /dev/null
+++ b/inputs/input21
@@ -0,0 +1,108 @@
+../.. => .#./.../###
+#./.. => .#./##./#..
+##/.. => #.#/#../###
+.#/#. => ##./..#/###
+##/#. => .#./#../..#
+##/## => #../..#/#.#
+.../.../... => .###/.#.#/.###/##.#
+#../.../... => .##./##../##../#.##
+.#./.../... => .#.#/#.#./..#./..#.
+##./.../... => ###./#.##/...#/#.##
+#.#/.../... => .#.#/.#../.###/.###
+###/.../... => ..##/#.#./..../##.#
+.#./#../... => #.../..../..../....
+##./#../... => ...#/..#./.###/#.#.
+..#/#../... => #.../#.##/###./##..
+#.#/#../... => .##./#..#/#..#/..##
+.##/#../... => .#.#/#.##/..##/.#.#
+###/#../... => #.#./.###/..#./#.#.
+.../.#./... => #..#/..../.##./.#.#
+#../.#./... => .#../.##./.#.#/...#
+.#./.#./... => ##.#/...#/.##./...#
+##./.#./... => ..#./#.#./#.##/####
+#.#/.#./... => ..##/#..#/.###/....
+###/.#./... => .#../#..#/#.../..#.
+.#./##./... => ..##/#.#./####/###.
+##./##./... => ...#/.#../####/#..#
+..#/##./... => ..##/##../###./....
+#.#/##./... => ..##/#.../.#../.##.
+.##/##./... => #.../##../#.##/...#
+###/##./... => .#../####/#.##/#.##
+.../#.#/... => #..#/####/###./#.#.
+#../#.#/... => #.../##.#/#.../.#..
+.#./#.#/... => ##.#/##.#/..#./..#.
+##./#.#/... => .###/..#./.#../.###
+#.#/#.#/... => .###/##../..#./..#.
+###/#.#/... => ##../.#../.#../.#..
+.../###/... => ..#./#.#./..#./#..#
+#../###/... => ..../#.#./##.#/..##
+.#./###/... => ..#./#.#./..##/.#..
+##./###/... => .##./..##/#..#/#.#.
+#.#/###/... => ###./###./#.##/..##
+###/###/... => ##.#/..../.##./.#..
+..#/.../#.. => .###/####/..../##.#
+#.#/.../#.. => ##../###./#..#/...#
+.##/.../#.. => ###./#..#/###./...#
+###/.../#.. => #.../#..#/##.#/.##.
+.##/#../#.. => ..##/####/..##/#...
+###/#../#.. => #.../..../...#/..##
+..#/.#./#.. => ####/#.#./..../.#.#
+#.#/.#./#.. => .##./.#.#/##.#/.##.
+.##/.#./#.. => ###./.#.#/###./##.#
+###/.#./#.. => #.##/..##/#.#./##.#
+.##/##./#.. => ..../..##/#.#./.##.
+###/##./#.. => #.#./#..#/#..#/###.
+#../..#/#.. => ..../####/#..#/.###
+.#./..#/#.. => .###/#.../#.../#.##
+##./..#/#.. => ####/##.#/###./####
+#.#/..#/#.. => .#../##.#/#..#/#..#
+.##/..#/#.. => ..##/##.#/#.##/###.
+###/..#/#.. => ##.#/####/##.#/.#..
+#../#.#/#.. => .###/#..#/.##./.###
+.#./#.#/#.. => #.##/.##./.#../..#.
+##./#.#/#.. => ###./..#./.##./##..
+..#/#.#/#.. => .###/.#.#/#.#./##..
+#.#/#.#/#.. => #..#/.###/.##./....
+.##/#.#/#.. => ###./.###/#.##/.###
+###/#.#/#.. => ####/.###/..../.##.
+#../.##/#.. => ##.#/..../#.../..#.
+.#./.##/#.. => #.../..../...#/###.
+##./.##/#.. => ###./.#../..##/...#
+#.#/.##/#.. => #.../...#/..#./.###
+.##/.##/#.. => ###./..../##.#/...#
+###/.##/#.. => ##.#/##../###./.##.
+#../###/#.. => ..#./#.../..##/#.##
+.#./###/#.. => ...#/.##./.#../.#..
+##./###/#.. => ##.#/.#.#/###./....
+..#/###/#.. => #.##/#.../####/.##.
+#.#/###/#.. => .#.#/...#/#..#/..#.
+.##/###/#.. => .##./#..#/#..#/.#.#
+###/###/#.. => ###./####/#.##/#...
+.#./#.#/.#. => ###./#..#/...#/...#
+##./#.#/.#. => #.#./#.##/#.../#..#
+#.#/#.#/.#. => .#.#/#.##/..../.#..
+###/#.#/.#. => #.#./.#../.###/#.#.
+.#./###/.#. => #.../.###/##../##.#
+##./###/.#. => .###/#.../####/.#.#
+#.#/###/.#. => #..#/####/#.#./#...
+###/###/.#. => .#../..../.##./.#.#
+#.#/..#/##. => ##../###./...#/###.
+###/..#/##. => .##./###./.###/#.##
+.##/#.#/##. => ..../##.#/#..#/#...
+###/#.#/##. => .###/##../..../..#.
+#.#/.##/##. => ####/.###/##../...#
+###/.##/##. => #.##/..##/..#./#..#
+.##/###/##. => ..../#.##/#.../#.##
+###/###/##. => ..../#..#/#.##/#.##
+#.#/.../#.# => #.../##.#/..../.#.#
+###/.../#.# => ##../##../#.#./.##.
+###/#../#.# => .##./.#../#.##/.##.
+#.#/.#./#.# => #.../.#../####/#.##
+###/.#./#.# => .###/##.#/#.../#.#.
+###/##./#.# => .##./.##./.###/.#.#
+#.#/#.#/#.# => ####/####/###./.##.
+###/#.#/#.# => #.#./.###/...#/.#.#
+#.#/###/#.# => .###/..#./..../.##.
+###/###/#.# => #.#./##.#/..#./..#.
+###/#.#/### => ###./#.../##../##..
+###/###/### => ##.#/.#.#/#.#./...#
diff --git a/inputs/input21_test b/inputs/input21_test
new file mode 100644
index 0000000..53105fb
--- /dev/null
+++ b/inputs/input21_test
@@ -0,0 +1,2 @@
+../.# => ##./#../...
+.#./..#/### => #..#/..../..../#..#
\ No newline at end of file