diff --git a/2017/d21/ex1/ex1.py b/2017/d21/ex1/ex1.py
new file mode 100755
index 0000000..b5bda88
--- /dev/null
+++ b/2017/d21/ex1/ex1.py
@@ -0,0 +1,88 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from collections.abc import Iterator
+
+# 2x2 *and* 3x3 grid
+Grid = tuple[tuple[bool, ...], ...]
+START = ".#./..#/###"
+
+
+def solve(input: str) -> int:
+    def parse_pattern(input: str) -> Grid:
+        return tuple(tuple(c == "#" for c in line) for line in input.split("/"))
+
+    def parse(input: str) -> dict[Grid, Grid]:
+        return {
+            start: end
+            for start, end in (
+                map(parse_pattern, line.split(" => ")) for line in input.splitlines()
+            )
+        }
+
+    def permutations(grid: Grid) -> set[Grid]:
+        def iter_rotated(grid: Grid) -> Iterator[Grid]:
+            for _ in range(4):
+                yield grid
+                grid = tuple(
+                    tuple(grid[len(grid) - y - 1][x] for y in range(len(grid[0])))
+                    for x in range(len(grid))
+                )
+
+        def iter_flipped(grid: Grid) -> Iterator[Grid]:
+            yield grid
+            yield grid[::-1]
+            yield tuple(line[::-1] for line in grid)
+
+        return {
+            flip for rotated in iter_rotated(grid) for flip in iter_flipped(rotated)
+        }
+
+    def normalize_rules(raw_rules: dict[Grid, Grid]) -> dict[Grid, Grid]:
+        res: dict[Grid, Grid] = {}
+        for pattern, grid in raw_rules.items():
+            for perm in permutations(pattern):
+                res[perm] = grid
+        return res
+
+    def enhance(rules: dict[Grid, Grid], rounds: int) -> list[list[bool]]:
+        def step(grid: list[list[bool]]) -> list[list[bool]]:
+            is_even = len(grid) % 2 == 0
+            step_size = 2 if is_even else 3
+            next_size = len(grid) * (step_size + 1) // step_size
+            res = [[False for _ in range(next_size)] for _ in range(next_size)]
+
+            for x, y in itertools.product(range(len(grid) // step_size), repeat=2):
+                elem = tuple(
+                    tuple(
+                        grid[x * step_size + dx][y * step_size + dy]
+                        for dy in range(step_size)
+                    )
+                    for dx in range(step_size)
+                )
+                new_grid = rules[elem]
+                for dx, line in enumerate(new_grid):
+                    for dy, p in enumerate(line):
+                        res[x * len(new_grid) + dx][y * len(line) + dy] = p
+
+            return res
+
+        grid = [[p for p in line] for line in parse_pattern(START)]
+        for _ in range(rounds):
+            grid = step(grid)
+        return grid
+
+    raw_rules = parse(input)
+    norm_rules = normalize_rules(raw_rules)
+    art = enhance(norm_rules, 5)
+    return sum(itertools.chain.from_iterable(art))
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()