diff --git a/2016/d21/ex2/ex2.py b/2016/d21/ex2/ex2.py
new file mode 100755
index 0000000..13a9c81
--- /dev/null
+++ b/2016/d21/ex2/ex2.py
@@ -0,0 +1,101 @@
+#!/usr/bin/env python
+
+import enum
+import sys
+from typing import NamedTuple
+
+
+class Op(enum.StrEnum):
+    SWAP_POSITION = "swap position"
+    SWAP_LETTER = "swap letter"
+    ROTATE_LEFT = "rotate left"
+    ROTATE_RIGHT = "rotate right"
+    ROTATE_BASED = "rotate based"
+    REVERSE_POSITION = "reverse positions"
+    MOVE_POSITION = "move position"
+
+
+class Instruction(NamedTuple):
+    op: Op
+    x: str
+    y: str | None
+
+    @classmethod
+    def from_str(cls, input: str) -> "Instruction":
+        split_input = input.split()
+        op = Op(" ".join(split_input[:2]))
+        x: str
+        y: str | None
+        match op:
+            case Op.ROTATE_LEFT | Op.ROTATE_RIGHT:
+                x, y = split_input[2], None
+            case Op.ROTATE_BASED:
+                x, y = split_input[-1], None
+            case _:
+                x, y = split_input[2], split_input[-1]
+        return cls(op, x, y)
+
+    def undo(self, password: str) -> str:
+        letters = list(password)
+        match self.op:
+            case Op.SWAP_POSITION:
+                assert self.y is not None  # Sanity check
+                x, y = int(self.x), int(self.y)
+                letters[x], letters[y] = letters[y], letters[x]
+            case Op.SWAP_LETTER:
+                assert self.y is not None  # Sanity check
+                x, y = letters.index(self.x), letters.index(self.y)
+                letters[x], letters[y] = letters[y], letters[x]
+            case Op.ROTATE_LEFT:
+                x = int(self.x)
+                x %= len(letters)
+                letters = letters[-x:] + letters[:-x]
+            case Op.ROTATE_RIGHT:
+                x = int(self.x)
+                x %= len(letters)
+                letters = letters[x:] + letters[:x]
+            case Op.ROTATE_BASED:
+                # original: 0 1 2 3 4 5 6 7
+                #   result: 1 3 5 7 2 4 6 0
+                # rotation: 1 2 3 4 6 7 0 1
+                x = {
+                    1: 1,
+                    3: 2,
+                    5: 3,
+                    7: 4,
+                    2: 6,
+                    4: 7,
+                    6: 0,
+                    0: 1,
+                }[letters.index(self.x)]
+                letters = letters[x:] + letters[:x]
+            case Op.REVERSE_POSITION:
+                assert self.y is not None  # Sanity check
+                x, y = int(self.x), int(self.y)
+                letters[x : y + 1] = letters[x : y + 1][::-1]
+            case Op.MOVE_POSITION:
+                assert self.y is not None  # Sanity check
+                x, y = int(self.x), int(self.y)
+                letter = letters.pop(y)
+                letters.insert(x, letter)
+        return "".join(letters)
+
+
+def solve(input: str) -> str:
+    def parse(input: str) -> list[Instruction]:
+        return [Instruction.from_str(line) for line in input.splitlines()]
+
+    instructions = parse(input)
+    password = "fbgdceah"
+    for instruction in instructions[::-1]:
+        password = instruction.undo(password)
+    return password
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()