diff --git a/2017/d25/ex1/ex1.py b/2017/d25/ex1/ex1.py
new file mode 100755
index 0000000..b57dc3f
--- /dev/null
+++ b/2017/d25/ex1/ex1.py
@@ -0,0 +1,73 @@
+#!/usr/bin/env python
+
+import collections
+import enum
+import sys
+from typing import NamedTuple
+
+
+class Direction(enum.StrEnum):
+    LEFT = "left"
+    RIGHT = "right"
+
+    def apply(self, pos: int) -> int:
+        return pos + 1 if self == Direction.RIGHT else pos - 1
+
+
+class Rules(NamedTuple):
+    zero_write: bool
+    zero_dir: Direction
+    zero_transition: str
+
+    one_write: bool
+    one_dir: Direction
+    one_transition: str
+
+    @classmethod
+    def from_str(cls, input: str) -> "Rules":
+        lines = input.splitlines()
+        return cls(
+            zero_write=bool(int(lines[1][-2])),
+            zero_dir=Direction(lines[2][:-1].split()[-1]),
+            zero_transition=lines[3][-2],
+            one_write=bool(int(lines[5][-2])),
+            one_dir=Direction(lines[6][:-1].split()[-1]),
+            one_transition=lines[7][-2],
+        )
+
+    def apply(self, tape: dict[int, bool], pos: int) -> tuple[str, int]:
+        if tape[pos]:
+            tape[pos] = self.one_write
+            return self.one_transition, self.one_dir.apply(pos)
+        else:
+            tape[pos] = self.zero_write
+            return self.zero_transition, self.zero_dir.apply(pos)
+
+
+def solve(input: str) -> int:
+    def parse_rules(input: str) -> tuple[str, Rules]:
+        assert input.startswith("In state ")  # Sanity check
+        state, lines = input.split("\n", 1)
+        return state[-2], Rules.from_str(lines)
+
+    def parse(input: str) -> tuple[str, int, dict[str, Rules]]:
+        first, *rest = input.split("\n\n")
+        raw_state, raw_checksum = first.splitlines()
+        rules = {state: rules for state, rules in map(parse_rules, rest)}
+        return raw_state[-2], int(raw_checksum.split()[-2]), rules
+
+    state, iterations, rules = parse(input)
+    pos = 0
+    tape: dict[int, bool] = collections.defaultdict(bool)
+    for _ in range(iterations):
+        state, pos = rules[state].apply(tape, pos)
+    return sum(tape.values())
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()