2018: d12: ex1: add solution

2018/d12/ex1/ex1.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python
+
+import sys
+
+
+def solve(input: str) -> int:
+    def parse_rules(input: list[str]) -> dict[str, str]:
+        return {key: val for key, val in map(lambda s: s.split(" => "), input)}
+
+    def parse(input: str) -> tuple[str, dict[str, str]]:
+        init, rules = input.split("\n\n")
+        return init.split(": ")[1], parse_rules(rules.splitlines())
+
+    def step(state: str, rules: dict[str, str]) -> str:
+        state = "...." + state + "...."
+        return "".join(
+            rules.get(state[i - 2 : i + 2 + 1], ".") for i in range(2, len(state) - 2)
+        )
+
+    def count_pots(state: str) -> int:
+        return sum(
+            i for i, c in enumerate(state, start=-(len(state) - 100) // 2) if c == "#"
+        )
+
+    state, rules = parse(input)
+    assert len(state) == 100  # Sanity check for `count_pots`
+    for _ in range(20):
+        state = step(state, rules)
+    return count_pots(state)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()