2024: d19: ex2: add solution

2024/d19/ex2/ex2.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+
+import functools
+import sys
+
+
+def solve(input: str) -> int:
+    def parse(input: str) -> tuple[frozenset[str], list[str]]:
+        towels, patterns = input.split("\n\n")
+        return frozenset(towels.split(", ")), patterns.splitlines()
+
+    @functools.cache
+    def count_ways(pattern: str, towels: frozenset[str]) -> int:
+        if pattern == "":
+            return 1
+        return sum(
+            count_ways(pattern.removeprefix(towel), towels)
+            for towel in towels
+            if pattern.startswith(towel)
+        )
+
+    towels, patterns = parse(input)
+    return sum(count_ways(pattern, towels) for pattern in patterns)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()