2025: d11: ex2: add solution

2025/d11/ex2/ex2.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+
+import functools
+import itertools
+import math
+import sys
+
+
+def solve(input: list[str]) -> int:
+    def parse(input: list[str]) -> dict[str, set[str]]:
+        return {
+            dev: set(outputs.split(" "))
+            for dev, outputs in map(lambda s: s.split(": "), input)
+        }
+
+    def count_paths(graph: dict[str, set[str]], start: str, end: str) -> int:
+        @functools.cache
+        def rec(start: str, end: str) -> int:
+            if start == end:
+                return 1
+            return sum(rec(n, end) for n in graph.get(start, []))
+
+        # Input is assured to be a DAG, so a simple recursive count is enough
+        return rec(start, end)
+
+    graph = parse(input)
+    return sum(
+        math.prod(count_paths(graph, a, b) for a, b in itertools.pairwise(path))
+        for path in (
+            ["svr", "dac", "fft", "out"],
+            ["svr", "fft", "dac", "out"],
+        )
+    )
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()