2023: d12: ex1: add solution

2023/d12/ex1/ex1.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+
+import functools
+import sys
+
+
+def solve(input: list[str]) -> int:
+    def parse(input: list[str]) -> list[tuple[str, tuple[int, ...]]]:
+        return [
+            (record, tuple(map(int, groups.split(","))))
+            for record, groups in map(str.split, input)
+        ]
+
+    @functools.cache
+    def solve_line(record: str, groups: tuple[int, ...]) -> int:
+        # Empty string must have no groups left, or it's not a solve
+        if len(record) == 0:
+            return 1 if len(groups) == 0 else 0
+        # Empty groups must not contain any broken spring, or it's not a solve
+        if len(groups) == 0:
+            return 1 if all(c in (".", "?") for c in record) else 0
+        # Skip working springs
+        if record[0] == ".":
+            return solve_line(record[1:], groups)
+        # Try with a '.' (and discard it directly), or a '#' for the unknown springs
+        if record[0] == "?":
+            return solve_line(record[1:], groups) + solve_line("#" + record[1:], groups)
+        # We start with a '#', check that the group is long enough
+        if len(record) < groups[0] or any(c == "." for c in record[: groups[0]]):
+            return 0
+        # And check that we _can_ separate the group from the next one
+        if len(record) > groups[0] and record[groups[0]] == "#":
+            return 0
+        # Now recurse
+        return solve_line(record[groups[0] + 1 :], groups[1:])
+
+    lines = parse(input)
+    return sum(solve_line(record, groups) for record, groups in lines)
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()