2024: d10: ex2: add solution

2024/d10/ex2/ex2.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python
+
+import sys
+from collections.abc import Iterator
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+    def neighbours(self) -> Iterator["Point"]:
+        for dx, dy in (
+            (-1, 0),
+            (1, 0),
+            (0, -1),
+            (0, 1),
+        ):
+            yield Point(self.x + dx, self.y + dy)
+
+
+TopoMap = dict[Point, int]
+
+
+def solve(input: str) -> int:
+    def parse(input: list[str]) -> TopoMap:
+        return {
+            Point(x, y): int(c)
+            for x, line in enumerate(input)
+            for y, c in enumerate(line)
+        }
+
+    def find_trail_heads(map: TopoMap) -> set[Point]:
+        return {p for p, height in map.items() if height == 0}
+
+    def rate_trail(map: TopoMap, start: Point) -> int:
+        def helper(pos: Point) -> int:
+            if map[pos] == 9:
+                return 1
+            res = 0
+            for n in pos.neighbours():
+                if map.get(n, -1) != (map[pos] + 1):
+                    continue
+                res += helper(n)
+            return res
+
+        assert map[start] == 0  # Sanity check
+        return helper(start)
+
+    map = parse(input.splitlines())
+    trail_heads = find_trail_heads(map)
+    return sum(rate_trail(map, head) for head in sorted(trail_heads))
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()