2021: d09: ex2: add solution

2021/d09/ex2/ex2.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python
+
+import sys
+from typing import Iterator, List, Tuple
+
+HeightMap = List[List[int]]
+Point = Tuple[int, int]
+
+
+def solve(input: List[str]) -> int:
+    height_map = [[int(c) for c in line] for line in input]
+
+    def neighbours_of(point: Point) -> Iterator[Point]:
+        for dx, dy in (-1, 0), (1, 0), (0, -1), (0, 1):
+            x, y = point[0] + dx, point[1] + dy
+            if x < 0 or x >= len(height_map):
+                continue
+            if y < 0 or y >= len(height_map[0]):
+                continue
+            yield x, y
+
+    def is_low_point(point: Point) -> bool:
+        for neighbour in neighbours_of(point):
+            if height_map[neighbour[0]][neighbour[1]] <= height_map[point[0]][point[1]]:
+                return False
+        return True
+
+    def explore_bassin(point: Point) -> int:
+        to_be_seen = set(neighbours_of(point))
+        explored = {point}
+        bassin = {point}
+
+        while len(to_be_seen) > 0:
+            new = to_be_seen.pop()
+            explored.add(new)
+            if height_map[new[0]][new[1]] == 9:
+                continue
+            bassin.add(new)
+            for neighbour in neighbours_of(new):
+                if neighbour in explored:
+                    continue
+                to_be_seen.add(neighbour)
+
+        return len(bassin)
+
+    points = [(x, y) for x in range(len(height_map)) for y in range(len(height_map[0]))]
+    low_points = filter(is_low_point, points)
+
+    bassin_sizes = sorted(map(explore_bassin, low_points), reverse=True)
+
+    return bassin_sizes[0] * bassin_sizes[1] * bassin_sizes[2]
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()