2025: d04: ex2: add solution

2025/d04/ex2/ex2.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+def solve(input: list[str]) -> int:
+    def parse(input: list[str]) -> set[Point]:
+        return {
+            Point(x, y)
+            for x, line in enumerate(input)
+            for y, c in enumerate(line)
+            if c == "@"
+        }
+
+    def is_accessible(point: Point, rolls: set[Point]) -> bool:
+        assert point in rolls  # Sanity check
+        return (
+            sum(
+                Point(point.x + dx, point.y + dy) in rolls
+                for dx, dy in itertools.product(range(-1, 1 + 1), repeat=2)
+                if dx != 0 or dy != 0
+            )
+            < 4
+        )
+
+    rolls = parse(input)
+    removed = 0
+    while accessible := {p for p in rolls if is_accessible(p, rolls)}:
+        removed += len(accessible)
+        rolls -= accessible
+    return removed
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()