From 68d99c78904c8bf111f219f1444ce96c56ff5226 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Wed, 21 May 2025 23:06:42 +0100
Subject: [PATCH] 2015: d18: ex2: add solution

---
 2015/d18/ex2/ex2.py | 70 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100755 2015/d18/ex2/ex2.py

diff --git a/2015/d18/ex2/ex2.py b/2015/d18/ex2/ex2.py
new file mode 100755
index 0000000..f39b6da
--- /dev/null
+++ b/2015/d18/ex2/ex2.py
@@ -0,0 +1,70 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from collections.abc import Iterator
+from typing import NamedTuple
+
+
+class Point(NamedTuple):
+    x: int
+    y: int
+
+
+def solve(input: str) -> int:
+    def parse(input: str) -> set[Point]:
+        return {
+            Point(x, y)
+            for x, line in enumerate(input.splitlines())
+            for y, c in enumerate(line)
+            if c == "#"
+        }
+
+    def neighbours(p: Point) -> Iterator[Point]:
+        for dx, dy in itertools.product((-1, 0, 1), repeat=2):
+            if dx == 0 and dy == 0:
+                continue
+            yield Point(p.x + dx, p.y + dy)
+
+    def step(lights: set[Point], dimensions: Point) -> set[Point]:
+        res = {
+            # Four corners are stuck on
+            Point(0, 0),
+            Point(0, dimensions.y - 1),
+            Point(dimensions.x - 1, 0),
+            Point(dimensions.x - 1, dimensions.y - 1),
+        }
+        for p in map(
+            Point._make,
+            itertools.product(range(dimensions.x), range(dimensions.y)),
+        ):
+            num_alive = sum(n in lights for n in neighbours(p))
+            if p in lights and num_alive not in (2, 3):
+                continue
+            if p not in lights and num_alive != 3:
+                continue
+            res.add(p)
+
+        return res
+
+    lights = parse(input)
+    dimensions = Point(100, 100)
+    lights |= {
+        # Four corners are stuck on
+        Point(0, 0),
+        Point(0, dimensions.y - 1),
+        Point(dimensions.x - 1, 0),
+        Point(dimensions.x - 1, dimensions.y - 1),
+    }
+    for _ in range(100):
+        lights = step(lights, dimensions)
+    return len(lights)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()