From c1f47d34ad28c0386ab3bfd31b1afb2771309c23 Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Mon, 30 Dec 2024 19:41:08 -0500
Subject: [PATCH] 2018: d18: ex1: add solution

---
 2018/d18/ex1/ex1.py | 68 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100755 2018/d18/ex1/ex1.py

diff --git a/2018/d18/ex1/ex1.py b/2018/d18/ex1/ex1.py
new file mode 100755
index 0000000..8e6b249
--- /dev/null
+++ b/2018/d18/ex1/ex1.py
@@ -0,0 +1,68 @@
+#!/usr/bin/env python
+
+import enum
+import itertools
+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 itertools.product(range(-1, 1 + 1), repeat=2):
+            if dx == 0 and dy == 0:
+                continue
+            yield Point(self.x + dx, self.y + dy)
+
+
+class Cell(enum.StrEnum):
+    OPEN = "."
+    TREE = "|"
+    LUMBERYARD = "#"
+
+
+def solve(input: str) -> int:
+    def parse(input: list[str]) -> dict[Point, Cell]:
+        return {
+            Point(x, y): Cell(c)
+            for x, line in enumerate(input)
+            for y, c in enumerate(line)
+        }
+
+    def step_cell(p: Point, grid: dict[Point, Cell]) -> Cell:
+        neighbours = (n for n in p.neighbours() if n in grid)
+        if grid[p] == Cell.OPEN:
+            trees = sum(grid[n] == Cell.TREE for n in neighbours)
+            return Cell.TREE if trees >= 3 else Cell.OPEN
+        if grid[p] == Cell.TREE:
+            lumberyards = sum(grid[n] == Cell.LUMBERYARD for n in neighbours)
+            return Cell.LUMBERYARD if lumberyards >= 3 else Cell.TREE
+        if grid[p] == Cell.LUMBERYARD:
+            continues = {Cell.TREE, Cell.LUMBERYARD} <= {grid[n] for n in neighbours}
+            return Cell.LUMBERYARD if continues else Cell.OPEN
+        assert False  # Sanity check
+
+    def step(grid: dict[Point, Cell]) -> dict[Point, Cell]:
+        res: dict[Point, Cell] = {}
+        for p in map(Point._make, itertools.product(range(50), repeat=2)):
+            res[p] = step_cell(p, grid)
+        return res
+
+    grid = parse(input.splitlines())
+    for _ in range(10):
+        grid = step(grid)
+    trees = sum(c == Cell.TREE for c in grid.values())
+    lumberyards = sum(c == Cell.LUMBERYARD for c in grid.values())
+    return trees * lumberyards
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()