2018: d18: ex2: add solution

2018/d18/ex2/ex2.py

@@ -0,0 +1,90 @@
+#!/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
+
+    def frozen(grid: dict[Point, Cell]) -> tuple[Cell, ...]:
+        return tuple(grid[p] for p in sorted(grid.keys()))
+
+    def thawed(hashed_grid: tuple[Cell, ...]) -> dict[Point, Cell]:
+        return {Point(i // 50, i % 50): c for i, c in enumerate(hashed_grid)}
+
+    def do_cycles(grid: dict[Point, Cell], end: int) -> dict[Point, Cell]:
+        hashed_grid = frozen(grid)
+        cache = {hashed_grid: 0}
+        t = 0
+        while t < end:
+            hashed_grid = frozen(step(thawed(hashed_grid)))
+            t += 1
+            if hashed_grid in cache:
+                previous_t = cache[hashed_grid]
+                cycle_length = t - previous_t
+                num_cycles = (end - t) // cycle_length
+                t += num_cycles * cycle_length
+            else:
+                cache[hashed_grid] = t
+
+        return thawed(hashed_grid)
+
+    grid = parse(input.splitlines())
+    grid = do_cycles(grid, 1000000000)
+    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()