2019: d24: ex1: add solution

2019/d24/ex1/ex1.py

@@ -0,0 +1,72 @@
+#!/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 neighbours(self) -> Iterator["Point"]:
+        for dx, dy in (
+            (-1, 0),
+            (1, 0),
+            (0, -1),
+            (0, 1),
+        ):
+            yield Point(self.x + dx, self.y + dy)
+
+
+DIMS = 5
+
+
+def solve(input: str) -> int:
+    def parse(input: list[str]) -> frozenset[Point]:
+        return frozenset(
+            Point(x, y)
+            for x, line in enumerate(input)
+            for y, c in enumerate(line)
+            if c == "#"
+        )
+
+    def step(bugs: frozenset[Point]) -> frozenset[Point]:
+        res: set[Point] = set()
+        for p in map(Point._make, itertools.product(range(DIMS), range(DIMS))):
+            neighbours = len(set(p.neighbours()) & bugs)
+            # Bug dies if not exactly one neighbour
+            if p in bugs and neighbours != 1:
+                continue
+            # An empty space stays empty if it doesn't have 1 or 2 neighbours
+            if p not in bugs and neighbours not in (1, 2):
+                continue
+            res.add(p)
+        return frozenset(res)
+
+    def biodiversity(bugs: frozenset[Point]) -> int:
+        res = 0
+        for x, y in itertools.product(range(DIMS), range(DIMS)):
+            res |= (Point(x, y) in bugs) << (x * DIMS + y)
+        return res
+
+    bugs = parse(input.splitlines())
+    layouts: set[frozenset[Point]] = set()
+
+    while True:
+        layouts.add(frozenset(bugs))
+        bugs = step(bugs)
+        if bugs in layouts:
+            return biodiversity(bugs)
+
+    assert False  # Sanity check
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()