From 856efa526ef261dc165843e9b080a3a4af4996bf Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Thu, 24 Dec 2020 13:26:52 +0100
Subject: [PATCH] 2020: d24: ex2: add solution

---
 2020/d24/ex2/ex2.py | 75 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100755 2020/d24/ex2/ex2.py

diff --git a/2020/d24/ex2/ex2.py b/2020/d24/ex2/ex2.py
new file mode 100755
index 0000000..ef749f9
--- /dev/null
+++ b/2020/d24/ex2/ex2.py
@@ -0,0 +1,75 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from collections import defaultdict
+from typing import Dict, Iterator, List, Tuple
+
+Offset = Tuple[int, int]
+Grid = Dict[Offset, bool]
+
+DELTAS = {
+    "nw": (0, -1),
+    "ne": (1, -1),
+    "e": (1, 0),
+    "se": (0, 1),
+    "sw": (-1, 1),
+    "w": (-1, 0),
+}
+
+
+def to_offset(path: str) -> Offset:
+    offset = 0, 0
+    i = 0
+    while i < len(path):
+        direction = path[i]
+        i += 1
+        if direction in ["s", "n"]:
+            direction += path[i]
+            i += 1
+        x, y = offset
+        dx, dy = DELTAS[direction]
+        offset = x + dx, y + dy
+    return offset
+
+
+def neighbours(tile: Offset) -> Iterator[Offset]:
+    x, y = tile
+    for dx, dy in DELTAS.values():
+        yield x + dx, y + dy
+
+
+def step(blacks: Grid) -> Grid:
+    to_visit = set(itertools.chain.from_iterable(neighbours(tile) for tile in blacks))
+    to_visit |= {tile for tile in blacks}
+    res: Grid = defaultdict(bool)
+
+    for tile in to_visit:
+        num_neighbours = sum(blacks[n] for n in neighbours(tile))
+        if blacks[tile]:
+            res[tile] = num_neighbours in (1, 2)
+        else:
+            res[tile] = num_neighbours == 2
+
+    return res
+
+
+def solve(raw: List[str]) -> int:
+    blacks: Grid = defaultdict(bool)
+
+    for offset in map(to_offset, raw):
+        blacks[offset] = not blacks[offset]
+
+    for __ in range(100):
+        blacks = step(blacks)
+
+    return sum(blacks.values())
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()