1 files changed, 90 insertions, 0 deletions
diff --git a/20/py/d17.py b/20/py/d17.py
new file mode 100644
index 0000000..5ec1419
--- /dev/null
+++ b/20/py/d17.py
@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+import aoc20
+import sys
+import functools
+from collections import defaultdict
+
+
+@functools.cache
+def neighbours3(p):
+    res = set()
+    for dx in range(-1, 1+1):
+        for dy in range(-1, 1+1):
+            for dz in range(-1, 1+1):
+                if dx == dy == dz == 0:
+                    continue
+                res.add((p[0] + dx, p[1] + dy, p[2] + dz))
+    return res
+
+
+def pt1(_in):
+    active = set()
+    board = defaultdict(lambda: 0)
+    for y in range(len(_in)):
+        for x in range(len(_in[y])):
+            if _in[y][x] == "#":
+                active.add((x, y, 0))
+                for n in neighbours3((x, y, 0)):
+                    board[n] += 1
+
+    for _ in range(6):
+        new_board = defaultdict(lambda: 0)
+        new_active = set()
+        for p, deg in board.items():
+            if p in active and 2 <= deg <= 3:
+                new_active.add(p)
+                for n in neighbours3(p):
+                    new_board[n] += 1
+            elif p not in active and deg == 3:
+                new_active.add(p)
+                for n in neighbours3(p):
+                    new_board[n] += 1
+        board = new_board
+        active = new_active
+    return(len(active))
+
+
+@functools.cache
+def neighbours4(p):
+    res = set()
+    for dx in range(-1, 1+1):
+        for dy in range(-1, 1+1):
+            for dz in range(-1, 1+1):
+                for dw in range(-1, 1+1):
+                    if dx == dy == dz == dw == 0:
+                        continue
+                    res.add((p[0] + dx, p[1] + dy, p[2] + dz, p[3] + dw))
+    return res
+
+
+def pt2(_in):
+    active = set()
+    board = defaultdict(lambda: 0)
+    for y in range(len(_in)):
+        for x in range(len(_in[y])):
+            if _in[y][x] == "#":
+                active.add((x, y, 0, 0))
+                for n in neighbours4((x, y, 0, 0)):
+                    board[n] += 1
+
+    for _ in range(6):
+        new_board = defaultdict(lambda: 0)
+        new_active = set()
+        for p, deg in board.items():
+            if p in active and 2 <= deg <= 3:
+                new_active.add(p)
+                for n in neighbours4(p):
+                    new_board[n] += 1
+            elif p not in active and deg == 3:
+                new_active.add(p)
+                for n in neighbours4(p):
+                    new_board[n] += 1
+        board = new_board
+        active = new_active
+    return(len(active))
+
+
+if __name__ == "__main__":
+    input = aoc20.read_input(sys.argv[1:], 17)
+    print(pt1(input))
+    print(pt2(input))