day 18 python fix a flood fill bug

not sure how this ever worked

day18/sol.py

@@ -53,11 +53,10 @@ def flood(lava, border):
         x |= roll(a, 1, +1)
         x |= roll(a, 2, -1)
         x |= roll(a, 2, +1)
-        x = x & ~border
+        x &= ~lava
-        x = x & ~lava
+        b = a | x
-        if (a == x).all():
+        if (a == b).all():
-            a |= border
             return a
-        a = x
+        a = b
 
 solve()