day 12, knowing what part1 has built, part2 was longer than expected

d12/run.py

@@ -0,0 +1,56 @@
+import sys
+from grid import *
+
+G = [list(l) for l in sys.stdin.read().strip().split("\n")]
+
+def build_zone(G, x:int, y:int, zone:set[tuple[int,int]]):
+    zone.add((x, y))
+    v = get_in_grid(G, x, y)
+    for dx, dy in MOVE4:
+        xn = x +dx
+        yn = y + dy
+        if (xn, yn) not in zone:
+            if get_in_grid(G, xn, yn) == v:
+                build_zone(G, xn, yn, zone)
+
+def perimeter(zone:set[tuple[int,int]]) -> int:
+    return sum (
+        (x+dx, y+dy) not in zone
+        for (x,y) in zone
+        for (dx, dy) in MOVE4)
+
+def corners(zone:set[tuple[int,int]]) -> int:
+    corners = 0
+    for x,y in zone:
+        # define tile neighbourhood
+        [N,E,S,W] = [(x+dx,y+dy)in zone for (dx,dy) in MOVE4]
+        [SE,NW,NE,SW] = [(x+dx,y+dy)in zone for (dx,dy) in DIAG4]
+        # alone == no adjacent
+        if not (W or N or E or S): corners+=4
+        # touches zone on just 1 side
+        if (W+N+E+S==1): corners+=2
+        # outside corners
+        if (S and E and not N and not W): corners+=1
+        if (S and W and not N and not E): corners+=1
+        if (N and E and not S and not W): corners+=1
+        if (N and W and not S and not E): corners+=1
+        # inside corners
+        if (E and N and not NE): corners+=1
+        if (E and S and not SE): corners+=1
+        if (W and N and not NW): corners+=1
+        if (W and S and not SW): corners+=1
+    return corners
+
+T1 = T2 = 0
+checked = set()
+W, H = grid_geom(G)
+for y in range(W):
+    for x in range(H):
+        if (x,y) in checked: continue
+        zone = set()
+        build_zone(G, x, y, zone)
+        area = len(zone)
+        T1 += area * perimeter(zone)
+        T2 += area * corners(zone)
+        checked |= zone
+print(T1, T2)