Day 4: Part 2

day4/main.py

@@ -1,5 +1,5 @@
 with open("day4/input.txt") as file:
-    paperrolls = [str(line.strip("\n")) for line in file.readlines()]
+    paperrolls = [list(line.strip("\n")) for line in file.readlines()]
 
 
 h = len(paperrolls)
@@ -7,18 +7,32 @@ w = len(paperrolls[0]) if h>0 else 0
 
 dirs = [(-1,-1),(-1,0),(-1,1),(0,-1),(0,1),(1,-1),(1,0),(1,1)]
 
-def neighbors_count(r,c):
+def neighbor_count(g, r, c):
     cnt = 0
     for dr, dc in dirs:
         rr, cc = r+dr, c+dc
-        if 0 <= rr < h and 0 <= cc < w and paperrolls[rr][cc] == '@':
+        if 0 <= rr < h and 0 <= cc < w and g[rr][cc] == '@':
             cnt += 1
     return cnt
 
-accessible = 0
+# total rolls
+total = sum(row.count('@') for row in paperrolls)
+
+# compute 4-core complement by iterative simultaneous removal
+g = [row[:] for row in paperrolls]
+removed = 0
+while True:
+    to_remove = []
     for r in range(h):
         for c in range(w):
-        if paperrolls[r][c] == '@' and neighbors_count(r,c) < 4:
+            if g[r][c] == '@' and neighbor_count(g, r, c) < 4:
-            accessible += 1
+                to_remove.append((r,c))
+    if not to_remove:
+        break
+    removed += len(to_remove)
+    for r,c in to_remove:
+        g[r][c] = '.'
 
-print(accessible)
+print("Total rolls:", total)
+print("Removable (eventually):", removed)
+print("Remaining in 4-core:", total - removed)