Add solution to 2025-12-04

Author: fuglede

2025/day04/solutions.py

@@ -0,0 +1,15 @@
+with open("input") as f:
+    ls = f.read().strip().split("\n")
+
+paper = {i + 1j * j for i, l in enumerate(ls) for j, c in enumerate(l) if c == "@"}
+octdir = {1, 1j, -1, -1j, 1 + 1j, 1 - 1j, -1 + 1j, -1 - 1j}
+removed = []
+while to_remove := {z for z in paper if len({z + dz for dz in octdir} & paper) < 4}:
+    removed.append(len(to_remove))
+    paper -= to_remove
+
+# Part 1
+print(removed[0])
+
+# Part 2
+print(sum(removed))