Solve day 4 part 2

day_4.cpp

@@ -8,14 +8,8 @@ struct position
 	int y;
 };
 
-int main()
+int remove_paper_rolls(std::vector<std::string> &lines)
 {
-	std::ifstream file("input.txt");
-
-	std::vector<std::string> lines;
-	std::string line;
-	while (getline(file, line)) lines.push_back(line);
-
 	const position positions[] = {
 		{ -1, -1 },
 		{ 0, -1 },
@@ -27,26 +21,57 @@ int main()
 		{ 1, 1 },
 	};
 
-	int result = 0;
+	int removed_count = 0;
+
+	std::vector<std::string> new_lines;
 
 	for (int y = 0; y < lines.size(); y++) {
+		new_lines.push_back("");
+
 		for (int x = 0; x < lines[y].size(); x++) {
-			if (lines[y][x] == '.') continue;
+			if (lines[y][x] == '.') {
+				new_lines[y].append(".");
+				continue;
+			}
 
 			int amount = 0;
 
 			for (position pos : positions) {
 				int check_x = x + pos.x, check_y = y + pos.y;
 
-				if (check_x < 0 || check_y < 0 || check_x >= lines[y].size() || check_y >= line.size())
+				if (check_x < 0 || check_y < 0 || check_x >= lines[y].size() || check_y >= lines.size())
 					continue;
 
 				if (lines[check_y][check_x] == '@') amount++;
 			}
 
-			if (amount < 4) result++;
+			if (amount < 4) {
+				new_lines[y].append(".");
+				removed_count++;
+			} else {
+				new_lines[y].append("@");
+			}
 		}
 	}
 
-	std::cout << result << std::endl;
+	lines = new_lines;
+
+	return removed_count;
+}
+
+int main()
+{
+	std::ifstream file("input.txt");
+
+	std::vector<std::string> lines;
+	std::string line;
+	while (getline(file, line)) lines.push_back(line);
+
+	int removed = 0, total_removed = 0;
+	do {
+		removed = remove_paper_rolls(lines);
+		total_removed += removed;
+	} while (removed > 0);
+
+	std::cout << total_removed << std::endl;
 }