day[04]: final touches to Matrix API

src/04/solution.cxx

···
 
   unsigned long height() { return data.size(); }
 
-  std::string operator[](unsigned long idx) { return data[idx]; }
+  std::string &operator[](unsigned long idx) { return data[idx]; }
+
+  const std::string &operator[](unsigned long idx) const { return data[idx]; }
 
   std::string get_surrounding(unsigned long row, unsigned long col) {
     std::string sur = "";
···
     unsigned long wid = width();
     auto hi = height();
 
-    if (row > 0 && col - 1 >= 0) {
+    if (row > 0 && col > 0) {
       sur += data[row - 1][col - 1];
     }
     if (row > 0) {
···
     return sur;
   }
 
-  void set(unsigned long row, unsigned long col, char ch) {
-    data[row][col] = ch;
-  }
-
 private:
   std::vector<std::string> data{};
 };
···
       if (count >= 4) {
         continue;
       }
-      auto roll = Roll {
-        .row = row,
-        .col = col,
+      auto roll = Roll{
+          .row = row,
+          .col = col,
       };
       rolls.push_back(roll);
     }
···
 
 int remove_rolls(Matrix &data, std::span<Roll> removables) {
   for (auto roll : removables) {
-    data.set(roll.row, roll.col, '.');
+    data[roll.row][roll.col] = '.';
   }
 
   return removables.size();