day[04]: using `size_t` for managing sizes

src/04/solution.cxx

···
+#include <cstddef>
 #include <filesystem>
 #include <fstream>
 #include <print>
···
     }
   };
 
-  unsigned long width() { return data[0].length(); }
+  std::size_t width() { return data[0].length(); }
 
-  unsigned long height() { return data.size(); }
+  std::size_t height() { return data.size(); }
 
-  std::string &operator[](unsigned long idx) { return data[idx]; }
+  std::string &operator[](std::size_t idx) { return data[idx]; }
 
-  const std::string &operator[](unsigned long idx) const { return data[idx]; }
+  const std::string &operator[](std::size_t idx) const { return data[idx]; }
 
-  std::string get_surrounding(unsigned long row, unsigned long col) {
+  std::string get_surrounding(std::size_t row, std::size_t col) {
     std::string sur = "";
     sur.resize(8);
-    unsigned long wid = width();
+    std::size_t wid = width();
     auto hi = height();
 
     if (row > 0 && col > 0) {
···
 };
 
 struct Roll {
-  unsigned long row;
-  unsigned long col;
+  std::size_t row;
+  std::size_t col;
 };
 
 std::vector<Roll> get_removable_rolls(Matrix &data) {
   std::vector<Roll> rolls{};
-  for (unsigned long row = 0; row < data.height(); ++row) {
-    for (unsigned long col = 0; col < data.width(); ++col) {
+  for (std::size_t row = 0; row < data.height(); ++row) {
+    for (std::size_t col = 0; col < data.width(); ++col) {
       if (data[row][col] != '@') {
         continue;
       }