Make Grid iterable

lib/include/aocpp/Grid.hpp

@@ -1,7 +1,6 @@
 #ifndef AOCPP_GRID_HPP_
 #define AOCPP_GRID_HPP_
 
-#include <concepts>
 #include <iostream>
 #include <stdexcept>
 #include <type_traits>
@@ -12,6 +11,31 @@
 
 namespace aocpp {
 
+struct Grid;
+
+struct GridElement {
+  const Coord pos;
+  char elt;
+};
+
+class GridIterator {
+  Grid const& grid_;
+  std::int64_t x_, y_;
+
+  public:
+  GridIterator(Grid const& grid, std::int64_t x, std::int64_t y)
+  : grid_{grid}, x_{x}, y_{y} {}
+
+  auto operator++() -> GridIterator&;
+
+  auto operator*() -> GridElement;
+
+  auto operator==(GridIterator const& rhs) const -> bool
+  {
+    return x_ == rhs.x_ && y_ == rhs.y_;
+  }
+};
+
 struct Grid {
   std::vector<std::string> rows;
   std::size_t columns;
@@ -57,28 +81,16 @@ struct Grid {
         }
         result.rows.emplace_back(std::move(line));
       }
-    } else {
-      result.columns = 0;
     }
     return result;
   }
 
-  /**
-   * @brief Apply the callback function across all elements of the grid.
-   * 
-   * @tparam F Type of callback
-   * @param f Callback function
-   */
-  template <std::invocable<Coord, char> F>
-  auto each(F f = F{}) const -> void {
-    auto const h = rows.size();
-    for (std::size_t y = 0; y < h; y++) {
-      auto const& row = rows[y];
-      auto const w = row.size();
-      for (std::size_t x = 0; x < w; x++) {
-        f(Coord{static_cast<std::int64_t>(x), static_cast<std::int64_t>(y)}, row[x]);
-      }
-    }
+  auto begin() const -> GridIterator {
+    return GridIterator{*this, 0, 0};
+  }
+
+  auto end() const -> GridIterator {
+    return GridIterator{*this, 0, static_cast<std::int64_t>(rows.size())};
   }
 };