Started prototyping a viewport iterator but couldn't get it right so I'll test drive it next.

gui.cpp

@@ -98,7 +98,8 @@ void MapViewUI::draw_map() {
   size_t end_y = std::min(size_t(height), $game_map.height() - start.y);
 
   for(size_t y = 0; y < end_y; ++y) {
-    for(size_t x = 0; x < end_x; ++x) {
+    for(size_t x = 0; x < end_x; ++x)
+    {
       const TileCell& tile = tiles.at(start.x+x, start.y+y);
       // light value is an integer that's a percent
       float light_value = debug.LIGHT ? 80 * PERCENT : lighting[start.y+y][start.x+x] * PERCENT;

matrix.hpp

@@ -78,6 +78,41 @@ namespace matrix {
     }
   };
 
+  template<typename MAT>
+  struct viewport_t {
+    Point start;
+    // this is the point in the map
+    size_t x;
+    size_t y;
+    // this is the point inside the box, start at 0
+    size_t view_x = ~0;
+    size_t view_y = ~0;
+    // viewport width/height
+    size_t width;
+    size_t height;
+
+    viewport_t(MAT &mat, Point start, int max_x, int max_y) :
+      start(start),
+      x(start.x-1),
+      y(start.y-1)
+    {
+      width = std::min(size_t(max_x), matrix::width(mat) - start.x);
+      height = std::min(size_t(max_y), matrix::height(mat) - start.y);
+      fmt::println("viewport_t max_x, max_y {},{} vs matrix {},{}, x={}, y={}",
+          max_x, max_y, matrix::width(mat), matrix::height(mat), x, y);
+    }
+
+    bool next() {
+      y = next_y(x, y);
+      x = next_x(x, width);
+      view_x = next_x(view_x, width);
+      view_y = next_y(view_x, view_y);
+      return at_end(y, height);
+    }
+  };
+
+  using viewport = viewport_t<Matrix>;
+
   using each_cell = each_cell_t<Matrix>;
 
   template<typename MAT>