Iterators are now working far more reliably and have more extensive tests that randomize inputs and fuzz them to check they keep working.

matrix.hpp

@@ -12,7 +12,6 @@ namespace matrix {
     size_t y = ~0;
     size_t width = 0;
     size_t height = 0;
-    int cell = 0;
 
     each_cell(Matrix &mat);
     bool next();
@@ -24,13 +23,25 @@ namespace matrix {
     size_t y = ~0;
     size_t width = 0;
     size_t height = 0;
-    int cell = 0;
     bool row = false;
 
     each_row(Matrix &mat);
     bool next();
   };
 
+  struct in_box {
+    size_t x = 0; // these are set in constructor
+    size_t y = 0; // again, no fancy ~ trick needed
+    size_t left = 0;
+    size_t top = 0;
+    size_t right = 0;
+    size_t bottom = 0;
+
+    in_box(Matrix &mat, size_t x, size_t y, size_t size);
+    bool next();
+    void dump();
+  };
+
   /*
    * Just a quick thing to reset a matrix to a value.
    */