With some help from chat I sorted out how to randomize rooms without overlap.

src/algos/maze.hpp

@@ -9,25 +9,28 @@ namespace maze {
     Matrix& $walls;
     std::vector<Room>& $rooms;
     std::vector<Point>& $dead_ends;
+    std::unordered_map<Point, bool> $ends_map;
 
     Builder(Map& map) :
       $walls(map.$walls), $rooms(map.$rooms), $dead_ends(map.$dead_ends)
     {
-      init();
+      clear();
     }
 
-    void init();
+    void clear();
     void hunt_and_kill(Point on={1,1});
     void place_rooms();
     void ensure_doors();
     void enclose();
-    void randomize_rooms();
+    void randomize_rooms(size_t room_size);
     void inner_donut(float outer_rad, float inner_rad);
     void inner_box(size_t outer_size, size_t inner_size);
     void divide(Point start, Point end);
     void remove_dead_ends();
     void dump(const std::string& msg);
     void perimeter(size_t x, size_t y, size_t width, size_t height, std::function<void(size_t x, size_t y)> cb);
-    void open_box(size_t outer_size, size_t inner_size);
+    void open_box(size_t outer_size);
+    void add_dead_end(Point at);
+    bool room_should_exist(Room& room);
   };
 }