save grid to custom format

fcore/quad-tree.sml

@@ -520,19 +520,25 @@ struct
         , Int.toString b
         , " "
         , Int.toString a
-        , " }"
+        , " } "
         ]
     in
       item :: acc
     end
 
-  fun toString (squares, size) =
+  fun toString (squares, canvasWidth, canvasHeight) =
     let
+      val size = Int.max (canvasWidth, canvasHeight)
       val qtree = buildTree (0, 0, size, squares)
       val bintree = merge (qtree, squares)
 
-      val vec = BinTree.foldr (toStringFolder, bintree, [])
+      val initial = ["}"]
+      val acc = BinTree.foldr (toStringFolder, bintree, initial)
+      val acc =
+        String.concat
+          [Int.toString canvasWidth, " ", Int.toString canvasHeight, " { "]
+        :: acc
     in
-      Vector.concat vec
+      String.concat acc
     end
 end