in generated export string, use structure name which is based on the name of the input file

fcore/common-update.sml

@@ -69,7 +69,7 @@ struct
       val squares = LayerTree.flatten (maxSide, layerTree)
 
       val exportString =
-        CollisionTree.toExportString (squares, canvasWidth, canvasHeight)
+        CollisionTree.toExportString (squares, canvasWidth, canvasHeight, openFilePath)
       val msg = EXPORT_SQUARES {output = exportString, filepath = openFilePath}
     in
       (model, [FILE msg])

fcore/quad-tree.sml

@@ -585,7 +585,7 @@ struct
       item :: acc
     end
 
-  fun toExportString (squares, canvasWidth, canvasHeight) =
+  fun toExportString (squares, canvasWidth, canvasHeight, filepath) =
     let
       val size = Int.max (canvasWidth, canvasHeight)
       val qtree = buildTree (0, 0, size, squares)
@@ -594,10 +594,12 @@ struct
       val f = toExportStringFolder (canvasWidth, canvasHeight)
       val coords = BinTree.foldr (f, bintree, [])
       val coords = String.concatWith ",\n" coords
+
+      val structureName = FileString.filenameToStructureName filepath
+      val structureStart = String.concat ["structure ", structureName, " = struct\n"]
     in
       String.concat
-        [ "structure AAA = \nstruct\n"
-
+        [ structureStart
         , "  fun xToNdc (xOffset, xpos, scale, halfWidth) =\n"
         , "    ((xpos * scale + xOffset) - halfWidth) / halfWidth\n\n"