preparation for functorising quad tree

fcore/quad-tree.sml

@@ -87,6 +87,11 @@ end
 
 structure CollisionTree =
 struct
+  (* ignoreData = any data we find in grid but which doesn't concern us
+   * so we omit from tree, to reduce number of items in tree,
+   * and therefore decrease the constant in O(n) *)
+  val ignoreData = 0
+
   local
     fun loopYAxis (x, y, eX, eY, yAxis, col) =
       if y > eY orelse y >= Vector.length yAxis then
@@ -129,12 +134,18 @@ struct
       let
         val yAxis = Vector.sub (grid, x)
         val data = Vector.sub (yAxis, y)
+      in
+        if data = ignoreData then
+          bintree
+        else
+          let
             val ex = x + size
             val ey = y + size
             val item = {x = x, y = y, ex = ex, ey = ey, data = data}
           in
             BinTree.insert (item, bintree)
           end
+      end
     else
       let
         (* handles odd-number divisions.
@@ -164,9 +175,6 @@ struct
       , xClickPoints
       , yClickPoints
       ) ({x, ex, y, ey, data}, acc) =
-      if data = 0 then
-        acc
-      else
       let
         val ex = if ex = x then x + 1 else ex
         val ey = if ey = y then y + 1 else ey