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
@@ -129,11 +134,17 @@ struct
       let
         val yAxis = Vector.sub (grid, x)
         val data = Vector.sub (yAxis, y)
-        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)
+        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
@@ -164,25 +175,22 @@ 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
+      let
+        val ex = if ex = x then x + 1 else ex
+        val ey = if ey = y then y + 1 else ey
 
-          val x = getClickPoint (xClickPoints, x)
-          val y = getClickPoint (yClickPoints, y)
-          val ex = getClickPoint (xClickPoints, ex)
-          val ey = getClickPoint (yClickPoints, ey)
+        val x = getClickPoint (xClickPoints, x)
+        val y = getClickPoint (yClickPoints, y)
+        val ex = getClickPoint (xClickPoints, ex)
+        val ey = getClickPoint (yClickPoints, ey)
 
-          val startX = Ndc.fromPixelX (x, windowWidth, windowHeight)
-          val endX = Ndc.fromPixelX (ex, windowWidth, windowHeight)
-          val startY = Ndc.fromPixelY (y, windowWidth, windowHeight)
-          val endY = Ndc.fromPixelY (ey, windowWidth, windowHeight)
-        in
-          Ndc.ltrbToVertexRgb (startX, startY, endX, endY, 0.0, 0.0, 0.0) :: acc
-        end
+        val startX = Ndc.fromPixelX (x, windowWidth, windowHeight)
+        val endX = Ndc.fromPixelX (ex, windowWidth, windowHeight)
+        val startY = Ndc.fromPixelY (y, windowWidth, windowHeight)
+        val endY = Ndc.fromPixelY (ey, windowWidth, windowHeight)
+      in
+        Ndc.ltrbToVertexRgb (startX, startY, endX, endY, 0.0, 0.0, 0.0) :: acc
+      end
   in
     fun toTriangles
       ( windowWidth