make odd-number-division case more explicit when building quad tree

fcore/quad-tree.sml

@@ -29,15 +29,24 @@ struct
     | NODE {x = ox, y = oy, ex = oex, ey = oey, data = oldData, left, right} =>
         let
           val dir =
-            if x < ox then LESS
+            if x < ox then
-            else if x > ox then GREATER
+              LESS
-            else if y < oy then LESS
+            else if x > ox then
-            else if y > oy then GREATER
+              GREATER
-            else if ex < oex then LESS
+            else
-            else if ex > oex then GREATER
+              (if y < oy then
-            else if ey < oey then LESS
+                 LESS
+               else if y > oy then
+                 GREATER
+               else
+                 (if ex < oex then
+                    LESS
+                  else if ex > oex then
+                    GREATER
+                  else
+                    (if ey < oey then LESS
                      else if ey > oey then GREATER
-            else EQUAL
+                     else EQUAL)))
         in
           case dir of
             LESS =>
@@ -147,19 +156,36 @@ struct
           end
       end
     else
+      (if size mod 2 = 0 orelse size = 1 then
          let
-        (* handles odd-number divisions.
+           val halfSize = size div 2
-         * For example, `7 div 2` is 3 because of integer division. *)
-        val halfSize =
-          if size = 1 orelse size mod 2 = 0 then size div 2
-          else (size + 1) div 2
-
            val bintree = build (x, y, halfSize, grid, bintree)
            val bintree = build (x + halfSize, y, halfSize, grid, bintree)
            val bintree = build (x, y + halfSize, halfSize, grid, bintree)
          in
            build (x + halfSize, y + halfSize, halfSize, grid, bintree)
          end
+       else
+         (* handles odd-number divisions.
+          * For example, `7 div 2` is 3 because of integer division. 
+          * We would not cover every pixel unless we handle odd numbers specially. *)
+         let
+           val halfSizeBefore = size div 2
+           val halfSizeAfter = (size + 1) div 2
+           val bintree = build (x, y, halfSizeAfter, grid, bintree)
+           val bintree = build
+             (x + halfSizeBefore, y, halfSizeAfter, grid, bintree)
+           val bintree = build
+             (x, y + halfSizeBefore, halfSizeAfter, grid, bintree)
+         in
+           build
+             ( x + halfSizeBefore
+             , y + halfSizeBefore
+             , halfSizeAfter
+             , grid
+             , bintree
+             )
+         end)
 
   local
     fun getClickPoint (clickPoints, pos) =