implement functions to get leftmost and rightmost x coordinates with mergeable items

fcore/quad-tree.sml

@@ -301,13 +301,9 @@ struct
     case tree of
       EMPTY => NONE
     | LEAF (item as {x = ix, y = iy, ex = iex, ey = iey, data = oldData}) =>
-        if data = oldData then
-          (* data matches *)
         if (x >= ix andalso x <= iex) andalso (y >= iy andalso y <= iey) then
           (* search coordinates are in item *)
-            SOME item
+          if data = oldData then (* data matches *) SOME item else NONE
-          else
-            NONE
         else
           NONE
     | NODE {tl, tr, bl, br} =>
@@ -357,4 +353,37 @@ struct
               (* bottom right *)
               getItemWithDataAt (x, y, qmx, qmy, halfSizeAfter, br, data)
           end
+
+  fun getLeftmostX (rootSize, rootTree, prevItem) =
+    let
+      val {x = prevX, y = prevY, ex = prevEx, ey = prevEy, data = prevData} =
+        prevItem
+    in
+      case
+        getItemWithDataAt (prevX - 1, prevY, 0, 0, rootSize, rootTree, prevData)
+      of
+        SOME (newItem as {y = newY, ey = newEy, data = newData, ...}) =>
+          if prevY = newY andalso newEy = prevEy andalso prevData = newData then
+            getLeftmostX (rootSize, rootTree, newItem)
+          else
+            prevX
+      | NONE => prevX
+    end
+
+  fun getRightmostX (rootSize, rootTree, prevItem) =
+    let
+      val {x = prevX, y = prevY, ex = prevEx, ey = prevEy, data = prevData} =
+        prevItem
+    in
+      case
+        getItemWithDataAt
+          (prevEx + 1, prevY, 0, 0, rootSize, rootTree, prevData)
+      of
+        SOME (newItem as {y = newY, ey = newEy, data = newData, ...}) =>
+          if prevY = newY andalso newEy = prevEy andalso prevData = newData then
+            getRightmostX (rootSize, rootTree, newItem)
+          else
+            prevEx
+      | NONE => prevEx
+    end
 end