add function to get the appropriate scroll column

fcore/text-scroll.sml

@@ -0,0 +1,22 @@
+structure TextScroll =
+struct
+  structure TC = TextConstants
+
+  (* Preqreuisite: move buffer to cursorIdx *)
+  fun getScrollColumn (buffer, cursorIdx, windowWidth) =
+    let
+      val startOfLine = Cursor.vi0 (buffer, cursorIdx)
+      val columnDifference = cursorIdx - startOfLine
+    in
+      if columnDifference = 0 then
+        0
+      else
+        let
+          val howManyColumnsCanWeFit =
+            if windowWidth >= TC.textLineWidth then TC.textLineCount
+            else windowWidth div TC.xSpace
+        in
+          columnDifference mod howManyColumnsCanWeFit
+        end
+    end
+end