done refactoring 'startOfCurrentWORD' function, with tests still passing

fcore/cursor-dfa/vi-WORD-dfa.sml

@@ -108,16 +108,14 @@ struct
              end
        end)
 
-  structure StartOfCurrentWORD =
-    MakePrevDfaLoop
-      (struct
-         val startState = startState
+  val startOfNextWORD = StartOfNextWORD.next
+  val endOfPrevWORD = EndOfPrevWORD.prev
 
-         fun fStart (idx, absIdx, str, tl, currentState, counter) =
+  fun loopStartOfCurrentWORD (idx, absIdx, str, tl, currentState, counter) =
     if idx < 0 then
       case tl of
         str :: tl =>
-                 fStart
+          loopStartOfCurrentWORD
             (String.size str - 1, absIdx, str, tl, currentState, counter)
       | [] => 0
     else
@@ -129,18 +127,33 @@ struct
           if counter - 1 = 0 then
             absIdx + 1
           else
-                   fStart
+            loopStartOfCurrentWORD
               (idx - 1, absIdx - 1, str, tl, startState, counter - 1)
         else
-                 fStart (idx - 1, absIdx - 1, str, tl, newState, counter)
+          loopStartOfCurrentWORD
+            (idx - 1, absIdx - 1, str, tl, newState, counter)
       end
-       end)
-
-  val startOfNextWORD = StartOfNextWORD.next
-  val endOfPrevWORD = EndOfPrevWORD.prev
 
+  (* we do not rely on MakeDfaLoop functor because we want to 
+   * ignore the character the cursor is currently on,
+   * and start tracking state from the character before the current one. *)
   fun startOfCurrentWORD (lineGap: LineGap.t, cursorIdx) =
-    let val lineGap = LineGap.goToIdx (cursorIdx - 1, lineGap)
-    in StartOfCurrentWORD.prev (lineGap, cursorIdx - 1)
+    let
+      val {idx = bufferIdx, leftStrings, ...} = lineGap
+      val strIdx = cursorIdx - bufferIdx - 1
+      val absIdx = cursorIdx - 1
+    in
+      if strIdx < 0 then
+        case leftStrings of
+          lhd :: ltl =>
+            loopStartOfCurrentWORD
+              (String.size lhd - 1, absIdx, lhd, ltl, startState, 1)
+        | [] => 0
+      else
+        case #rightStrings lineGap of
+          rhd :: _ =>
+            loopStartOfCurrentWORD
+              (strIdx, absIdx, rhd, leftStrings, startState, 1)
+        | [] => Int.max (0, cursorIdx - 2)
     end
 end