reimplement 'f' move motion more efficiently, not leaving the LineGap.t data structure while iterating to next count

fcore/cursor.sml

@@ -307,6 +307,55 @@ struct
   fun toNextChr (lineGap, cursorIdx, chr) =
     nextChr (lineGap, cursorIdx, chr, startToNextChr)
 
+  structure ToNextChr =
+    MakeIfCharFolderNext
+      (struct
+         type env = {findChr: char, count: int}
+
+         fun helpToNextChr
+           (strPos, str, absIdx, stl, lastFoundIdx, findChr, count) =
+           if strPos = String.size str then
+             case stl of
+               str :: stl =>
+                 helpToNextChr
+                   (0, str, absIdx, stl, lastFoundIdx, findChr, count)
+             | [] => lastFoundIdx
+           else
+             let
+               val chr = String.sub (str, strPos)
+             in
+               if chr = findChr then
+                 if count - 1 = 0 then
+                   absIdx
+                 else
+                   helpToNextChr
+                     ( strPos + 1
+                     , str
+                     , absIdx + 1
+                     , stl
+                     , absIdx
+                     , findChr
+                     , count - 1
+                     )
+               else
+                 helpToNextChr
+                   ( strPos + 1
+                   , str
+                   , absIdx + 1
+                   , stl
+                   , lastFoundIdx
+                   , findChr
+                   , count
+                   )
+             end
+
+         fun fStart (strPos, str, _, absIdx, stl, _, {findChr, count}) =
+           (* we want to start iterating from char after cursor *)
+           helpToNextChr (strPos + 1, str, absIdx + 1, stl, ~1, findChr, count)
+       end)
+
+  val toNextChrNew = ToNextChr.foldNext
+
   structure ToPrevChr =
     MakeIfCharFolderPrev
       (struct

fcore/normal-mode/normal-mode.sml

@@ -571,9 +571,7 @@ struct
         parseMoveToChr (1, app, Cursor.tillPrevChr, chrCmd)
     | #"y" => ParseYank.parseYank (strPos, str, count, app, chrCmd, time)
     | #"d" => ParseDelete.parseDelete (strPos, str, count, app, chrCmd, time)
-    | #"f" =>
-        (* to chr, forward *)
-        parseMoveToChr (count, app, Cursor.toNextChr, chrCmd)
+    | #"f" => (* to chr, forward *) NormalMove.toNextChr (app, count, chrCmd)
     | #"F" =>
         (* to chr, backward *)
         parseMoveToChr (count, app, Cursor.toPrevChr, chrCmd)

fcore/normal-mode/normal-move.sml

@@ -516,4 +516,18 @@ struct
         NormalFinish.buildTextAndClear
           (app, buffer, newCursorIdx, searchList, [], bufferModifyTime)
     end
+
+  fun toNextChr (app: app_type, count, chr) =
+    let
+      val {cursorIdx, buffer, searchList, bufferModifyTime, ...} = app
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+      val newCursorIdx =
+        Cursor.toNextChrNew (buffer, cursorIdx, {findChr = chr, count = count})
+    in
+      if newCursorIdx = ~1 then
+        NormalFinish.clearMode app
+      else
+        NormalFinish.buildTextAndClear
+          (app, buffer, newCursorIdx, searchList, [], bufferModifyTime)
+    end
 end