reimplement vi's 'j' motion, and remove the original function for this motion which was in cursor.sml

fcore/normal-mode/normal-mode.sml

@@ -147,7 +147,7 @@ struct
   fun parseChr (app: app_type, count, chr, str, time) =
     case chr of
       #"h" => MoveViH.move (app, count)
-    | #"j" => MoveViJ.move (app, count)
+    | #"j" => NormalMove.moveCursorDown (app, count)
     | #"k" => NormalMove.moveCursorUp (app, count)
     | #"l" => MoveViL.move (app, count)
     | #"w" => MoveToNextWord.move (app, count)

fcore/normal-mode/normal-move.sml

@@ -132,6 +132,7 @@ struct
         else
           LineGap.idxToLineNumber (cursorIdx, buffer)
       val newCursorLineNumber = Int.max (cursorLineNumber - count, 0)
+      val column = if newCursorLineNumber = 0 then column - 1 else column
 
       val buffer = LineGap.goToLine (newCursorLineNumber, buffer)
       val lineIdx = LineGap.lineNumberToIdx (newCursorLineNumber, buffer)
@@ -143,7 +144,6 @@ struct
       val buffer = LineGap.goToIdx (lineIdx, buffer)
       val endOfLineIdx = Cursor.viDlr (buffer, lineIdx, 1)
 
-      val column = if newCursorLineNumber = 0 then column - 1 else column
       val cursorIdx = Int.min (endOfLineIdx, lineIdx + column)
 
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
@@ -188,6 +188,94 @@ struct
         )
     end
 
+  fun moveCursorDown (app: app_type, count) =
+    let
+      val
+        { windowWidth
+        , windowHeight
+        , cursorIdx
+        , buffer
+        , startLine = prevLineNumber
+        , searchList
+        , searchString
+        , bufferModifyTime
+        , visualScrollColumn = prevScrollColumn
+        , ...
+        } = app
+
+      (* calculate new idx to move to *)
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+      val startOfLine = Cursor.vi0 (buffer, cursorIdx)
+      val column = cursorIdx - startOfLine
+
+      val cursorLineNumber =
+        if Cursor.isNextChrEndOfLine (buffer, cursorIdx) then
+          LineGap.idxToLineNumber (cursorIdx + 1, buffer)
+        else
+          LineGap.idxToLineNumber (cursorIdx, buffer)
+
+      val newCursorLineNumber = cursorLineNumber + count
+      val newCursorLineNumber =
+        if cursorLineNumber >= #lineLength buffer - 2 then
+          Int.max (0, #lineLength buffer - 2)
+        else
+          newCursorLineNumber
+
+      val buffer = LineGap.goToLine (newCursorLineNumber, buffer)
+      val lineIdx = LineGap.lineNumberToIdx (newCursorLineNumber, buffer)
+      val buffer = LineGap.goToIdx (lineIdx, buffer)
+      val lineIdx =
+        if Cursor.isNextChrEndOfLine (buffer, lineIdx) then lineIdx
+        else lineIdx + 1
+
+      val buffer = LineGap.goToIdx (lineIdx, buffer)
+      val endOfLineIdx = Cursor.viDlr (buffer, lineIdx, 1)
+
+      val cursorIdx = Int.min (endOfLineIdx, lineIdx + column)
+
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+
+      (* create draw message *)
+      val visualScrollColumn =
+        TextScroll.getScrollColumn
+          (buffer, cursorIdx, windowWidth, prevScrollColumn)
+
+      val startLine =
+        TextScroll.getStartLine
+          (prevLineNumber, newCursorLineNumber, windowHeight)
+
+      val buffer = LineGap.goToLine (startLine, buffer)
+
+      val drawMsg = NormalModeTextBuilder.build
+        ( startLine
+        , cursorIdx
+        , buffer
+        , windowWidth
+        , windowHeight
+        , searchList
+        , searchString
+        , visualScrollColumn
+        )
+      val drawMsg = Vector.concat drawMsg
+      val drawMsg = DrawMsg.DRAW_TEXT drawMsg
+      val drawMsg = [MailboxType.DRAW drawMsg]
+
+      val mode = NORMAL_MODE ""
+    in
+      NormalModeWith.bufferAndCursorIdx
+        ( app
+        , buffer
+        , cursorIdx
+        , mode
+        , startLine
+        , searchList
+        , drawMsg
+        , bufferModifyTime
+        , visualScrollColumn
+        )
+    end
+
+
   fun moveToLine (app: app_type, reqLine) =
     let
       val reqLine = reqLine - 1