progress with viJ to move cursor downwards

fcore/app-update.sml

@@ -26,15 +26,13 @@ struct
       (* move LineGap to cursorIdx, which is necessary for finding newCursorIdx *)
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val cursorIdx = Cursor.viL (buffer, cursorIdx)
-      val preferredColumn = Cursor.getCursorColumn (buffer, cursorIdx)
 
       (* move LineGap to first line displayed on screen, and build new text *)
       val buffer = LineGap.goToLine (startLine, buffer)
       val drawMsg = TextBuilder.build
         (startLine, cursorIdx, buffer, windowWidth, windowHeight)
 
-      val newApp =
-        AppWith.bufferAndCursorIdx (app, buffer, cursorIdx, preferredColumn)
+      val newApp = AppWith.bufferAndCursorIdx (app, buffer, cursorIdx)
     in
       (newApp, drawMsg)
     end
@@ -45,14 +43,28 @@ struct
 
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val cursorIdx = Cursor.viH (buffer, cursorIdx)
-      val preferredColumn = Cursor.getCursorColumn (buffer, cursorIdx)
 
       val buffer = LineGap.goToLine (startLine, buffer)
       val drawMsg = TextBuilder.build
         (startLine, cursorIdx, buffer, windowWidth, windowHeight)
 
-      val newApp =
-        AppWith.bufferAndCursorIdx (app, buffer, cursorIdx, preferredColumn)
+      val newApp = AppWith.bufferAndCursorIdx (app, buffer, cursorIdx)
+    in
+      (newApp, drawMsg)
+    end
+
+  fun moveDown (app: app_type) =
+    let
+      val {buffer, windowWidth, windowHeight, startLine, cursorIdx, ...} = app
+
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+      val cursorIdx = Cursor.viJ (buffer, cursorIdx)
+
+      val buffer = LineGap.goToLine (startLine, buffer)
+      val drawMsg = TextBuilder.build
+        (startLine, cursorIdx, buffer, windowWidth, windowHeight)
+
+      val newApp = AppWith.bufferAndCursorIdx (app, buffer, cursorIdx)
     in
       (newApp, drawMsg)
     end
@@ -60,6 +72,7 @@ struct
   fun handleChr (app: app_type, chr) =
     case chr of
       #"h" => moveLeft app
+    | #"j" => moveDown app
     | #"l" => moveRight app
     | _ => (app, [])