add two more test cases for 'dj' motion that were failing when added, and modify 'MakeNormalDelete.deleteLineDown' to handle them

fcore/normal-mode/make-normal-delete.sml

@@ -413,7 +413,18 @@ struct
            * to the first column of the now-last line *)
           val newEndIdx = #textLength buffer - 1
         in
-          if startIdx >= newEndIdx then
+          if newEndIdx < 0 then
+            (* deleted whole file; add newline to the end *)
+            let val buffer = LineGap.append ("\n", buffer)
+            in finishAfterDeletingBuffer (app, 0, buffer, time, initialMsg)
+            end
+          else if newEndIdx = 0 then
+            (* there is only one char left in the file *)
+            finishAfterDeletingBuffer (app, 0, buffer, time, initialMsg)
+          else if startIdx >= newEndIdx then
+            (* deleted the last part of the file such that the cursor's idx
+             * now refers to an index that no longer exists.
+             * Have to move cursor to the last line of the file. *)
             let
               val buffer = LineGap.goToIdx (newEndIdx, buffer)
             in

temp.txt

@@ -1,3 +1,4 @@
 hello
 world
-
+trello
+brillo

test/normal-delete.sml

@@ -386,6 +386,56 @@ struct
            in
              Expect.isTrue (stringIsExpected andalso cursorIdxIsExpected)
            end)
+    , test
+        "leaves a newline at the end when deleting the whole file \
+        \and file ends with a newline"
+        (fn _ =>
+           let
+             (* arrange *)
+             val originalString = "hello\nworld\n"
+             val originalIdx = 0
+
+             val app = TestUtils.init originalString
+             val app = AppWith.idx (app, originalIdx)
+
+             (* act *)
+             val {cursorIdx, buffer, ...} = TestUtils.updateMany (app, "33dj")
+
+             (* assert *)
+             val expectedString = "\n"
+             val actualString = LineGap.toString buffer
+             val stringIsExpected = expectedString = actualString
+
+             val expectedCursorIdx = 0
+             val cursorIdxIsExpected = expectedCursorIdx = cursorIdx
+           in
+             Expect.isTrue (stringIsExpected andalso cursorIdxIsExpected)
+           end)
+    , test
+        "leaves a newline at the end when deleting the whole file \
+        \and file does not end with a newline"
+        (fn _ =>
+           let
+             (* arrange *)
+             val originalString = "hello\nworld"
+             val originalIdx = 0
+
+             val app = TestUtils.init originalString
+             val app = AppWith.idx (app, originalIdx)
+
+             (* act *)
+             val {cursorIdx, buffer, ...} = TestUtils.updateMany (app, "33dj")
+
+             (* assert *)
+             val expectedString = "\n"
+             val actualString = LineGap.toString buffer
+             val stringIsExpected = expectedString = actualString
+
+             val expectedCursorIdx = 0
+             val cursorIdxIsExpected = expectedCursorIdx = cursorIdx
+           in
+             Expect.isTrue (stringIsExpected andalso cursorIdxIsExpected)
+           end)
     ]
 
   val tests = [dhDelete, dlDelete, djDelete]