Use em_delete in key_delete (#504)

* Test existing behavior

Typing Ctrl-D ends editing but typing <Del> does not.

Also renamed a test that is not testing ed_delete_next_char but
key_delete.

* Check if line empty first in em_delete

By distributivity of AND over OR, we can factor out this condition. This
will make the next commit simpler.

* Use em_delete in key_delete

When the editing mode is emacs, use `em_delete` in `key_delete`. We need
to add a condition though to `em_delete`, because it implements both
`delete-char` and `end-of-file`. We only want the `end-of-file` behavior
is the key is really Ctrl-D.

This matches the behavior of the <Del> key with readline, i.e. deleting
the next character if there is one, but not moving the cursor, while not
finishing the editing if there are no characters.

lib/reline/line_editor.rb

@@ -1944,8 +1944,10 @@ def finish
   end
 
   private def key_delete(key)
-    if @config.editing_mode_is?(:vi_insert, :emacs)
+    if @config.editing_mode_is?(:vi_insert)
       ed_delete_next_char(key)
+    elsif @config.editing_mode_is?(:emacs)
+      em_delete(key)
     end
   end
 
@@ -2651,7 +2653,7 @@ def finish
   alias_method :kill_whole_line, :em_kill_line
 
   private def em_delete(key)
-    if (not @is_multiline and @line.empty?) or (@is_multiline and @line.empty? and @buffer_of_lines.size == 1)
+    if @line.empty? and (not @is_multiline or @buffer_of_lines.size == 1) and key == "\C-d".ord
       @line = nil
       if @buffer_of_lines.size > 1
         scroll_down(@highest_in_all - @first_line_started_from)

test/reline/test_key_actor_emacs.rb

@@ -428,6 +428,12 @@ def test_em_delete_for_mbchar_by_plural_code_points
     assert_line("き\u3099")
   end
 
+  def test_em_delete_ends_editing
+    input_keys("\C-d") # quit from inputing
+    assert_line(nil)
+    assert(@line_editor.finished?)
+  end
+
   def test_ed_clear_screen
     refute(@line_editor.instance_variable_get(:@cleared))
     input_keys("\C-l", false)
@@ -449,7 +455,7 @@ def test_ed_clear_screen_with_inputed
     assert_line('abc')
   end
 
-  def test_ed_delete_next_char
+  def test_key_delete
     input_keys('abc')
     assert_cursor(3)
     assert_cursor_max(3)
@@ -459,6 +465,25 @@ def test_ed_delete_next_char
     assert_line('abc')
   end
 
+  def test_key_delete_does_not_end_editing
+    @line_editor.input_key(Reline::Key.new(:key_delete, :key_delete, false))
+    assert_cursor(0)
+    assert_cursor_max(0)
+    assert_line('')
+    refute(@line_editor.finished?)
+  end
+
+  def test_key_delete_preserves_cursor
+    input_keys('abc')
+    input_keys("\C-b", false)
+    assert_cursor(2)
+    assert_cursor_max(3)
+    @line_editor.input_key(Reline::Key.new(:key_delete, :key_delete, false))
+    assert_cursor(2)
+    assert_cursor_max(2)
+    assert_line('ab')
+  end
+
   def test_em_next_word
     assert_byte_pointer_size('')
     assert_cursor(0)