Improve dedent behavior (#1232)

* tmp add code for dedent

* finish normal_mode with dedent behavior

* use function pointer

* rebase from origin

* check dedent condition inside normal_mode implementation

* using if let...

* fix check

* using char_is_whitespace instead of ch.is_whitespace

* fix clippy

* abstract restore_indent function

helix-term/src/commands.rs

@@ -3610,6 +3610,7 @@ fn normal_mode(cx: &mut Context) {
 
     doc.mode = Mode::Normal;
 
+    try_restore_indent(doc, view.id);
     doc.append_changes_to_history(view.id);
 
     // if leaving append mode, move cursor back by 1
@@ -3627,6 +3628,36 @@ fn normal_mode(cx: &mut Context) {
     }
 }
 
+fn try_restore_indent(doc: &mut Document, view_id: ViewId) {
+    let doc_changes = doc.changes().changes();
+    let text = doc.text().slice(..);
+    let pos = doc.selection(view_id).primary().cursor(text);
+    let mut can_restore_indent = false;
+
+    // Removes trailing whitespace if insert mode is exited after starting a blank new line.
+    use helix_core::chars::char_is_whitespace;
+    use helix_core::Operation;
+    if let [Operation::Retain(move_pos), Operation::Insert(ref inserted_str), Operation::Retain(_)] =
+        doc_changes
+    {
+        if move_pos + inserted_str.len32() as usize == pos
+            && inserted_str.starts_with('\n')
+            && inserted_str.chars().skip(1).all(char_is_whitespace)
+        {
+            can_restore_indent = true;
+        }
+    }
+
+    if can_restore_indent {
+        let transaction =
+            Transaction::change_by_selection(doc.text(), doc.selection(view_id), |range| {
+                let line_start_pos = text.line_to_char(range.cursor_line(text));
+                (line_start_pos, pos, None)
+            });
+        doc.apply(&transaction, view_id);
+    }
+}
+
 // Store a jump on the jumplist.
 fn push_jump(editor: &mut Editor) {
     let (view, doc) = current!(editor);

helix-view/src/document.rs

@@ -889,6 +889,10 @@ impl Document {
         self.indent_style.as_str()
     }
 
+    pub fn changes(&self) -> &ChangeSet {
+        &self.changes
+    }
+
     #[inline]
     /// File path on disk.
     pub fn path(&self) -> Option<&PathBuf> {