Implement indent-aware delete (#1120)

* delete character backward can make undent behavior

* improve to handle mixed indentation

helix-term/src/commands.rs

@@ -10,10 +10,11 @@ use helix_core::{
     object, pos_at_coords,
     regex::{self, Regex, RegexBuilder},
     register::Register,
-    search, selection, surround, textobject, LineEnding, Position, Range, Rope, RopeGraphemes,
-    RopeSlice, Selection, SmallVec, Tendril, Transaction,
+    search, selection, surround, textobject,
+    unicode::width::UnicodeWidthChar,
+    LineEnding, Position, Range, Rope, RopeGraphemes, RopeSlice, Selection, SmallVec, Tendril,
+    Transaction,
 };
-
 use helix_view::{
     clipboard::ClipboardType,
     document::{Mode, SCRATCH_BUFFER_NAME},
@@ -4014,19 +4015,70 @@ pub mod insert {
         doc.apply(&transaction, view.id);
     }
 
-    // TODO: handle indent-aware delete
     pub fn delete_char_backward(cx: &mut Context) {
         let count = cx.count();
         let (view, doc) = current!(cx.editor);
         let text = doc.text().slice(..);
+        let indent_unit = doc.indent_unit();
+        let tab_size = doc.tab_width();
+
         let transaction =
             Transaction::change_by_selection(doc.text(), doc.selection(view.id), |range| {
                 let pos = range.cursor(text);
+                let line_start_pos = text.line_to_char(range.cursor_line(text));
+                // considier to delete by indent level if all characters before `pos` are indent units.
+                let fragment = Cow::from(text.slice(line_start_pos..pos));
+                if !fragment.is_empty() && fragment.chars().all(|ch| ch.is_whitespace()) {
+                    if text.get_char(pos.saturating_sub(1)) == Some('\t') {
+                        // fast path, delete one char
+                        (
+                            graphemes::nth_prev_grapheme_boundary(text, pos, 1),
+                            pos,
+                            None,
+                        )
+                    } else {
+                        let unit_len = indent_unit.chars().count();
+                        // NOTE: indent_unit always contains 'only spaces' or 'only tab' according to `IndentStyle` definition.
+                        let unit_size = if indent_unit.starts_with('\t') {
+                            tab_size * unit_len
+                        } else {
+                            unit_len
+                        };
+                        let width: usize = fragment
+                            .chars()
+                            .map(|ch| {
+                                if ch == '\t' {
+                                    tab_size
+                                } else {
+                                    // it can be none if it still meet control characters other than '\t'
+                                    // here just set the width to 1 (or some value better?).
+                                    ch.width().unwrap_or(1)
+                                }
+                            })
+                            .sum();
+                        let mut drop = width % unit_size; // round down to nearest unit
+                        if drop == 0 {
+                            drop = unit_size
+                        }; // if it's already at a unit, consume a whole unit
+                        let mut chars = fragment.chars().rev();
+                        let mut start = pos;
+                        for _ in 0..drop {
+                            // delete up to `drop` spaces
+                            match chars.next() {
+                                Some(' ') => start -= 1,
+                                _ => break,
+                            }
+                        }
+                        (start, pos, None) // delete!
+                    }
+                } else {
+                    // delete char
                     (
                         graphemes::nth_prev_grapheme_boundary(text, pos, count),
                         pos,
                         None,
                     )
+                }
             });
         doc.apply(&transaction, view.id);
     }