Performant and correct set_spans_truncated

the previous implementation used set_string_truncated. This is not only
awkward with this kind of "streaming" string (and therefore lead to an
inefficient and incorrect initial implementation) but that function also
truncates strings of width 1 when there is only a single char available.

The implementation here is performant, correct and also handles the
single width case correctly.

helix-tui/src/buffer.rs

@@ -434,28 +434,44 @@ pub fn set_string_truncated_at_end(
     }
 
     pub fn set_spans_truncated(&mut self, x: u16, y: u16, spans: &Spans, width: u16) -> (u16, u16) {
-        let mut remaining_width = width;
-        let mut alt_x = x;
-        let (text, styles) =
-            spans
-                .0
-                .iter()
-                .fold((String::new(), vec![]), |(mut s, mut h), span| {
-                    s.push_str(span.content.as_ref());
-                    let mut styles = span
-                        .styled_graphemes(span.style)
-                        .map(|grapheme| grapheme.style)
-                        .collect();
-                    h.append(&mut styles);
+        // prevent panic if out of range
+        if !self.in_bounds(x, y) || width == 0 {
+            return (x, y);
+        }
 
-                    let w = span.width() as u16;
-                    alt_x = alt_x + w;
-                    remaining_width = remaining_width.saturating_sub(w);
+        let mut x_offset = x as usize;
+        let max_offset = min(self.area.right(), width.saturating_add(x));
+        let mut start_index = self.index_of(x, y);
+        let mut index = self.index_of(max_offset as u16, y);
 
-                    (s, h)
-                });
-        self.set_string_truncated(x, y, &text, width.into(), |idx| styles[idx], true, true);
-        (x, y)
+        let content_width = spans.width();
+        let truncated = content_width > width as usize;
+        if truncated {
+            self.content[start_index].set_symbol("…");
+            start_index += 1;
+        } else {
+            index -= width as usize - content_width;
+        }
+        for span in spans.0.iter().rev() {
+            for s in span.content.graphemes(true).rev() {
+                let width = s.width();
+                if width == 0 {
+                    continue;
+                }
+                let start = index - width;
+                if start < start_index {
+                    break;
+                }
+                self.content[start].set_symbol(s);
+                self.content[start].set_style(span.style);
+                for i in start + 1..index {
+                    self.content[i].reset();
+                }
+                index -= width;
+                x_offset += width;
+            }
+        }
+        (x_offset as u16, y)
     }
 
     pub fn set_spans(&mut self, x: u16, y: u16, spans: &Spans, width: u16) -> (u16, u16) {