Fix prompt truncation for non-ASCII lines

The prompt was previously assuming that each grapheme cluster in the
line was single-width and single-byte. Lines like the one in the new
integration test would cause panics because the anchor attempted to
slice into a character.

This change rewrites the anchor and truncation code in the prompt to
account for Unicode segmentation and width. Now multi-width graphemes
can be hidden by multiple consecutive elipses - for example "十" is
hidden by "……" (2-width).

Co-authored-by: Narazaki, Shuji <shujinarazaki@protonmail.com>

helix-term/src/ui/prompt.rs

@@ -12,7 +12,9 @@ use tui::text::Span;
 use tui::widgets::{Block, Widget};
 
 use helix_core::{
-    unicode::segmentation::GraphemeCursor, unicode::width::UnicodeWidthStr, Position,
+    unicode::segmentation::{GraphemeCursor, UnicodeSegmentation},
+    unicode::width::UnicodeWidthStr,
+    Position,
 };
 use helix_view::{
     graphics::{CursorKind, Margin, Rect},
@@ -535,21 +537,51 @@ impl Prompt {
             .into();
             text.render(self.line_area, surface, cx);
         } else {
-            if self.line.len() < self.line_area.width as usize {
+            let line_width = self.line_area.width as usize;
+
+            if self.line.width() < line_width {
                 self.anchor = 0;
-            } else if self.cursor < self.anchor {
-                self.anchor = self.cursor;
-            } else if self.cursor - self.anchor > self.line_area.width as usize {
-                self.anchor = self.cursor - self.line_area.width as usize;
+            } else if self.cursor <= self.anchor {
+                // Ensure the grapheme under the cursor is in view.
+                self.anchor = self.line[..self.cursor]
+                    .grapheme_indices(true)
+                    .next_back()
+                    .map(|(i, _)| i)
+                    .unwrap_or_default();
+            } else if self.line[self.anchor..self.cursor].width() > line_width {
+                // Set the anchor to the last grapheme cluster before the width is exceeded.
+                let mut width = 0;
+                self.anchor = self.line[..self.cursor]
+                    .grapheme_indices(true)
+                    .rev()
+                    .find_map(|(idx, g)| {
+                        width += g.width();
+                        if width > line_width {
+                            Some(idx + g.len())
+                        } else {
+                            None
+                        }
+                    })
+                    .unwrap();
             }
 
             self.truncate_start = self.anchor > 0;
-            self.truncate_end = self.line.len() - self.anchor > self.line_area.width as usize;
+            self.truncate_end = self.line[self.anchor..].width() > line_width;
 
             // if we keep inserting characters just before the end elipsis, we move the anchor
             // so that those new characters are displayed
-            if self.truncate_end && self.cursor - self.anchor >= self.line_area.width as usize {
-                self.anchor += 1;
+            if self.truncate_end && self.line[self.anchor..self.cursor].width() >= line_width {
+                // Move the anchor forward by one non-zero-width grapheme.
+                self.anchor += self.line[self.anchor..]
+                    .grapheme_indices(true)
+                    .find_map(|(idx, g)| {
+                        if g.width() > 0 {
+                            Some(idx + g.len())
+                        } else {
+                            None
+                        }
+                    })
+                    .unwrap();
             }
 
             surface.set_string_anchored(
@@ -558,7 +590,7 @@ impl Prompt {
                 self.truncate_start,
                 self.truncate_end,
                 &self.line.as_str()[self.anchor..],
-                self.line_area.width as usize - self.truncate_end as usize,
+                line_width,
                 |_| prompt_color,
             );
         }
@@ -734,17 +766,21 @@ impl Component for Prompt {
             .clip_left(self.prompt.len() as u16)
             .clip_right(if self.prompt.is_empty() { 2 } else { 0 });
 
-        let anchor = self.anchor.min(self.line.len().saturating_sub(1));
-        let mut col = area.left() as usize
-            + UnicodeWidthStr::width(&self.line[anchor..self.cursor.max(anchor)]);
+        let mut col = area.left() as usize + self.line[self.anchor..self.cursor].width();
 
         // ensure the cursor does not go beyond elipses
-        if self.truncate_end && self.cursor - self.anchor >= self.line_area.width as usize {
+        if self.truncate_end
+            && self.line[self.anchor..self.cursor].width() >= self.line_area.width as usize
+        {
             col -= 1;
         }
 
         if self.truncate_start && self.cursor == self.anchor {
-            col += 1;
+            col += self.line[self.cursor..]
+                .graphemes(true)
+                .next()
+                .unwrap()
+                .width();
         }
 
         let line = area.height as usize - 1;

helix-term/tests/test/commands.rs

@@ -820,3 +820,25 @@ async fn macro_play_within_macro_record() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread")]
+async fn global_search_with_multibyte_chars() -> anyhow::Result<()> {
+    // Assert that `helix_term::commands::global_search` handles multibyte characters correctly.
+    test((
+        indoc! {"\
+            // Hello world!
+            // #[|
+            ]#
+            "},
+        // start global search
+        " /«十分に長い マルチバイトキャラクター列» で検索<ret><esc>",
+        indoc! {"\
+            // Hello world!
+            // #[|
+            ]#
+            "},
+    ))
+    .await?;
+
+    Ok(())
+}

helix-tui/src/buffer.rs

@@ -326,43 +326,44 @@ impl Buffer {
             return (x, y);
         }
 
-        let max_offset = min(
-            self.area.right() as usize - 1,
-            width.saturating_add(x as usize),
-        );
-        let mut start_index = self.index_of(x, y);
-        let mut end_index = self.index_of(max_offset as u16, y);
-
-        if truncate_end {
-            self.content[end_index].set_symbol("…");
-            end_index -= 1;
-        }
+        let mut index = self.index_of(x, y);
+        let mut rendered_width = 0;
+        let mut graphemes = string.grapheme_indices(true);
 
         if truncate_start {
-            self.content[start_index].set_symbol("…");
-            start_index += 1;
+            for _ in 0..graphemes.next().map(|(_, g)| g.width()).unwrap_or_default() {
+                self.content[index].set_symbol("…");
+                index += 1;
+                rendered_width += 1;
+            }
         }
 
-        let graphemes = string.grapheme_indices(true);
-
-        for (byte_offset, s) in graphemes.skip(truncate_start as usize) {
-            if start_index > end_index {
+        for (byte_offset, s) in graphemes {
+            let grapheme_width = s.width();
+            if truncate_end && rendered_width + grapheme_width >= width {
                 break;
             }
-            let width = s.width();
-            if width == 0 {
+            if grapheme_width == 0 {
                 continue;
             }
 
-            self.content[start_index].set_symbol(s);
-            self.content[start_index].set_style(style(byte_offset));
+            self.content[index].set_symbol(s);
+            self.content[index].set_style(style(byte_offset));
 
             // Reset following cells if multi-width (they would be hidden by the grapheme):
-            for i in start_index + 1..start_index + width {
+            for i in index + 1..index + grapheme_width {
                 self.content[i].reset();
             }
 
-            start_index += width;
+            index += grapheme_width;
+            rendered_width += grapheme_width;
+        }
+
+        if truncate_end {
+            for _ in 0..width.saturating_sub(rendered_width) {
+                self.content[index].set_symbol("…");
+                index += 1;
+            }
         }
 
         (x, y)