Implement support for Ambiguous = Wide (#406)

Does what it says on the tin. It's just a lot in the can.

Closes #115

src/bin/edit/documents.rs

@@ -199,6 +199,13 @@ impl DocumentManager {
         Ok(self.list.front_mut().unwrap())
     }
 
+    pub fn reflow_all(&self) {
+        for doc in &self.list {
+            let mut tb = doc.buffer.borrow_mut();
+            tb.reflow();
+        }
+    }
+
     pub fn open_for_reading(path: &Path) -> apperr::Result<File> {
         File::open(path).map_err(apperr::Error::from)
     }

src/bin/edit/main.rs

@@ -23,12 +23,12 @@ use draw_menubar::*;
 use draw_statusbar::*;
 use edit::arena::{self, Arena, ArenaString, scratch_arena};
 use edit::framebuffer::{self, IndexedColor};
-use edit::helpers::{KIBI, MEBI, MetricFormatter, Rect, Size};
+use edit::helpers::{CoordType, KIBI, MEBI, MetricFormatter, Rect, Size};
 use edit::input::{self, kbmod, vk};
 use edit::oklab::oklab_blend;
 use edit::tui::*;
 use edit::vt::{self, Token};
-use edit::{apperr, arena_format, base64, path, sys};
+use edit::{apperr, arena_format, base64, path, sys, unicode};
 use localization::*;
 use state::*;
 
@@ -79,7 +79,7 @@ fn run() -> apperr::Result<()> {
     let mut input_parser = input::Parser::new();
     let mut tui = Tui::new()?;
 
-    let _restore = setup_terminal(&mut tui, &mut vt_parser);
+    let _restore = setup_terminal(&mut tui, &mut state, &mut vt_parser);
 
     state.menubar_color_bg = oklab_blend(
         tui.indexed(IndexedColor::Background),
@@ -502,7 +502,7 @@ impl Drop for RestoreModes {
     }
 }
 
-fn setup_terminal(tui: &mut Tui, vt_parser: &mut vt::Parser) -> RestoreModes {
+fn setup_terminal(tui: &mut Tui, state: &mut State, vt_parser: &mut vt::Parser) -> RestoreModes {
     sys::write_stdout(concat!(
         // 1049: Alternative Screen Buffer
         //   I put the ASB switch in the beginning, just in case the terminal performs
@@ -517,6 +517,12 @@ fn setup_terminal(tui: &mut Tui, vt_parser: &mut vt::Parser) -> RestoreModes {
         "\x1b]4;8;?;9;?;10;?;11;?;12;?;13;?;14;?;15;?\x07",
         // OSC 10 and 11 queries for the current foreground and background colors.
         "\x1b]10;?\x07\x1b]11;?\x07",
+        // Test whether ambiguous width characters are two columns wide.
+        // We use "…", because it's the most common ambiguous width character we use,
+        // and the old Windows conhost doesn't actually use wcwidth, it measures the
+        // actual display width of the character and assigns it columns accordingly.
+        // We detect it by writing the character and asking for the cursor position.
+        "\r…\x1b[6n",
         // CSI c reports the terminal capabilities.
         // It also helps us to detect the end of the responses, because not all
         // terminals support the OSC queries, but all of them support CSI c.
@@ -527,6 +533,7 @@ fn setup_terminal(tui: &mut Tui, vt_parser: &mut vt::Parser) -> RestoreModes {
     let mut osc_buffer = String::new();
     let mut indexed_colors = framebuffer::DEFAULT_THEME;
     let mut color_responses = 0;
+    let mut ambiguous_width = 1;
 
     while !done {
         let scratch = scratch_arena(None);
@@ -537,7 +544,12 @@ fn setup_terminal(tui: &mut Tui, vt_parser: &mut vt::Parser) -> RestoreModes {
         let mut vt_stream = vt_parser.parse(&input);
         while let Some(token) = vt_stream.next() {
             match token {
-                Token::Csi(state) if state.final_byte == 'c' => done = true,
+                Token::Csi(csi) => match csi.final_byte {
+                    'c' => done = true,
+                    // CPR (Cursor Position Report) response.
+                    'R' => ambiguous_width = csi.params[1] as CoordType - 1,
+                    _ => {}
+                },
                 Token::Osc { mut data, partial } => {
                     if partial {
                         osc_buffer.push_str(data);
@@ -594,6 +606,11 @@ fn setup_terminal(tui: &mut Tui, vt_parser: &mut vt::Parser) -> RestoreModes {
         }
     }
 
+    if ambiguous_width == 2 {
+        unicode::setup_ambiguous_width(2);
+        state.documents.reflow_all();
+    }
+
     if color_responses == indexed_colors.len() {
         tui.setup_indexed_colors(indexed_colors);
     }

src/buffer/mod.rs

@@ -427,7 +427,7 @@ impl TextBuffer {
             false
         } else {
             self.margin_enabled = enabled;
-            self.reflow(true);
+            self.reflow();
             true
         }
     }
@@ -482,7 +482,7 @@ impl TextBuffer {
             false
         } else {
             self.width = width;
-            self.reflow(true);
+            self.reflow();
             true
         }
     }
@@ -499,7 +499,7 @@ impl TextBuffer {
             false
         } else {
             self.tab_size = width;
-            self.reflow(true);
+            self.reflow();
             true
         }
     }
@@ -524,7 +524,7 @@ impl TextBuffer {
         self.ruler = column;
     }
 
-    fn reflow(&mut self, force: bool) {
+    pub fn reflow(&mut self) {
         // +1 onto logical_lines, because line numbers are 1-based.
         // +1 onto log10, because we want the digit width and not the actual log10.
         // +3 onto log10, because we append " | " to the line numbers to form the margin.
@@ -536,24 +536,25 @@ impl TextBuffer {
 
         let text_width = self.text_width();
         // 2 columns are required, because otherwise wide glyphs wouldn't ever fit.
-        let word_wrap_column =
+        self.word_wrap_column =
             if self.word_wrap_enabled && text_width >= 2 { text_width } else { 0 };
 
-        if force || self.word_wrap_column > word_wrap_column {
-            self.word_wrap_column = word_wrap_column;
-
-            if self.cursor.offset != 0 {
-                self.cursor = self
-                    .cursor_move_to_logical_internal(Default::default(), self.cursor.logical_pos);
-            }
-
-            // Recalculate the line statistics.
-            if self.word_wrap_enabled {
-                let end = self.cursor_move_to_logical_internal(self.cursor, Point::MAX);
-                self.stats.visual_lines = end.visual_pos.y + 1;
+        // Recalculate the cursor position.
+        self.cursor = self.cursor_move_to_logical_internal(
+            if self.word_wrap_column > 0 {
+                Default::default()
             } else {
-                self.stats.visual_lines = self.stats.logical_lines;
-            }
+                self.goto_line_start(self.cursor, self.cursor.logical_pos.y)
+            },
+            self.cursor.logical_pos,
+        );
+
+        // Recalculate the line statistics.
+        if self.word_wrap_column > 0 {
+            let end = self.cursor_move_to_logical_internal(self.cursor, Point::MAX);
+            self.stats.visual_lines = end.visual_pos.y + 1;
+        } else {
+            self.stats.visual_lines = self.stats.logical_lines;
         }
 
         self.cursor_for_rendering = None;
@@ -583,7 +584,7 @@ impl TextBuffer {
         self.set_selection(None);
         self.search = None;
         self.mark_as_clean();
-        self.reflow(true);
+        self.reflow();
     }
 
     /// Copies the contents of the buffer into a string.
@@ -2312,9 +2313,7 @@ impl TextBuffer {
         }
 
         self.search = None;
-
-        // Also takes care of clearing `cursor_for_rendering`.
-        self.reflow(false);
+        self.cursor_for_rendering = None;
     }
 
     /// Undo the last edit operation.
@@ -2428,8 +2427,7 @@ impl TextBuffer {
             }
         }
 
-        // Also takes care of clearing `cursor_for_rendering`.
-        self.reflow(false);
+        self.cursor_for_rendering = None;
     }
 
     /// For interfacing with ICU.

src/unicode/measurement.rs

@@ -9,6 +9,25 @@ use crate::document::ReadableDocument;
 use crate::helpers::{CoordType, Point};
 use crate::simd::{memchr2, memrchr2};
 
+// On one hand it's disgusting that I wrote this as a global variable, but on the
+// other hand, this isn't a public library API, and it makes the code a lot cleaner,
+// because we don't need to inject this once-per-process value everywhere.
+static mut AMBIGUOUS_WIDTH: usize = 1;
+
+/// Sets the width of "ambiguous" width characters as per "UAX #11: East Asian Width".
+///
+/// Defaults to 1.
+pub fn setup_ambiguous_width(ambiguous_width: CoordType) {
+    unsafe { AMBIGUOUS_WIDTH = ambiguous_width as usize };
+}
+
+#[inline]
+fn ambiguous_width() -> usize {
+    // SAFETY: This is a global variable that is set once per process.
+    // It is never changed after that, so this is safe to call.
+    unsafe { AMBIGUOUS_WIDTH }
+}
+
 /// Stores a position inside a [`ReadableDocument`].
 ///
 /// The cursor tracks both the absolute byte-offset,
@@ -40,16 +59,25 @@ pub struct Cursor {
 /// Your entrypoint to navigating inside a [`ReadableDocument`].
 #[derive(Clone)]
 pub struct MeasurementConfig<'doc> {
-    buffer: &'doc dyn ReadableDocument,
+    cursor: Cursor,
     tab_size: CoordType,
     word_wrap_column: CoordType,
-    cursor: Cursor,
+    buffer: &'doc dyn ReadableDocument,
 }
 
 impl<'doc> MeasurementConfig<'doc> {
     /// Creates a new [`MeasurementConfig`] for the given document.
     pub fn new(buffer: &'doc dyn ReadableDocument) -> Self {
-        Self { buffer, tab_size: 8, word_wrap_column: 0, cursor: Default::default() }
+        Self { cursor: Default::default(), tab_size: 8, word_wrap_column: 0, buffer }
+    }
+
+    /// Sets the initial cursor to the given position.
+    ///
+    /// WARNING: While the code doesn't panic if the cursor is invalid,
+    /// the results will obviously be complete garbage.
+    pub fn with_cursor(mut self, cursor: Cursor) -> Self {
+        self.cursor = cursor;
+        self
     }
 
     /// Sets the tab size.
@@ -68,31 +96,13 @@ impl<'doc> MeasurementConfig<'doc> {
         self
     }
 
-    /// Sets the initial cursor to the given position.
-    ///
-    /// WARNING: While the code doesn't panic if the cursor is invalid,
-    /// the results will obviously be complete garbage.
-    pub fn with_cursor(mut self, cursor: Cursor) -> Self {
-        self.cursor = cursor;
-        self
-    }
-
     /// Navigates **forward** to the given absolute offset.
     ///
     /// # Returns
     ///
     /// The cursor position after the navigation.
     pub fn goto_offset(&mut self, offset: usize) -> Cursor {
-        self.cursor = Self::measure_forward(
-            self.tab_size,
-            self.word_wrap_column,
-            offset,
-            Point::MAX,
-            Point::MAX,
-            self.cursor,
-            self.buffer,
-        );
-        self.cursor
+        self.measure_forward(offset, Point::MAX, Point::MAX)
     }
 
     /// Navigates **forward** to the given logical position.
@@ -103,16 +113,7 @@ impl<'doc> MeasurementConfig<'doc> {
     ///
     /// The cursor position after the navigation.
     pub fn goto_logical(&mut self, logical_target: Point) -> Cursor {
-        self.cursor = Self::measure_forward(
-            self.tab_size,
-            self.word_wrap_column,
-            usize::MAX,
-            logical_target,
-            Point::MAX,
-            self.cursor,
-            self.buffer,
-        );
-        self.cursor
+        self.measure_forward(usize::MAX, logical_target, Point::MAX)
     }
 
     /// Navigates **forward** to the given visual position.
@@ -123,16 +124,7 @@ impl<'doc> MeasurementConfig<'doc> {
     ///
     /// The cursor position after the navigation.
     pub fn goto_visual(&mut self, visual_target: Point) -> Cursor {
-        self.cursor = Self::measure_forward(
-            self.tab_size,
-            self.word_wrap_column,
-            usize::MAX,
-            Point::MAX,
-            visual_target,
-            self.cursor,
-            self.buffer,
-        );
-        self.cursor
+        self.measure_forward(usize::MAX, Point::MAX, visual_target)
     }
 
     /// Returns the current cursor position.
@@ -149,27 +141,24 @@ impl<'doc> MeasurementConfig<'doc> {
     // the wrap exists on both lines and it'll default to wrapping. `goto_visual` however will always
     // try to return a Y position that matches the requested position, so that Home/End works properly.
     fn measure_forward(
-        tab_size: CoordType,
-        word_wrap_column: CoordType,
+        &mut self,
         offset_target: usize,
         logical_target: Point,
         visual_target: Point,
-        cursor: Cursor,
-        buffer: &dyn ReadableDocument,
     ) -> Cursor {
-        if cursor.offset >= offset_target
-            || cursor.logical_pos >= logical_target
-            || cursor.visual_pos >= visual_target
+        if self.cursor.offset >= offset_target
+            || self.cursor.logical_pos >= logical_target
+            || self.cursor.visual_pos >= visual_target
         {
-            return cursor;
+            return self.cursor;
         }
 
-        let mut offset = cursor.offset;
-        let mut logical_pos_x = cursor.logical_pos.x;
-        let mut logical_pos_y = cursor.logical_pos.y;
-        let mut visual_pos_x = cursor.visual_pos.x;
-        let mut visual_pos_y = cursor.visual_pos.y;
-        let mut column = cursor.column;
+        let mut offset = self.cursor.offset;
+        let mut logical_pos_x = self.cursor.logical_pos.x;
+        let mut logical_pos_y = self.cursor.logical_pos.y;
+        let mut visual_pos_x = self.cursor.visual_pos.x;
+        let mut visual_pos_y = self.cursor.visual_pos.y;
+        let mut column = self.cursor.column;
 
         let mut logical_target_x = Self::calc_target_x(logical_target, logical_pos_y);
         let mut visual_target_x = Self::calc_target_x(visual_target, visual_pos_y);
@@ -177,7 +166,7 @@ impl<'doc> MeasurementConfig<'doc> {
         // wrap_opp = Wrap Opportunity
         // These store the position and column of the last wrap opportunity. If `word_wrap_column` is
         // zero (word wrap disabled), all grapheme clusters are a wrap opportunity, because none are.
-        let mut wrap_opp = cursor.wrap_opp;
+        let mut wrap_opp = self.cursor.wrap_opp;
         let mut wrap_opp_offset = offset;
         let mut wrap_opp_logical_pos_x = logical_pos_x;
         let mut wrap_opp_visual_pos_x = visual_pos_x;
@@ -209,7 +198,7 @@ impl<'doc> MeasurementConfig<'doc> {
             loop {
                 if !chunk_iter.has_next() {
                     cold_path();
-                    chunk_iter = Utf8Chars::new(buffer.read_forward(chunk_range.end), 0);
+                    chunk_iter = Utf8Chars::new(self.buffer.read_forward(chunk_range.end), 0);
                     chunk_range = chunk_range.end..chunk_range.end + chunk_iter.len();
                 }
 
@@ -219,7 +208,8 @@ impl<'doc> MeasurementConfig<'doc> {
                 // Similar applies to the width.
                 props_last_char = props_next_cluster;
                 offset_next_cluster = chunk_range.start + chunk_iter.offset();
-                width += ucd_grapheme_cluster_character_width(props_next_cluster) as CoordType;
+                width += ucd_grapheme_cluster_character_width(props_next_cluster, ambiguous_width())
+                    as CoordType;
 
                 // The `Document::read_forward` interface promises us that it will not split
                 // grapheme clusters across chunks. Therefore, we can safely break here.
@@ -252,10 +242,10 @@ impl<'doc> MeasurementConfig<'doc> {
 
             // Tabs require special handling because they can have a variable width.
             if props_last_char == ucd_tab_properties() {
-                // SAFETY: `tab_size` is clamped to >= 1 in `with_tab_size`.
+                // SAFETY: `self.tab_size` is clamped to >= 1 in `with_tab_size`.
                 // This assert ensures that Rust doesn't insert panicking null checks.
-                unsafe { std::hint::assert_unchecked(tab_size >= 1) };
-                width = tab_size - (column % tab_size);
+                unsafe { std::hint::assert_unchecked(self.tab_size >= 1) };
+                width = self.tab_size - (column % self.tab_size);
             }
 
             // Hard wrap: Both the logical and visual position advance by one line.
@@ -290,7 +280,7 @@ impl<'doc> MeasurementConfig<'doc> {
 
             // Since this code above may need to revert to a previous `wrap_opp_*`,
             // it must be done before advancing / checking for `ucd_line_break_joins`.
-            if word_wrap_column > 0 && visual_pos_x + width > word_wrap_column {
+            if self.word_wrap_column > 0 && visual_pos_x + width > self.word_wrap_column {
                 if !wrap_opp {
                     // Otherwise, the lack of a wrap opportunity means that a single word
                     // is wider than the word wrap column. We need to force-break the word.
@@ -342,7 +332,7 @@ impl<'doc> MeasurementConfig<'doc> {
             visual_pos_x += width;
             column += width;
 
-            if word_wrap_column > 0
+            if self.word_wrap_column > 0
                 && !ucd_line_break_joins(props_current_cluster, props_next_cluster)
             {
                 wrap_opp = true;
@@ -355,7 +345,7 @@ impl<'doc> MeasurementConfig<'doc> {
 
         // If we're here, we hit our target. Now the only question is:
         // Is the word we're currently on so wide that it will be wrapped further down the document?
-        if word_wrap_column > 0 {
+        if self.word_wrap_column > 0 {
             if !wrap_opp {
                 // If the current laid-out line had no wrap opportunities, it means we had an input
                 // such as "fooooooooooooooooooooo" at a `word_wrap_column` of e.g. 10. The word
@@ -386,7 +376,8 @@ impl<'doc> MeasurementConfig<'doc> {
                     loop {
                         if !chunk_iter.has_next() {
                             cold_path();
-                            chunk_iter = Utf8Chars::new(buffer.read_forward(chunk_range.end), 0);
+                            chunk_iter =
+                                Utf8Chars::new(self.buffer.read_forward(chunk_range.end), 0);
                             chunk_range = chunk_range.end..chunk_range.end + chunk_iter.len();
                         }
 
@@ -396,8 +387,10 @@ impl<'doc> MeasurementConfig<'doc> {
                         // Similar applies to the width.
                         props_last_char = props_next_cluster;
                         offset_next_cluster = chunk_range.start + chunk_iter.offset();
-                        width +=
-                            ucd_grapheme_cluster_character_width(props_next_cluster) as CoordType;
+                        width += ucd_grapheme_cluster_character_width(
+                            props_next_cluster,
+                            ambiguous_width(),
+                        ) as CoordType;
 
                         // The `Document::read_forward` interface promises us that it will not split
                         // grapheme clusters across chunks. Therefore, we can safely break here.
@@ -431,10 +424,10 @@ impl<'doc> MeasurementConfig<'doc> {
 
                     // Tabs require special handling because they can have a variable width.
                     if props_last_char == ucd_tab_properties() {
-                        // SAFETY: `tab_size` is clamped to >= 1 in `with_tab_size`.
+                        // SAFETY: `self.tab_size` is clamped to >= 1 in `with_tab_size`.
                         // This assert ensures that Rust doesn't insert panicking null checks.
-                        unsafe { std::hint::assert_unchecked(tab_size >= 1) };
-                        width = tab_size - (column % tab_size);
+                        unsafe { std::hint::assert_unchecked(self.tab_size >= 1) };
+                        width = self.tab_size - (column % self.tab_size);
                     }
 
                     // Hard wrap: Both the logical and visual position advance by one line.
@@ -444,7 +437,7 @@ impl<'doc> MeasurementConfig<'doc> {
 
                     visual_pos_x_lookahead += width;
 
-                    if visual_pos_x_lookahead > word_wrap_column {
+                    if visual_pos_x_lookahead > self.word_wrap_column {
                         visual_pos_x -= wrap_opp_visual_pos_x;
                         visual_pos_y += 1;
                         break;
@@ -467,13 +460,12 @@ impl<'doc> MeasurementConfig<'doc> {
             }
         }
 
-        Cursor {
-            offset,
-            logical_pos: Point { x: logical_pos_x, y: logical_pos_y },
-            visual_pos: Point { x: visual_pos_x, y: visual_pos_y },
-            column,
-            wrap_opp,
-        }
+        self.cursor.offset = offset;
+        self.cursor.logical_pos = Point { x: logical_pos_x, y: logical_pos_y };
+        self.cursor.visual_pos = Point { x: visual_pos_x, y: visual_pos_y };
+        self.cursor.column = column;
+        self.cursor.wrap_opp = wrap_opp;
+        self.cursor
     }
 
     #[inline]

tools/grapheme-table-gen/src/main.rs

@@ -247,12 +247,15 @@ Usage: grapheme-table-gen [options...] <ucd.nounihan.grouped.xml>
                         Expose tab and linefeed as grapheme cluster properties
   --no-ambiguous        Treat all ambiguous characters as narrow
   --line-breaks         Store and expose line break information
+
+Download ucd.nounihan.grouped.xml at:
+  https://www.unicode.org/Public/UCD/latest/ucdxml/ucd.nounihan.grouped.zip
 ";
 
 fn main() -> anyhow::Result<()> {
     let mut args = pico_args::Arguments::from_env();
     if args.contains(["-h", "--help"]) {
-        eprint!("{}", HELP);
+        eprint!("{HELP}");
         return Ok(());
     }
 
@@ -310,7 +313,7 @@ fn main() -> anyhow::Result<()> {
         for s in &out.trie.stages {
             actual = s.values[actual as usize + ((cp >> s.shift) & s.mask)];
         }
-        assert_eq!(expected.value(), actual, "trie sanity check failed for U+{:04X}", cp);
+        assert_eq!(expected.value(), actual, "trie sanity check failed for U+{cp:04X}");
     }
     for (cp, &expected) in out.ucd.values[..0x80].iter().enumerate() {
         let last = out.trie.stages.last().unwrap();
@@ -318,8 +321,7 @@ fn main() -> anyhow::Result<()> {
         assert_eq!(
             expected.value(),
             actual,
-            "trie sanity check failed for direct ASCII mapping of U+{:04X}",
-            cp
+            "trie sanity check failed for direct ASCII mapping of U+{cp:04X}"
         );
     }
 
@@ -372,7 +374,7 @@ fn generate_c(out: Output) -> String {
     for table in &out.rules_gc {
         buf.push_str("    {\n");
         for &r in table {
-            _ = writeln!(buf, "        0b{:032b},", r);
+            _ = writeln!(buf, "        0b{r:032b},");
         }
         buf.push_str("    },\n");
     }
@@ -443,15 +445,38 @@ fn generate_c(out: Output) -> String {
         {{
             return state == 3;
         }}
-        inline int ucd_grapheme_cluster_character_width(const int val)
-        {{
-            return val >> {1};
-        }}
         ",
         out.ucd.packing.mask_cluster_break,
-        out.ucd.packing.shift_character_width,
     );
 
+    if out.arg_no_ambiguous {
+        _ = writedoc!(
+            buf,
+            "
+            inline int ucd_grapheme_cluster_character_width(const int val)
+            {{
+                return val >> {};
+            }}
+            ",
+            out.ucd.packing.shift_character_width,
+        );
+    } else {
+        _ = writedoc!(
+            buf,
+            "
+            inline int ucd_grapheme_cluster_character_width(const int val, int ambiguous_width)
+            {{
+                int w = val >> {};
+                if (w == 3) {{
+                    w = ambiguous_width;
+                }}
+                return w;
+            }}
+            ",
+            out.ucd.packing.shift_character_width,
+        );
+    }
+
     if out.arg_line_breaks {
         _ = writedoc!(
             buf,
@@ -546,7 +571,7 @@ fn generate_rust(out: Output) -> String {
     for table in &out.rules_gc {
         buf.push_str("    [\n");
         for &r in table {
-            _ = writeln!(buf, "        0b{:032b},", r);
+            _ = writeln!(buf, "        0b{r:032b},");
         }
         buf.push_str("    ],\n");
     }
@@ -622,15 +647,43 @@ fn generate_rust(out: Output) -> String {
         pub fn ucd_grapheme_cluster_joins_done(state: u32) -> bool {{
             state == 3
         }}
-        #[inline(always)]
-        pub fn ucd_grapheme_cluster_character_width(val: usize) -> usize {{
-            val >> {1}
-        }}
         ",
         out.ucd.packing.mask_cluster_break,
-        out.ucd.packing.shift_character_width,
     );
 
+    if out.arg_no_ambiguous {
+        _ = writedoc!(
+            buf,
+            "
+            #[inline(always)]
+            pub fn ucd_grapheme_cluster_character_width(val: usize) -> usize {{
+                val >> {}
+            }}
+            ",
+            out.ucd.packing.shift_character_width,
+        );
+    } else {
+        // `cold_path()` ensures that LLVM emits a branch instead of a conditional move.
+        // This improves performance, as ambiguous characters are rare.
+        // `> 2` is used instead of `== 3`, because this way the compiler can immediately
+        // test whether `val > (2 << shift_character_width)` before shifting.
+        _ = writedoc!(
+            buf,
+            "
+            #[inline(always)]
+            pub fn ucd_grapheme_cluster_character_width(val: usize, ambiguous_width: usize) -> usize {{
+                let mut w = val >> {};
+                if w > 2 {{
+                    cold_path();
+                    w = ambiguous_width;
+                }}
+                w
+            }}
+            ",
+            out.ucd.packing.shift_character_width,
+        );
+    }
+
     if out.arg_line_breaks {
         _ = writedoc!(
             buf,
@@ -681,6 +734,17 @@ fn generate_rust(out: Output) -> String {
         );
     }
 
+    if !out.arg_no_ambiguous {
+        _ = writedoc!(
+            buf,
+            "
+            #[cold]
+            #[inline(always)]
+            fn cold_path() {{}}
+            "
+        );
+    }
+
     buf.push_str("// END: Generated by grapheme-table-gen\n");
     buf
 }