merman_render/text/

deterministic.rs

//! Deterministic text measurement and wrapping fallback.

use super::heuristic::append_text_width_em;
use super::line_break::html_break_spaces_segments;
use super::{
    TextMeasurer, TextMetrics, TextStyle, WrapMode, trim_end_html_collapsible_ascii_whitespace,
    trim_html_collapsible_ascii_whitespace,
};
use std::sync::Arc;
use unicode_segmentation::UnicodeSegmentation;

type TextWidthPxFn = Arc<dyn Fn(&str, &TextStyle) -> f64 + Send + Sync>;

#[derive(Clone)]
struct WidthCallbackTextMeasurer {
    width_px: TextWidthPxFn,
    line_height_factor: f64,
}

#[derive(Clone, Copy, Default)]
struct LineWidthAccumulator {
    heuristic_em: f64,
    char_count: usize,
}

#[derive(Clone, Copy)]
enum LineWidthSource<'a> {
    Heuristic,
    UniformAdvanceEm(f64),
    Callback(&'a TextWidthPxFn),
}

#[derive(Clone, Copy)]
struct LineWidthModel<'a> {
    font_size: f64,
    source: LineWidthSource<'a>,
    style: &'a TextStyle,
}

impl LineWidthModel<'_> {
    fn width_px(self, text: &str) -> f64 {
        if let LineWidthSource::Callback(width_px) = self.source {
            return width_px(text, self.style);
        }

        let mut width = LineWidthAccumulator::default();
        self.append_builtin(&mut width, text);
        self.finish_builtin(width)
    }

    fn candidate_width_px(
        self,
        width: LineWidthAccumulator,
        candidate: &str,
        appended: &str,
    ) -> (LineWidthAccumulator, f64) {
        if let LineWidthSource::Callback(width_px) = self.source {
            return (width, width_px(candidate, self.style));
        }

        let mut width = width;
        self.append_builtin(&mut width, appended);
        (width, self.finish_builtin(width))
    }

    fn append_builtin(self, width: &mut LineWidthAccumulator, text: &str) {
        match self.source {
            LineWidthSource::Heuristic => append_text_width_em(&mut width.heuristic_em, text),
            LineWidthSource::UniformAdvanceEm(_) => {
                width.char_count = width.char_count.saturating_add(text.chars().count());
            }
            LineWidthSource::Callback(_) => unreachable!("callback widths use complete strings"),
        }
    }

    fn finish_builtin(self, width: LineWidthAccumulator) -> f64 {
        match self.source {
            LineWidthSource::Heuristic => width.heuristic_em * self.font_size,
            LineWidthSource::UniformAdvanceEm(advance_em) => {
                width.char_count as f64 * self.font_size * advance_em
            }
            LineWidthSource::Callback(_) => unreachable!("callback widths use complete strings"),
        }
    }
}

#[derive(Debug, Clone, Default)]
pub struct DeterministicTextMeasurer {
    pub char_width_factor: f64,
    pub line_height_factor: f64,
}

impl DeterministicTextMeasurer {
    /// Uses caller-supplied widths with Merman's wrapping behavior.
    ///
    /// Uses `width_px` to measure complete candidate strings while Merman retains its wrapping and
    /// line-height behavior. Vertical metrics and baseline offsets remain deterministic
    /// approximations; the callback does not provide glyph bounds or font ascent/descent.
    ///
    /// The callback must return a finite, non-negative width in CSS pixels. Use
    /// [`crate::environment::HostTextMeasurer`] when measurement can fail or vary by operation.
    pub fn with_width_callback(
        self,
        width_px: impl Fn(&str, &TextStyle) -> f64 + Send + Sync + 'static,
    ) -> Arc<dyn TextMeasurer + Send + Sync> {
        Arc::new(WidthCallbackTextMeasurer {
            width_px: Arc::new(width_px),
            line_height_factor: self.line_height_factor,
        })
    }

    fn collapse_svg_text_whitespace(text: &str) -> String {
        let mut collapsed = String::with_capacity(text.len());
        let mut pending_space = false;

        for ch in text.chars() {
            if matches!(ch, ' ' | '\t' | '\r' | '\n') {
                pending_space = !collapsed.is_empty();
            } else {
                if pending_space {
                    collapsed.push(' ');
                    pending_space = false;
                }
                collapsed.push(ch);
            }
        }

        collapsed
    }

    fn replace_br_variants(text: &str) -> String {
        let mut out = String::with_capacity(text.len());
        let mut i = 0usize;
        while i < text.len() {
            let Some(rest) = text.get(i..) else {
                break;
            };

            // Mirror Mermaid's `lineBreakRegex = /<br\\s*\\/?>/gi` behavior:
            // - allow ASCII whitespace between `br` and the optional `/` or `>`
            // - do NOT accept extra characters (e.g. `<br \\t/>` should *not* count as a break)
            if rest.starts_with('<') {
                let bytes = text.as_bytes();
                if i + 3 < bytes.len()
                    && matches!(bytes[i + 1], b'b' | b'B')
                    && matches!(bytes[i + 2], b'r' | b'R')
                {
                    let mut j = i + 3;
                    while j < bytes.len() && matches!(bytes[j], b' ' | b'\t' | b'\r' | b'\n') {
                        j += 1;
                    }
                    if j < bytes.len() && bytes[j] == b'/' {
                        j += 1;
                    }
                    if j < bytes.len() && bytes[j] == b'>' {
                        out.push('\n');
                        i = j + 1;
                        continue;
                    }
                }
            }

            let Some(ch) = rest.chars().next() else {
                break;
            };
            out.push(ch);
            i += ch.len_utf8();
        }
        out
    }

    fn normalized_text_lines_with(
        text: &str,
        trailing_line_is_empty: impl Fn(&str) -> bool,
    ) -> Vec<String> {
        let t = Self::replace_br_variants(text);
        let mut out = t.split('\n').map(|s| s.to_string()).collect::<Vec<_>>();

        // Mermaid often produces labels with a trailing newline (e.g. YAML `|` block scalars from
        // FlowDB). The rendered label does not keep an extra blank line at the end, so we trim
        // trailing empty lines to keep height parity.
        while out.len() > 1 && out.last().is_some_and(|s| trailing_line_is_empty(s)) {
            out.pop();
        }

        if out.is_empty() {
            vec!["".to_string()]
        } else {
            out
        }
    }

    pub fn normalized_text_lines(text: &str) -> Vec<String> {
        Self::normalized_text_lines_with(text, |line| {
            trim_html_collapsible_ascii_whitespace(line).is_empty()
        })
    }

    pub(crate) fn split_line_to_words(text: &str) -> Vec<String> {
        // Mirrors Mermaid's `splitLineToWords` fallback behavior when `Intl.Segmenter` is absent:
        // split by spaces, then re-add the spaces as separate tokens (preserving multiple spaces).
        let parts = text.split(' ').collect::<Vec<_>>();
        let mut out: Vec<String> = Vec::new();
        for part in parts {
            if !part.is_empty() {
                out.push(part.to_string());
            }
            out.push(" ".to_string());
        }
        while out.last().is_some_and(|s| s == " ") {
            out.pop();
        }
        out
    }

    fn wrapped_line_has_visible_content(text: &str) -> bool {
        !trim_html_collapsible_ascii_whitespace(text).is_empty()
    }

    fn split_token_to_width<'t>(
        token: &'t str,
        max_width_px: f64,
        width_model: LineWidthModel<'_>,
    ) -> (&'t str, &'t str) {
        let mut split_at = 0;
        let mut width = LineWidthAccumulator::default();
        let mut width_px = 0.0;
        let mut has_positive_advance = false;

        for (index, grapheme) in token.grapheme_indices(true) {
            let candidate_end = index + grapheme.len();
            let (candidate, candidate_width_px) =
                width_model.candidate_width_px(width, &token[..candidate_end], grapheme);
            if has_positive_advance
                && candidate_width_px > width_px
                && candidate_width_px > max_width_px
            {
                break;
            }

            split_at = candidate_end;
            width = candidate;
            width_px = candidate_width_px;
            has_positive_advance |= candidate_width_px > 0.0;
        }

        // A visible grapheme wider than the whole line must still make progress. Leading
        // zero-width graphemes stay attached to it instead of becoming an orphan line.
        if split_at == 0 {
            split_at = token.graphemes(true).next().map_or(0, str::len);
        }
        token.split_at(split_at)
    }

    fn wrap_line(
        line: &str,
        max_width_px: f64,
        break_long_words: bool,
        wrap_mode: WrapMode,
        html_break_spaces_active: bool,
        width_model: LineWidthModel<'_>,
    ) -> Vec<String> {
        if !max_width_px.is_finite() || max_width_px <= 0.0 {
            return vec![line.to_string()];
        }

        let tokens = match (wrap_mode, html_break_spaces_active) {
            (WrapMode::HtmlLike, true) => html_break_spaces_segments(line)
                .into_iter()
                .map(str::to_string)
                .collect(),
            _ => Self::split_line_to_words(line),
        };
        let mut tokens = std::collections::VecDeque::from(tokens);
        let mut out: Vec<String> = Vec::new();
        let mut cur = String::new();
        let mut cur_width = LineWidthAccumulator::default();

        while let Some(tok) = tokens.pop_front() {
            if cur.is_empty() && tok == " " && !html_break_spaces_active {
                continue;
            }

            let previous_len = cur.len();
            cur.push_str(&tok);
            let (candidate_width, candidate_width_px) =
                width_model.candidate_width_px(cur_width, &cur, &tok);
            if candidate_width_px <= max_width_px {
                cur_width = candidate_width;
                continue;
            }
            cur.truncate(previous_len);

            let current_line_has_content = if html_break_spaces_active {
                !cur.is_empty()
            } else {
                Self::wrapped_line_has_visible_content(&cur)
            };
            if current_line_has_content {
                out.push(if html_break_spaces_active {
                    std::mem::take(&mut cur)
                } else {
                    let line = trim_end_html_collapsible_ascii_whitespace(&cur).to_string();
                    cur.clear();
                    line
                });
                cur_width = LineWidthAccumulator::default();
                tokens.push_front(tok);
                continue;
            }

            // `tok` itself does not fit on an empty line.
            if tok == " " && !html_break_spaces_active {
                continue;
            }
            if !break_long_words {
                out.push(tok);
            } else {
                // Split at the largest grapheme boundary that fits. Zero-width graphemes do not
                // consume line capacity, and an over-wide grapheme still advances the queue.
                let (head, tail) = Self::split_token_to_width(&tok, max_width_px, width_model);
                out.push(head.to_string());
                if !tail.is_empty() {
                    tokens.push_front(tail.to_string());
                }
            }
        }

        if html_break_spaces_active && !cur.is_empty() {
            out.push(cur);
        } else if Self::wrapped_line_has_visible_content(&cur) {
            out.push(trim_end_html_collapsible_ascii_whitespace(&cur).to_string());
        }

        if out.is_empty() {
            vec!["".to_string()]
        } else {
            out
        }
    }
}

impl TextMeasurer for DeterministicTextMeasurer {
    #[allow(private_interfaces)]
    fn begin_svg_text_computed_length(
        &self,
        style: &TextStyle,
    ) -> Option<crate::environment::BuiltinSvgComputedLength> {
        (self.char_width_factor == 0.0)
            .then(|| crate::environment::BuiltinSvgComputedLength::deterministic(style))
    }

    fn measure(&self, text: &str, style: &TextStyle) -> TextMetrics {
        self.measure_wrapped(text, style, None, WrapMode::SvgLike)
    }

    fn measure_wrapped(
        &self,
        text: &str,
        style: &TextStyle,
        max_width: Option<f64>,
        wrap_mode: WrapMode,
    ) -> TextMetrics {
        self.measure_wrapped_impl(text, style, max_width, wrap_mode)
            .0
    }

    fn measure_wrapped_with_raw_width(
        &self,
        text: &str,
        style: &TextStyle,
        max_width: Option<f64>,
        wrap_mode: WrapMode,
    ) -> (TextMetrics, Option<f64>) {
        self.measure_wrapped_impl(text, style, max_width, wrap_mode)
    }

    fn measure_mermaid_calculate_text_dimensions(
        &self,
        text: &str,
        style: &TextStyle,
    ) -> TextMetrics {
        let collapsed = Self::collapse_svg_text_whitespace(text);
        TextMetrics {
            width: self.measure_svg_simple_text_bbox_width_for_wrap_px(&collapsed, style),
            height: self.measure_svg_simple_text_bbox_height_px(&collapsed, style),
            line_count: 1,
        }
    }

    fn measure_svg_simple_text_bbox_height_px(&self, text: &str, style: &TextStyle) -> f64 {
        let t = trim_end_html_collapsible_ascii_whitespace(text);
        if t.is_empty() {
            return 0.0;
        }
        (style.font_size.max(1.0) * 1.1).max(0.0)
    }

    fn measure_svg_tspan_text_bbox_height_px(&self, text: &str, style: &TextStyle) -> f64 {
        if trim_end_html_collapsible_ascii_whitespace(text).is_empty() {
            0.0
        } else {
            super::svg_wrapped_first_line_bbox_height_px(style)
        }
    }
}

impl TextMeasurer for WidthCallbackTextMeasurer {
    fn measure(&self, text: &str, style: &TextStyle) -> TextMetrics {
        self.measure_wrapped(text, style, None, WrapMode::SvgLike)
    }

    fn measure_wrapped(
        &self,
        text: &str,
        style: &TextStyle,
        max_width: Option<f64>,
        wrap_mode: WrapMode,
    ) -> TextMetrics {
        DeterministicTextMeasurer::measure_wrapped_impl_with_width(
            text,
            style,
            max_width,
            wrap_mode,
            LineWidthSource::Callback(&self.width_px),
            self.line_height_factor,
        )
        .0
    }

    fn measure_wrapped_with_raw_width(
        &self,
        text: &str,
        style: &TextStyle,
        max_width: Option<f64>,
        wrap_mode: WrapMode,
    ) -> (TextMetrics, Option<f64>) {
        DeterministicTextMeasurer::measure_wrapped_impl_with_width(
            text,
            style,
            max_width,
            wrap_mode,
            LineWidthSource::Callback(&self.width_px),
            self.line_height_factor,
        )
    }

    fn measure_mermaid_calculate_text_dimensions(
        &self,
        text: &str,
        style: &TextStyle,
    ) -> TextMetrics {
        let collapsed = DeterministicTextMeasurer::collapse_svg_text_whitespace(text);
        TextMetrics {
            width: self.measure_svg_simple_text_bbox_width_for_wrap_px(&collapsed, style),
            height: self.measure_svg_simple_text_bbox_height_px(&collapsed, style),
            line_count: 1,
        }
    }

    fn measure_svg_simple_text_bbox_height_px(&self, text: &str, style: &TextStyle) -> f64 {
        let text = trim_end_html_collapsible_ascii_whitespace(text);
        if text.is_empty() {
            return 0.0;
        }
        (style.font_size.max(1.0) * 1.1).max(0.0)
    }

    fn measure_svg_tspan_text_bbox_height_px(&self, text: &str, style: &TextStyle) -> f64 {
        if trim_end_html_collapsible_ascii_whitespace(text).is_empty() {
            0.0
        } else {
            super::svg_wrapped_first_line_bbox_height_px(style)
        }
    }
}

impl DeterministicTextMeasurer {
    fn measure_wrapped_impl(
        &self,
        text: &str,
        style: &TextStyle,
        max_width: Option<f64>,
        wrap_mode: WrapMode,
    ) -> (TextMetrics, Option<f64>) {
        let width_source = if self.char_width_factor == 0.0 {
            LineWidthSource::Heuristic
        } else {
            LineWidthSource::UniformAdvanceEm(self.char_width_factor)
        };
        Self::measure_wrapped_impl_with_width(
            text,
            style,
            max_width,
            wrap_mode,
            width_source,
            self.line_height_factor,
        )
    }

    fn measure_wrapped_impl_with_width(
        text: &str,
        style: &TextStyle,
        max_width: Option<f64>,
        wrap_mode: WrapMode,
        width_source: LineWidthSource<'_>,
        configured_line_height_factor: f64,
    ) -> (TextMetrics, Option<f64>) {
        let default_line_height_factor = match wrap_mode {
            WrapMode::SvgLike | WrapMode::SvgLikeSingleRun => 1.1,
            WrapMode::HtmlLike => 1.5,
        };
        let line_height_factor = if configured_line_height_factor == 0.0 {
            default_line_height_factor
        } else {
            configured_line_height_factor
        };

        let font_size = style.font_size.max(1.0);
        let width_model = LineWidthModel {
            font_size,
            source: width_source,
            style,
        };
        let max_width = max_width.filter(|w| w.is_finite() && *w > 0.0);
        let break_long_words = matches!(wrap_mode, WrapMode::SvgLike | WrapMode::SvgLikeSingleRun);

        let raw_lines = Self::normalized_text_lines(text);
        let raw_width = (wrap_mode == WrapMode::HtmlLike || max_width.is_none()).then(|| {
            raw_lines
                .iter()
                .fold(0.0_f64, |width, line| width.max(width_model.width_px(line)))
        });
        let html_break_spaces_active = wrap_mode == WrapMode::HtmlLike
            && max_width
                .is_some_and(|max_width| raw_width.is_some_and(|raw_width| raw_width > max_width));
        let mut lines = Vec::new();
        for line in raw_lines {
            if let Some(w) = max_width {
                lines.extend(Self::wrap_line(
                    &line,
                    w,
                    break_long_words,
                    wrap_mode,
                    html_break_spaces_active,
                    width_model,
                ));
            } else {
                lines.push(line);
            }
        }

        let mut width = if max_width.is_none() {
            raw_width.expect("unwrapped measurement computes raw width")
        } else {
            lines
                .iter()
                .fold(0.0_f64, |width, line| width.max(width_model.width_px(line)))
        };
        if html_break_spaces_active && let Some(max_width) = max_width {
            // Mermaid switches overflowing HTML labels to a fixed-width table. Breakable text
            // therefore occupies the configured width, while an unbreakable segment may still
            // expand it through its min-content width.
            width = width.max(max_width);
        }
        let height = lines.len() as f64 * font_size * line_height_factor;
        let metrics = TextMetrics {
            width,
            height,
            line_count: lines.len(),
        };
        let raw_width_px = if wrap_mode == WrapMode::HtmlLike {
            Some(raw_width.expect("HTML measurement computes raw width"))
        } else {
            None
        };
        (metrics, raw_width_px)
    }
}

#[cfg(test)]
mod tests {
    use super::{DeterministicTextMeasurer, LineWidthModel, LineWidthSource};
    use crate::text::{TextMeasurer, TextStyle, WrapMode};
    use std::sync::{Arc, Mutex};
    use unicode_segmentation::UnicodeSegmentation;

    #[test]
    fn wrapping_uses_estimated_width_instead_of_character_count() {
        let measurer = DeterministicTextMeasurer::default();
        let metrics = measurer.measure_wrapped(
            "iiii WWWW",
            &TextStyle {
                font_size: 10.0,
                ..TextStyle::default()
            },
            Some(20.0),
            WrapMode::SvgLike,
        );

        assert_eq!(metrics.line_count, 3);
        assert!((metrics.width - 17.0).abs() < f64::EPSILON, "{metrics:?}");
    }

    #[test]
    fn long_word_splitting_keeps_zero_width_scalars_with_visible_text() {
        let (head, tail) = DeterministicTextMeasurer::split_token_to_width(
            "W\u{0301}W",
            8.0,
            LineWidthModel {
                font_size: 10.0,
                source: LineWidthSource::Heuristic,
                style: &TextStyle::default(),
            },
        );
        assert_eq!(head, "W\u{0301}");
        assert_eq!(tail, "W");

        let metrics = DeterministicTextMeasurer::default().measure_wrapped(
            "\u{0301}WW",
            &TextStyle {
                font_size: 10.0,
                ..TextStyle::default()
            },
            Some(8.5),
            WrapMode::SvgLike,
        );

        assert_eq!(metrics.line_count, 2);
        assert!((metrics.width - 8.5).abs() < f64::EPSILON, "{metrics:?}");
    }

    #[test]
    fn default_width_uses_sequence_aware_unicode_display_width() {
        let measurer = DeterministicTextMeasurer::default();
        let style = TextStyle {
            font_size: 16.0,
            ..TextStyle::default()
        };

        for sequence in ["👩‍🔬", "👨‍👩‍👧‍👦", "👍🏽", "🇨🇳", "1️⃣"]
        {
            let metrics = measurer.measure(sequence, &style);
            assert_eq!(metrics.width, 16.0, "sequence={sequence:?}");
        }
    }

    #[test]
    fn wrapping_splits_long_emoji_runs_only_at_grapheme_boundaries() {
        let measurer = DeterministicTextMeasurer::default();
        let style = TextStyle {
            font_size: 16.0,
            ..TextStyle::default()
        };
        let width_model = LineWidthModel {
            font_size: 16.0,
            source: LineWidthSource::Heuristic,
            style: &style,
        };

        for (text, first, second) in [
            ("👩‍🔬👨‍🔬", "👩‍🔬", "👨‍🔬"),
            ("👍🏽👍🏻", "👍🏽", "👍🏻"),
            ("🇨🇳🇺🇸", "🇨🇳", "🇺🇸"),
            ("1️⃣2️⃣", "1️⃣", "2️⃣"),
        ] {
            let (head, tail) =
                DeterministicTextMeasurer::split_token_to_width(text, 16.0, width_model);
            assert_eq!((head, tail), (first, second), "text={text:?}");

            let metrics = measurer.measure_wrapped(text, &style, Some(16.0), WrapMode::SvgLike);
            assert_eq!(metrics.line_count, 2, "text={text:?}: {metrics:?}");
            assert_eq!(metrics.width, 16.0, "text={text:?}: {metrics:?}");
        }
    }

    #[test]
    fn html_long_words_preserve_their_min_content_width() {
        let measurer = DeterministicTextMeasurer::default();
        let style = TextStyle {
            font_size: 10.0,
            ..TextStyle::default()
        };

        let metrics = measurer.measure_wrapped(
            "Supercalifragilisticexpialidocious",
            &style,
            Some(20.0),
            WrapMode::HtmlLike,
        );

        assert_eq!(metrics.line_count, 1);
        assert!(metrics.width > 20.0, "{metrics:?}");
    }

    #[test]
    fn html_wrapping_uses_unicode_soft_breaks_and_fixed_container_width() {
        let measurer = DeterministicTextMeasurer::default();
        let style = TextStyle {
            font_size: 10.0,
            ..TextStyle::default()
        };

        let cjk = measurer.measure_wrapped("负责人审批", &style, Some(25.0), WrapMode::HtmlLike);
        assert_eq!(cjk.line_count, 3);
        assert_eq!(cjk.width.to_bits(), 25.0_f64.to_bits());

        let breakable =
            measurer.measure_wrapped("alpha beta", &style, Some(40.0), WrapMode::HtmlLike);
        assert_eq!(breakable.line_count, 2);
        assert_eq!(breakable.width.to_bits(), 40.0_f64.to_bits());

        let hyphenated =
            measurer.measure_wrapped("half-rounded", &style, Some(21.5), WrapMode::HtmlLike);
        assert_eq!(hyphenated.line_count, 2);
        assert!(
            hyphenated.width > 21.5,
            "unbreakable segment keeps min-content width: {hyphenated:?}"
        );

        let preserved_spaces =
            measurer.measure_wrapped("a  b", &style, Some(7.0), WrapMode::HtmlLike);
        assert_eq!(preserved_spaces.line_count, 3);
        assert!(preserved_spaces.width > 7.0, "{preserved_spaces:?}");
    }

    #[test]
    fn repeated_measurement_is_bitwise_deterministic() {
        let measurer = DeterministicTextMeasurer::default();
        let style = TextStyle {
            font_size: 16.0,
            ..TextStyle::default()
        };
        let first = measurer.measure_wrapped(
            "Latin / 中文 / 🙂 / e\u{0301}",
            &style,
            Some(80.0),
            WrapMode::SvgLike,
        );
        let second = measurer.measure_wrapped(
            "Latin / 中文 / 🙂 / e\u{0301}",
            &style,
            Some(80.0),
            WrapMode::SvgLike,
        );

        assert_eq!(first.width.to_bits(), second.width.to_bits());
        assert_eq!(first.height.to_bits(), second.height.to_bits());
        assert_eq!(first.line_count, second.line_count);
    }

    #[test]
    fn exact_natural_width_does_not_trigger_a_spurious_wrap() {
        let measurer = DeterministicTextMeasurer::default();
        let style = TextStyle {
            font_size: 16.0,
            ..TextStyle::default()
        };
        let text = "FtQHasK pGRJ";

        for mode in [WrapMode::SvgLike, WrapMode::HtmlLike] {
            let natural = measurer.measure_wrapped(text, &style, None, mode);
            let fitted = measurer.measure_wrapped(text, &style, Some(natural.width), mode);
            assert_eq!(fitted.line_count, 1, "{mode:?}: {natural:?} -> {fitted:?}");
            assert_eq!(fitted.width.to_bits(), natural.width.to_bits());
        }
    }

    #[test]
    fn non_additive_callback_fits_at_exact_natural_width() {
        let measurer = DeterministicTextMeasurer::default().with_width_callback(|text, _| {
            let count = text.chars().count();
            count as f64 * 10.0 - count.saturating_sub(1) as f64
        });
        let style = TextStyle::default();
        let text = "A V";

        for mode in [WrapMode::SvgLike, WrapMode::HtmlLike] {
            let natural = measurer.measure_wrapped(text, &style, None, mode);
            assert_eq!(natural.width, 28.0);
            let fitted = measurer.measure_wrapped(text, &style, Some(natural.width), mode);
            assert_eq!(fitted.line_count, 1, "{mode:?}: {natural:?} -> {fitted:?}");
            assert_eq!(fitted.width.to_bits(), natural.width.to_bits());
        }
    }

    #[test]
    fn callback_preserves_html_fixed_width_and_long_word_overflow() {
        let measurer = DeterministicTextMeasurer::default()
            .with_width_callback(|text, _| text.graphemes(true).count() as f64 * 10.0);
        let style = TextStyle::default();

        let breakable =
            measurer.measure_wrapped("alpha beta", &style, Some(60.0), WrapMode::HtmlLike);
        assert_eq!(breakable.line_count, 2);
        assert_eq!(breakable.width, 60.0);

        let long_word =
            measurer.measure_wrapped("abcdefgh", &style, Some(30.0), WrapMode::HtmlLike);
        assert_eq!(long_word.line_count, 1);
        assert_eq!(long_word.width, 80.0);
    }

    #[test]
    fn callback_svg_wrapping_splits_only_at_grapheme_boundaries() {
        let measurer = DeterministicTextMeasurer::default()
            .with_width_callback(|text, _| text.graphemes(true).count() as f64 * 10.0);

        let metrics =
            measurer.measure_wrapped("👩‍🔬👨‍🔬", &TextStyle::default(), Some(10.0), WrapMode::SvgLike);

        assert_eq!(metrics.line_count, 2);
        assert_eq!(metrics.width, 10.0);
    }

    #[test]
    fn callback_receives_complete_text_and_returns_pixel_width() {
        let calls = Arc::new(Mutex::new(Vec::new()));
        let recorded = Arc::clone(&calls);
        let measurer =
            DeterministicTextMeasurer::default().with_width_callback(move |text, style| {
                recorded
                    .lock()
                    .unwrap()
                    .push((text.to_string(), style.font_size));
                37.0
            });
        let style = TextStyle {
            font_size: 99.0,
            ..TextStyle::default()
        };

        assert_eq!(measurer.measure("ffi", &style).width, 37.0);
        assert_eq!(&*calls.lock().unwrap(), &[("ffi".to_string(), 99.0)]);
    }
}