1 files changed, 75 insertions, 0 deletions
diff --git a/crates/typst-layout/src/math/accent.rs b/crates/typst-layout/src/math/accent.rs
new file mode 100644
index 00000000..9fa7a5a0
--- /dev/null
+++ b/crates/typst-layout/src/math/accent.rs
@@ -0,0 +1,75 @@
+use typst_library::diag::SourceResult;
+use typst_library::foundations::{Packed, StyleChain};
+use typst_library::layout::{Em, Frame, Point, Rel, Size};
+use typst_library::math::{Accent, AccentElem};
+
+use super::{
+    scaled_font_size, style_cramped, FrameFragment, GlyphFragment, MathContext,
+    MathFragment,
+};
+
+/// How much the accent can be shorter than the base.
+const ACCENT_SHORT_FALL: Em = Em::new(0.5);
+
+/// Lays out an [`AccentElem`].
+#[typst_macros::time(name = "math.accent", span = elem.span())]
+pub fn layout_accent(
+    elem: &Packed<AccentElem>,
+    ctx: &mut MathContext,
+    styles: StyleChain,
+) -> SourceResult<()> {
+    let cramped = style_cramped();
+    let base = ctx.layout_into_fragment(elem.base(), styles.chain(&cramped))?;
+
+    // Preserve class to preserve automatic spacing.
+    let base_class = base.class();
+    let base_attach = base.accent_attach();
+
+    let width = elem
+        .size(styles)
+        .unwrap_or(Rel::one())
+        .at(scaled_font_size(ctx, styles))
+        .relative_to(base.width());
+
+    // Forcing the accent to be at least as large as the base makes it too
+    // wide in many case.
+    let Accent(c) = elem.accent();
+    let glyph = GlyphFragment::new(ctx, styles, *c, elem.span());
+    let short_fall = ACCENT_SHORT_FALL.at(glyph.font_size);
+    let variant = glyph.stretch_horizontal(ctx, width, short_fall);
+    let accent = variant.frame;
+    let accent_attach = variant.accent_attach;
+
+    // Descent is negative because the accent's ink bottom is above the
+    // baseline. Therefore, the default gap is the accent's negated descent
+    // minus the accent base height. Only if the base is very small, we need
+    // a larger gap so that the accent doesn't move too low.
+    let accent_base_height = scaled!(ctx, styles, accent_base_height);
+    let gap = -accent.descent() - base.height().min(accent_base_height);
+    let size = Size::new(base.width(), accent.height() + gap + base.height());
+    let accent_pos = Point::with_x(base_attach - accent_attach);
+    let base_pos = Point::with_y(accent.height() + gap);
+    let baseline = base_pos.y + base.ascent();
+    let base_italics_correction = base.italics_correction();
+    let base_text_like = base.is_text_like();
+
+    let base_ascent = match &base {
+        MathFragment::Frame(frame) => frame.base_ascent,
+        _ => base.ascent(),
+    };
+
+    let mut frame = Frame::soft(size);
+    frame.set_baseline(baseline);
+    frame.push_frame(accent_pos, accent);
+    frame.push_frame(base_pos, base.into_frame());
+    ctx.push(
+        FrameFragment::new(ctx, styles, frame)
+            .with_class(base_class)
+            .with_base_ascent(base_ascent)
+            .with_italics_correction(base_italics_correction)
+            .with_accent_attach(base_attach)
+            .with_text_like(base_text_like),
+    );
+
+    Ok(())
+}