1 files changed, 136 insertions, 0 deletions
diff --git a/crates/typst-layout/src/math/frac.rs b/crates/typst-layout/src/math/frac.rs
new file mode 100644
index 00000000..50686333
--- /dev/null
+++ b/crates/typst-layout/src/math/frac.rs
@@ -0,0 +1,136 @@
+use typst_library::diag::SourceResult;
+use typst_library::foundations::{Content, Packed, StyleChain};
+use typst_library::layout::{Em, Frame, FrameItem, Point, Size};
+use typst_library::math::{BinomElem, FracElem};
+use typst_library::text::TextElem;
+use typst_library::visualize::{FixedStroke, Geometry};
+use typst_syntax::Span;
+
+use super::{
+    scaled_font_size, style_for_denominator, style_for_numerator, FrameFragment,
+    GlyphFragment, MathContext, DELIM_SHORT_FALL,
+};
+
+const FRAC_AROUND: Em = Em::new(0.1);
+
+/// Lays out a [`FracElem`].
+#[typst_macros::time(name = "math.frac", span = elem.span())]
+pub fn layout_frac(
+    elem: &Packed<FracElem>,
+    ctx: &mut MathContext,
+    styles: StyleChain,
+) -> SourceResult<()> {
+    layout_frac_like(
+        ctx,
+        styles,
+        elem.num(),
+        std::slice::from_ref(elem.denom()),
+        false,
+        elem.span(),
+    )
+}
+
+/// Lays out a [`BinomElem`].
+#[typst_macros::time(name = "math.binom", span = elem.span())]
+pub fn layout_binom(
+    elem: &Packed<BinomElem>,
+    ctx: &mut MathContext,
+    styles: StyleChain,
+) -> SourceResult<()> {
+    layout_frac_like(ctx, styles, elem.upper(), elem.lower(), true, elem.span())
+}
+
+/// Layout a fraction or binomial.
+fn layout_frac_like(
+    ctx: &mut MathContext,
+    styles: StyleChain,
+    num: &Content,
+    denom: &[Content],
+    binom: bool,
+    span: Span,
+) -> SourceResult<()> {
+    let font_size = scaled_font_size(ctx, styles);
+    let short_fall = DELIM_SHORT_FALL.at(font_size);
+    let axis = scaled!(ctx, styles, axis_height);
+    let thickness = scaled!(ctx, styles, fraction_rule_thickness);
+    let shift_up = scaled!(
+        ctx, styles,
+        text: fraction_numerator_shift_up,
+        display: fraction_numerator_display_style_shift_up,
+    );
+    let shift_down = scaled!(
+        ctx, styles,
+        text: fraction_denominator_shift_down,
+        display: fraction_denominator_display_style_shift_down,
+    );
+    let num_min = scaled!(
+        ctx, styles,
+        text: fraction_numerator_gap_min,
+        display: fraction_num_display_style_gap_min,
+    );
+    let denom_min = scaled!(
+        ctx, styles,
+        text: fraction_denominator_gap_min,
+        display: fraction_denom_display_style_gap_min,
+    );
+
+    let num_style = style_for_numerator(styles);
+    let num = ctx.layout_into_frame(num, styles.chain(&num_style))?;
+
+    let denom_style = style_for_denominator(styles);
+    let denom = ctx.layout_into_frame(
+        &Content::sequence(
+            // Add a comma between each element.
+            denom.iter().flat_map(|a| [TextElem::packed(','), a.clone()]).skip(1),
+        ),
+        styles.chain(&denom_style),
+    )?;
+
+    let around = FRAC_AROUND.at(font_size);
+    let num_gap = (shift_up - (axis + thickness / 2.0) - num.descent()).max(num_min);
+    let denom_gap =
+        (shift_down + (axis - thickness / 2.0) - denom.ascent()).max(denom_min);
+
+    let line_width = num.width().max(denom.width());
+    let width = line_width + 2.0 * around;
+    let height = num.height() + num_gap + thickness + denom_gap + denom.height();
+    let size = Size::new(width, height);
+    let num_pos = Point::with_x((width - num.width()) / 2.0);
+    let line_pos =
+        Point::new((width - line_width) / 2.0, num.height() + num_gap + thickness / 2.0);
+    let denom_pos = Point::new((width - denom.width()) / 2.0, height - denom.height());
+    let baseline = line_pos.y + axis;
+
+    let mut frame = Frame::soft(size);
+    frame.set_baseline(baseline);
+    frame.push_frame(num_pos, num);
+    frame.push_frame(denom_pos, denom);
+
+    if binom {
+        let mut left = GlyphFragment::new(ctx, styles, '(', span)
+            .stretch_vertical(ctx, height, short_fall);
+        left.center_on_axis(ctx);
+        ctx.push(left);
+        ctx.push(FrameFragment::new(ctx, styles, frame));
+        let mut right = GlyphFragment::new(ctx, styles, ')', span)
+            .stretch_vertical(ctx, height, short_fall);
+        right.center_on_axis(ctx);
+        ctx.push(right);
+    } else {
+        frame.push(
+            line_pos,
+            FrameItem::Shape(
+                Geometry::Line(Point::with_x(line_width)).stroked(
+                    FixedStroke::from_pair(
+                        TextElem::fill_in(styles).as_decoration(),
+                        thickness,
+                    ),
+                ),
+                span,
+            ),
+        );
+        ctx.push(FrameFragment::new(ctx, styles, frame));
+    }
+
+    Ok(())
+}