Implement fraction styles: vertical, skewed, and horizontal. (#6672)

crates/typst-eval/src/math.rs

@@ -123,9 +123,20 @@ impl Eval for ast::MathFrac<'_> {
     type Output = Content;
 
     fn eval(self, vm: &mut Vm) -> SourceResult<Self::Output> {
-        let num = self.num().eval_display(vm)?;
-        let denom = self.denom().eval_display(vm)?;
-        Ok(FracElem::new(num, denom).pack())
+        let num_expr = self.num();
+        let num = num_expr.eval_display(vm)?;
+        let denom_expr = self.denom();
+        let denom = denom_expr.eval_display(vm)?;
+
+        let num_depar =
+            matches!(num_expr, ast::Expr::Math(math) if math.was_deparenthesized());
+        let denom_depar =
+            matches!(denom_expr, ast::Expr::Math(math) if math.was_deparenthesized());
+
+        Ok(FracElem::new(num, denom)
+            .with_num_deparenthesized(num_depar)
+            .with_denom_deparenthesized(denom_depar)
+            .pack())
     }
 }
 

crates/typst-layout/src/math/frac.rs

@@ -1,7 +1,11 @@
 use typst_library::diag::SourceResult;
-use typst_library::foundations::{Content, Packed, Resolve, StyleChain, SymbolElem};
-use typst_library::layout::{Em, Frame, FrameItem, Point, Size};
-use typst_library::math::{BinomElem, EquationElem, FracElem, MathSize};
+use typst_library::foundations::{
+    Content, NativeElement, Packed, Resolve, StyleChain, SymbolElem,
+};
+use typst_library::layout::{Abs, Em, Frame, FrameItem, Point, Size};
+use typst_library::math::{
+    BinomElem, EquationElem, FracElem, FracStyle, LrElem, MathSize,
+};
 use typst_library::text::TextElem;
 use typst_library::visualize::{FixedStroke, Geometry};
 use typst_syntax::Span;
@@ -20,14 +24,28 @@ pub fn layout_frac(
     ctx: &mut MathContext,
     styles: StyleChain,
 ) -> SourceResult<()> {
-    layout_frac_like(
-        ctx,
-        styles,
-        &elem.num,
-        std::slice::from_ref(&elem.denom),
-        false,
-        elem.span(),
-    )
+    match elem.style.get(styles) {
+        FracStyle::Skewed => {
+            layout_skewed_frac(ctx, styles, &elem.num, &elem.denom, elem.span())
+        }
+        FracStyle::Horizontal => layout_horizontal_frac(
+            ctx,
+            styles,
+            &elem.num,
+            &elem.denom,
+            elem.span(),
+            elem.num_deparenthesized.get(styles),
+            elem.denom_deparenthesized.get(styles),
+        ),
+        FracStyle::Vertical => layout_vertical_frac_like(
+            ctx,
+            styles,
+            &elem.num,
+            std::slice::from_ref(&elem.denom),
+            false,
+            elem.span(),
+        ),
+    }
 }
 
 /// Lays out a [`BinomElem`].
@@ -37,11 +55,11 @@ pub fn layout_binom(
     ctx: &mut MathContext,
     styles: StyleChain,
 ) -> SourceResult<()> {
-    layout_frac_like(ctx, styles, &elem.upper, &elem.lower, true, elem.span())
+    layout_vertical_frac_like(ctx, styles, &elem.upper, &elem.lower, true, elem.span())
 }
 
-/// Layout a fraction or binomial.
-fn layout_frac_like(
+/// Layout a vertical fraction or binomial.
+fn layout_vertical_frac_like(
     ctx: &mut MathContext,
     styles: StyleChain,
     num: &Content,
@@ -143,3 +161,119 @@ fn layout_frac_like(
 
     Ok(())
 }
+
+// Lays out a horizontal fraction
+fn layout_horizontal_frac(
+    ctx: &mut MathContext,
+    styles: StyleChain,
+    num: &Content,
+    denom: &Content,
+    span: Span,
+    num_deparen: bool,
+    denom_deparen: bool,
+) -> SourceResult<()> {
+    let num = if num_deparen {
+        &LrElem::new(Content::sequence(vec![
+            SymbolElem::packed('('),
+            num.clone(),
+            SymbolElem::packed(')'),
+        ]))
+        .pack()
+    } else {
+        num
+    };
+    let num_frame = ctx.layout_into_fragment(num, styles)?;
+    ctx.push(num_frame);
+
+    let mut slash =
+        ctx.layout_into_fragment(&SymbolElem::packed('/').spanned(span), styles)?;
+    slash.center_on_axis();
+    ctx.push(slash);
+
+    let denom = if denom_deparen {
+        &LrElem::new(Content::sequence(vec![
+            SymbolElem::packed('('),
+            denom.clone(),
+            SymbolElem::packed(')'),
+        ]))
+        .pack()
+    } else {
+        denom
+    };
+    let denom_frame = ctx.layout_into_fragment(denom, styles)?;
+    ctx.push(denom_frame);
+
+    Ok(())
+}
+
+/// Lay out a skewed fraction.
+fn layout_skewed_frac(
+    ctx: &mut MathContext,
+    styles: StyleChain,
+    num: &Content,
+    denom: &Content,
+    span: Span,
+) -> SourceResult<()> {
+    // Font-derived constants
+    let constants = ctx.font().math();
+    let vgap = constants.skewed_fraction_vertical_gap.resolve(styles);
+    let hgap = constants.skewed_fraction_horizontal_gap.resolve(styles);
+    let axis = constants.axis_height.resolve(styles);
+
+    let num_style = style_for_numerator(styles);
+    let num_frame = ctx.layout_into_frame(num, styles.chain(&num_style))?;
+    let num_size = num_frame.size();
+    let denom_style = style_for_denominator(styles);
+    let denom_frame = ctx.layout_into_frame(denom, styles.chain(&denom_style))?;
+    let denom_size = denom_frame.size();
+
+    let short_fall = DELIM_SHORT_FALL.resolve(styles);
+
+    // Height of the fraction frame
+    // We recalculate this value below if the slash glyph overflows
+    let mut fraction_height = num_size.y + denom_size.y + vgap;
+
+    // Build the slash glyph to calculate its size
+    let mut slash_frag =
+        ctx.layout_into_fragment(&SymbolElem::packed('\u{2044}').spanned(span), styles)?;
+    slash_frag.stretch_vertical(ctx, fraction_height - short_fall);
+    slash_frag.center_on_axis();
+    let slash_frame = slash_frag.into_frame();
+
+    // Adjust the fraction height if the slash overflows
+    let slash_size = slash_frame.size();
+    let vertical_offset = Abs::zero().max(slash_size.y - fraction_height) / 2.0;
+    fraction_height.set_max(slash_size.y);
+
+    // Reference points for all three objects, used to place them in the frame.
+    let mut slash_up_left = Point::new(num_size.x + hgap / 2.0, fraction_height / 2.0)
+        - slash_size.to_point() / 2.0;
+    let mut num_up_left = Point::with_y(vertical_offset);
+    let mut denom_up_left = num_up_left + num_size.to_point() + Point::new(hgap, vgap);
+
+    // Fraction width
+    let fraction_width = (denom_up_left.x + denom_size.x)
+        .max(slash_up_left.x + slash_size.x)
+        + Abs::zero().max(-slash_up_left.x);
+    // We have to shift everything right to avoid going in the negatives for
+    // the x coordinate
+    let horizontal_offset = Point::with_x(Abs::zero().max(-slash_up_left.x));
+    slash_up_left += horizontal_offset;
+    num_up_left += horizontal_offset;
+    denom_up_left += horizontal_offset;
+
+    // Build the final frame
+    let mut fraction_frame = Frame::soft(Size::new(fraction_width, fraction_height));
+
+    // Baseline (use axis height to center slash on the axis)
+    fraction_frame.set_baseline(fraction_height / 2.0 + axis);
+
+    // Numerator, Denominator, Slash
+    fraction_frame.push_frame(num_up_left, num_frame);
+    fraction_frame.push_frame(denom_up_left, denom_frame);
+    fraction_frame.push_frame(slash_up_left, slash_frame);
+
+    ctx.push(FrameFragment::new(styles, fraction_frame));
+
+    Ok(())
+}

crates/typst-library/src/math/frac.rs

@@ -1,7 +1,7 @@
 use typst_syntax::Spanned;
 
 use crate::diag::bail;
-use crate::foundations::{Content, Value, elem};
+use crate::foundations::{Cast, Content, Value, elem};
 use crate::math::Mathy;
 
 /// A mathematical fraction.
@@ -26,6 +26,43 @@ pub struct FracElem {
     /// The fraction's denominator.
     #[required]
     pub denom: Content,
+
+    /// How the fraction shoud be laid out.
+    ///
+    /// ```example
+    /// #set math.frac(style: "skewed")
+    /// $ a / b $
+    /// $ frac(x, y, style: "vertical") $
+    /// ```
+    #[default(FracStyle::Vertical)]
+    pub style: FracStyle,
+
+    /// Whether the numerator was originally surrounded by parentheses
+    /// that were stripped by the parser.
+    #[internal]
+    #[parse(None)]
+    #[default(false)]
+    pub num_deparenthesized: bool,
+
+    /// Whether the denominator was originally surrounded by parentheses
+    /// that were stripped by the parser.
+    #[internal]
+    #[parse(None)]
+    #[default(false)]
+    pub denom_deparenthesized: bool,
+}
+
+/// Fraction style
+#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Hash, Cast)]
+pub enum FracStyle {
+    /// Stacked numerator and denominator with a bar.
+    #[default]
+    Vertical,
+    /// Numerator and denominator separated by a slash.
+    Skewed,
+    /// Numerator and denominator placed inline and parentheses are not
+    /// absorbed.
+    Horizontal,
 }
 
 /// A binomial expression.

crates/typst-library/src/text/font/mod.rs

@@ -379,6 +379,10 @@ impl FontMetrics {
                         .to_em(constants.fraction_denominator_gap_min().value),
                     fraction_denom_display_style_gap_min: font
                         .to_em(constants.fraction_denom_display_style_gap_min().value),
+                    skewed_fraction_vertical_gap: font
+                        .to_em(constants.skewed_fraction_vertical_gap().value),
+                    skewed_fraction_horizontal_gap: font
+                        .to_em(constants.skewed_fraction_horizontal_gap().value),
                     overbar_vertical_gap: font
                         .to_em(constants.overbar_vertical_gap().value),
                     overbar_rule_thickness: font
@@ -413,6 +417,8 @@ impl FontMetrics {
                 // - `flattened_accent_base_height` from Building Math Fonts
                 // - `overbar_rule_thickness` and `underbar_rule_thickness`
                 //   from our best guess
+                // - `skewed_fraction_vertical_gap` and `skewed_fraction_horizontal_gap`
+                //   from our best guess
                 // - `script_percent_scale_down` and
                 //   `script_script_percent_scale_down` from Building Math
                 //   Fonts as the defaults given in MathML Core have more
@@ -458,6 +464,8 @@ impl FontMetrics {
                     fraction_denominator_gap_min: metrics.underline.thickness,
                     fraction_denom_display_style_gap_min: 3.0
                         * metrics.underline.thickness,
+                    skewed_fraction_vertical_gap: Em::zero(),
+                    skewed_fraction_horizontal_gap: Em::new(0.5),
                     overbar_vertical_gap: 3.0 * metrics.underline.thickness,
                     overbar_rule_thickness: metrics.underline.thickness,
                     overbar_extra_ascender: metrics.underline.thickness,
@@ -553,6 +561,8 @@ pub struct MathConstants {
     pub fraction_rule_thickness: Em,
     pub fraction_denominator_gap_min: Em,
     pub fraction_denom_display_style_gap_min: Em,
+    pub skewed_fraction_vertical_gap: Em,
+    pub skewed_fraction_horizontal_gap: Em,
     pub overbar_vertical_gap: Em,
     pub overbar_rule_thickness: Em,
     pub overbar_extra_ascender: Em,

crates/typst-syntax/src/ast.rs

@@ -840,6 +840,16 @@ impl<'a> Math<'a> {
     pub fn exprs(self) -> impl DoubleEndedIterator<Item = Expr<'a>> {
         self.0.children().filter_map(Expr::cast_with_space)
     }
+
+    /// Whether this `Math` node was originally parenthesized.
+    pub fn was_deparenthesized(self) -> bool {
+        let mut iter = self.0.children();
+        matches!(iter.next().map(SyntaxNode::kind), Some(SyntaxKind::LeftParen))
+            && matches!(
+                iter.next_back().map(SyntaxNode::kind),
+                Some(SyntaxKind::RightParen)
+            )
+    }
 }
 
 node! {

tests/suite/math/frac.typ

@@ -45,3 +45,34 @@ $ a_1/b_2, 1/f(x), zeta(x)/2, "foo"[|x|]/2 \
 --- math-frac-gap ---
 // Test that the gap above and below the fraction rule is correct.
 $ sqrt(n^(2/3)) $
+
+--- math-frac-horizontal ---
+// Test that horizontal fractions look identical to inline math with `slash`
+#set math.frac(style: "horizontal")
+$ (a / b) / (c / (d / e)) $
+$ (a slash b) slash (c slash (d slash e)) $
+
+--- math-frac-horizontal-lr-paren ---
+// Test that parentheses are in a left-right pair even when rebuilt by a horizontal fraction
+#set math.frac(style: "horizontal")
+$ (#v(2em)) / n $
+
+--- math-frac-skewed ---
+// Test skewed fractions
+#set math.frac(style: "skewed")
+$ a / b,  a / (b / c) $
+
+--- math-frac-horizontal-explicit ---
+// Test that explicit fractions don't change parentheses
+#set math.frac(style: "horizontal")
+$ frac(a, (b + c)), frac(a, b + c) $
+
+--- math-frac-horizontal-nonparen-brackets ---
+// Test that non-parentheses left-right pairs remain untouched
+#set math.frac(style: "horizontal")
+$ [x+y] / {z} $
+
+--- math-frac-styles-inline ---
+// Test inline layout of styled fractions
+#set math.frac(style: "horizontal")
+$a/(b+c), frac(a, b+c, style: "skewed"), frac(a, b+c, style: "vertical")$