typst/library/src/math/frac.rs

use super::*;

const FRAC_AROUND: Em = Em::new(0.1);

/// # Fraction
/// A mathematical fraction.
///
/// ## Example
/// ```example
/// $ 1/2 < (x+1)/2 $
/// $ ((x+1)) / 2 = frac(a, b) $
/// ```
///
/// ## Syntax
/// This function also has dedicated syntax: Use a slash to turn neighbouring
/// expressions into a fraction. Multiple atoms can be grouped into a single
/// expression using round grouping parenthesis. Such parentheses are removed
/// from the output, but you can nest multiple to force them.
///
/// Display: Fraction
/// Category: math
#[element(LayoutMath)]
pub struct FracElem {
    /// The fraction's numerator.
    #[required]
    pub num: Content,

    /// The fraction's denominator.
    #[required]
    pub denom: Content,
}

impl LayoutMath for FracElem {
    fn layout_math(&self, ctx: &mut MathContext) -> SourceResult<()> {
        layout(ctx, &self.num(), &self.denom(), false, self.span())
    }
}

/// A binomial expression.
///
/// ## Example
/// ```example
/// $ binom(n, k) $
/// ```
///
/// Display: Binomial
/// Category: math
#[element(LayoutMath)]
pub struct BinomElem {
    /// The binomial's upper index.
    #[required]
    pub upper: Content,

    /// The binomial's lower index.
    #[required]
    pub lower: Content,
}

impl LayoutMath for BinomElem {
    fn layout_math(&self, ctx: &mut MathContext) -> SourceResult<()> {
        layout(ctx, &self.upper(), &self.lower(), true, self.span())
    }
}

/// Layout a fraction or binomial.
fn layout(
    ctx: &mut MathContext,
    num: &Content,
    denom: &Content,
    binom: bool,
    span: Span,
) -> SourceResult<()> {
    let short_fall = DELIM_SHORT_FALL.scaled(ctx);
    let axis = scaled!(ctx, axis_height);
    let thickness = scaled!(ctx, fraction_rule_thickness);
    let shift_up = scaled!(
        ctx,
        text: fraction_numerator_shift_up,
        display: fraction_numerator_display_style_shift_up,
    );
    let shift_down = scaled!(
        ctx,
        text: fraction_denominator_shift_down,
        display: fraction_denominator_display_style_shift_down,
    );
    let num_min = scaled!(
        ctx,
        text: fraction_numerator_gap_min,
        display: fraction_num_display_style_gap_min,
    );
    let denom_min = scaled!(
        ctx,
        text: fraction_denominator_gap_min,
        display: fraction_denom_display_style_gap_min,
    );

    ctx.style(ctx.style.for_numerator());
    let num = ctx.layout_frame(num)?;
    ctx.unstyle();

    ctx.style(ctx.style.for_denominator());
    let denom = ctx.layout_frame(denom)?;
    ctx.unstyle();

    let around = FRAC_AROUND.scaled(ctx);
    let num_gap = (shift_up - axis - num.descent()).max(num_min + thickness / 2.0);
    let denom_gap = (shift_down + axis - denom.ascent()).max(denom_min + thickness / 2.0);

    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::new(size);
    frame.set_baseline(baseline);
    frame.push_frame(num_pos, num);
    frame.push_frame(denom_pos, denom);

    if binom {
        ctx.push(
            GlyphFragment::new(ctx, '(', span).stretch_vertical(ctx, height, short_fall),
        );
        ctx.push(FrameFragment::new(ctx, frame));
        ctx.push(
            GlyphFragment::new(ctx, ')', span).stretch_vertical(ctx, height, short_fall),
        );
    } else {
        frame.push(
            line_pos,
            FrameItem::Shape(
                Geometry::Line(Point::with_x(line_width)).stroked(Stroke {
                    paint: TextElem::fill_in(ctx.styles()),
                    thickness,
                }),
                span,
            ),
        );
        ctx.push(FrameFragment::new(ctx, frame));
    }

    Ok(())
}