typst-layout 0.15.0

use typst_library::diag::SourceResult;
use typst_library::foundations::StyleChain;
use typst_library::layout::{
    Abs, Axis, Corners, Frame, FrameItem, Point, Rel, Sides, Size,
};
use typst_library::math::ir::{MathProperties, RadicalItem};
use typst_library::math::{EquationElem, MathSize};
use typst_library::text::TextElem;
use typst_library::visualize::{FixedStroke, Geometry};

use super::MathContext;
use super::fragment::FrameFragment;
use crate::shapes::styled_rect;

/// Lays out a [`RadicalItem`].
///
/// TeXbook page 443, page 360
/// See also: <https://www.w3.org/TR/mathml-core/#radicals-msqrt-mroot>
#[typst_macros::time(name = "math radical layout", span = props.span)]
pub fn layout_radical(
    item: &RadicalItem,
    ctx: &mut MathContext,
    styles: StyleChain,
    props: &MathProperties,
) -> SourceResult<()> {
    let radicand = ctx.layout_into_fragment(&item.radicand, styles)?.into_frame();
    let target = {
        let sqrt = ctx.layout_into_fragment(&item.sqrt, styles)?;
        let styles = item.sqrt.styles().unwrap_or(styles);
        let (font, size) = sqrt.font(ctx, styles);
        let thickness = font.math().radical_rule_thickness.at(size);
        let gap = match styles.get(EquationElem::size) {
            MathSize::Display => font.math().radical_display_style_vertical_gap,
            _ => font.math().radical_vertical_gap,
        }
        .at(size);
        radicand.height() + thickness + gap
    };

    // Layout root symbol.
    item.sqrt.set_stretch_relative_to(target, Axis::Y);
    let sqrt = ctx.layout_into_fragment(&item.sqrt, styles)?;
    let sqrt_styles = item.sqrt.styles().unwrap_or(styles);

    let (font, size) = sqrt.font(ctx, sqrt_styles);
    let thickness = font.math().radical_rule_thickness.at(size);
    let extra_ascender = font.math().radical_extra_ascender.at(size);
    let kern_before = font.math().radical_kern_before_degree.at(size);
    let kern_after = font.math().radical_kern_after_degree.at(size);
    let raise_factor = font.math().radical_degree_bottom_raise_percent;
    let gap = match sqrt_styles.get(EquationElem::size) {
        MathSize::Display => font.math().radical_display_style_vertical_gap,
        _ => font.math().radical_vertical_gap,
    }
    .at(size);

    let text_fill = sqrt
        .fill()
        .unwrap_or_else(|| sqrt_styles.get_ref(TextElem::fill).as_decoration());
    let line_width = radicand.width();
    let sqrt_stroke = sqrt.stroke();

    let sqrt = sqrt.into_frame();

    // Layout the index.
    let index = item
        .index
        .as_ref()
        .map(|index| ctx.layout_into_fragment(index, styles))
        .transpose()?
        .map(|frag| frag.into_frame());

    // TeXbook, page 443, item 11
    // Keep original gap, and then distribute any remaining free space
    // equally above and below.
    let gap = gap.max((sqrt.height() - thickness - radicand.height() + gap) / 2.0);

    let sqrt_ascent = radicand.ascent() + gap + thickness;
    let descent = sqrt.height() - sqrt_ascent;
    let inner_ascent = sqrt_ascent + extra_ascender;

    let mut sqrt_offset = Abs::zero();
    let mut shift_up = Abs::zero();
    let mut ascent = inner_ascent;

    if let Some(index) = &index {
        sqrt_offset = kern_before + index.width() + kern_after;
        // The formula below for how much raise the index by comes from
        // the TeXbook, page 360, in the definition of `\root`.
        // However, the `+ index.descent()` part is different from TeX.
        // Without it, descenders can collide with the surd, a rarity
        // in practice, but possible.  MS Word also adjusts index positions
        // for descenders.
        shift_up = raise_factor * (inner_ascent - descent) + index.descent();
        ascent.set_max(shift_up + index.ascent());
    }

    let sqrt_x = sqrt_offset.max(Abs::zero());
    let radicand_x = sqrt_x + sqrt.width();
    let radicand_y = ascent - radicand.ascent();
    let size = Size::new(radicand_x + line_width, ascent + descent);

    // The extra "- thickness" comes from the fact that the sqrt is placed
    // in `push_frame` with respect to its top, not its baseline.
    let sqrt_pos = Point::new(sqrt_x, radicand_y - gap - thickness);
    let line_pos = Point::new(radicand_x, radicand_y - gap - (thickness / 2.0));
    let radicand_pos = Point::new(radicand_x, radicand_y);

    let mut frame = Frame::soft(size);
    frame.set_baseline(ascent);

    if let Some(index) = index {
        let index_x = -sqrt_offset.min(Abs::zero()) + kern_before;
        let index_pos = Point::new(index_x, ascent - index.ascent() - shift_up);
        frame.push_frame(index_pos, index);
    }

    frame.push_frame(sqrt_pos, sqrt);

    // Add the horizontal line of the root symbol.
    if let Some(fixed_stroke) = sqrt_stroke {
        let sides = styled_rect(
            Size::new(line_width, thickness),
            &Corners::splat(Rel::<Abs>::zero()),
            Some(text_fill),
            &Sides::new(
                None, // Omit the left-side edge.
                Some(fixed_stroke.clone()),
                Some(fixed_stroke.clone()),
                Some(fixed_stroke),
            ),
        );
        frame.push_multiple(sides.into_iter().map(|shape| {
            (
                line_pos - Point::with_y(thickness / 2.0),
                FrameItem::Shape(shape, props.span),
            )
        }));
    } else {
        let line = FrameItem::Shape(
            Geometry::Line(Point::with_x(line_width))
                .stroked(FixedStroke::from_pair(text_fill, thickness)),
            props.span,
        );
        frame.push(line_pos, line);
    }

    frame.push_frame(radicand_pos, radicand);

    ctx.push(FrameFragment::new(props, styles, frame));
    Ok(())
}