typst/src/layout/pad.rs

use super::*;

/// A node that adds padding to its child.
#[derive(Debug, Clone, Eq, PartialEq)]
#[cfg_attr(feature = "layout-cache", derive(Hash))]
pub struct PadNode {
    /// The amount of padding.
    pub padding: Sides<Linear>,
    /// The child node whose sides to pad.
    pub child: LayoutNode,
}

impl Layout for PadNode {
    fn layout(
        &self,
        ctx: &mut LayoutContext,
        regions: &Regions,
    ) -> Vec<Constrained<Rc<Frame>>> {
        let mut frames = self.child.layout(
            ctx,
            &regions.map(|size| size - self.padding.resolve(size).size()),
        );

        for (Constrained { item: frame, constraints }, (current, base)) in
            frames.iter_mut().zip(regions.iter())
        {
            fn solve_axis(length: Length, padding: Linear) -> Length {
                (length + padding.abs) / (1.0 - padding.rel.get())
            }

            // Solve for the size `padded` that satisfies (approximately):
            // `padded - padding.resolve(padded).size() == size`
            let padded = Size::new(
                solve_axis(frame.size.width, self.padding.left + self.padding.right),
                solve_axis(frame.size.height, self.padding.top + self.padding.bottom),
            );

            let padding = self.padding.resolve(padded);
            let origin = Point::new(padding.left, padding.top);

            // Inflate min and max contraints by the padding.
            for spec in [&mut constraints.min, &mut constraints.max] {
                if let Some(horizontal) = spec.horizontal.as_mut() {
                    *horizontal += padding.size().width;
                }
                if let Some(vertical) = spec.vertical.as_mut() {
                    *vertical += padding.size().height;
                }
            }

            // Set exact and base constraints if the child had them.
            constraints.exact.horizontal.and_set(Some(current.width));
            constraints.exact.vertical.and_set(Some(current.height));
            constraints.base.horizontal.and_set(Some(base.width));
            constraints.base.vertical.and_set(Some(base.height));

            // Also set base constraints if the padding is relative.
            if self.padding.left.is_relative() || self.padding.right.is_relative() {
                constraints.base.horizontal = Some(base.width);
            }

            if self.padding.top.is_relative() || self.padding.bottom.is_relative() {
                constraints.base.vertical = Some(base.height);
            }

            // Create a new larger frame and place the child's frame inside it.
            let empty = Frame::new(padded, frame.baseline + origin.y);
            let prev = std::mem::replace(frame, Rc::new(empty));
            let new = Rc::make_mut(frame);
            new.push_frame(origin, prev);
        }

        frames
    }
}

impl From<PadNode> for LayoutNode {
    fn from(pad: PadNode) -> Self {
        Self::new(pad)
    }
}