typst/src/layout/stack.rs

use std::fmt::{self, Debug, Formatter};

use super::*;

/// A node that stacks its children.
#[derive(Debug, Hash)]
pub struct StackNode {
    /// The stacking direction.
    pub dir: Dir,
    /// The children to be stacked.
    pub children: Vec<StackChild>,
}

/// A child of a stack node.
#[derive(Hash)]
pub enum StackChild {
    /// Spacing between other nodes.
    Spacing(Spacing),
    /// Any block node and how to align it in the stack.
    Node(BlockNode, Align),
}

impl BlockLevel for StackNode {
    fn layout(
        &self,
        ctx: &mut LayoutContext,
        regions: &Regions,
    ) -> Vec<Constrained<Rc<Frame>>> {
        StackLayouter::new(self, regions.clone()).layout(ctx)
    }
}

impl Debug for StackChild {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::Spacing(v) => write!(f, "Spacing({:?})", v),
            Self::Node(node, _) => node.fmt(f),
        }
    }
}

/// Performs stack layout.
struct StackLayouter<'a> {
    /// The stack node to layout.
    stack: &'a StackNode,
    /// The axis of the block direction.
    axis: SpecAxis,
    /// Whether the stack should expand to fill the region.
    expand: Spec<bool>,
    /// The region to layout into.
    regions: Regions,
    /// The full size of `regions.current` that was available before we started
    /// subtracting.
    full: Size,
    /// The generic size used by the frames for the current region.
    used: Gen<Length>,
    /// The sum of fractional ratios in the current region.
    fr: Fractional,
    /// Spacing and layouted nodes.
    items: Vec<StackItem>,
    /// Finished frames for previous regions.
    finished: Vec<Constrained<Rc<Frame>>>,
}

/// A prepared item in a stack layout.
enum StackItem {
    /// Absolute spacing between other items.
    Absolute(Length),
    /// Fractional spacing between other items.
    Fractional(Fractional),
    /// A layouted child node.
    Frame(Rc<Frame>, Align),
}

impl<'a> StackLayouter<'a> {
    /// Create a new stack layouter.
    fn new(stack: &'a StackNode, mut regions: Regions) -> Self {
        // Disable expansion along the block axis for children.
        let axis = stack.dir.axis();
        let expand = regions.expand;
        regions.expand.set(axis, false);

        Self {
            stack,
            axis,
            expand,
            full: regions.current,
            regions,
            used: Gen::zero(),
            fr: Fractional::zero(),
            items: vec![],
            finished: vec![],
        }
    }

    /// Layout all children.
    fn layout(mut self, ctx: &mut LayoutContext) -> Vec<Constrained<Rc<Frame>>> {
        for child in &self.stack.children {
            match *child {
                StackChild::Spacing(Spacing::Linear(v)) => {
                    self.layout_absolute(v);
                }
                StackChild::Spacing(Spacing::Fractional(v)) => {
                    self.items.push(StackItem::Fractional(v));
                    self.fr += v;
                }
                StackChild::Node(ref node, align) => {
                    self.layout_node(ctx, node, align);
                }
            }
        }

        self.finish_region();
        self.finished
    }

    /// Layout absolute spacing.
    fn layout_absolute(&mut self, amount: Linear) {
        // Resolve the linear, limiting it to the remaining available space.
        let remaining = self.regions.current.get_mut(self.axis);
        let resolved = amount.resolve(self.full.get(self.axis));
        let limited = resolved.min(*remaining);
        *remaining -= limited;
        self.used.block += limited;
        self.items.push(StackItem::Absolute(resolved));
    }

    /// Layout a block node.
    fn layout_node(&mut self, ctx: &mut LayoutContext, node: &BlockNode, align: Align) {
        let frames = node.layout(ctx, &self.regions);
        let len = frames.len();
        for (i, frame) in frames.into_iter().enumerate() {
            // Grow our size.
            let size = frame.item.size.to_gen(self.axis);
            self.used.block += size.block;
            self.used.inline.set_max(size.inline);

            // Remember the frame and shrink available space in the region for the
            // following children.
            self.items.push(StackItem::Frame(frame.item, align));
            *self.regions.current.get_mut(self.axis) -= size.block;

            if i + 1 < len {
                self.finish_region();
            }
        }
    }

    /// Finish the frame for one region.
    fn finish_region(&mut self) {
        // Determine the size that remains for fractional spacing.
        let remaining = self.full.get(self.axis) - self.used.block;

        // Determine the size of the stack in this region dependening on whether
        // the region expands.
        let used = self.used.to_size(self.axis);
        let mut size = Size::new(
            if self.expand.x { self.full.w } else { used.w },
            if self.expand.y { self.full.h } else { used.h },
        );

        // Expand fully if there are fr spacings.
        let full = self.full.get(self.axis);
        if !self.fr.is_zero() && full.is_finite() {
            size.set(self.axis, full);
        }

        let mut output = Frame::new(size, size.h);
        let mut before = Length::zero();
        let mut ruler = Align::Start;
        let mut first = true;

        // Place all frames.
        for item in self.items.drain(..) {
            match item {
                StackItem::Absolute(v) => before += v,
                StackItem::Fractional(v) => {
                    let ratio = v / self.fr;
                    if remaining.is_finite() && ratio.is_finite() {
                        before += ratio * remaining;
                    }
                }
                StackItem::Frame(frame, align) => {
                    ruler = ruler.max(align);

                    let parent = size.to_gen(self.axis);
                    let child = frame.size.to_gen(self.axis);

                    // Align along the block axis.
                    let block = ruler.resolve(
                        self.stack.dir,
                        if self.stack.dir.is_positive() {
                            let after = self.used.block - before;
                            before .. parent.block - after
                        } else {
                            let before_with_self = before + child.block;
                            let after = self.used.block - before_with_self;
                            after .. parent.block - before_with_self
                        },
                    );

                    let pos = Gen::new(Length::zero(), block).to_point(self.axis);
                    if first {
                        // The baseline of the stack is that of the first frame.
                        output.baseline = pos.y + frame.baseline;
                        first = false;
                    }

                    output.push_frame(pos, frame);
                    before += child.block;
                }
            }
        }

        // Generate tight constraints for now.
        let mut cts = Constraints::new(self.expand);
        cts.exact = self.full.to_spec().map(Some);
        cts.base = self.regions.base.to_spec().map(Some);

        self.regions.next();
        self.full = self.regions.current;
        self.used = Gen::zero();
        self.fr = Fractional::zero();
        self.finished.push(output.constrain(cts));
    }
}