typst/library/src/layout/flow.rs

use typst::model::{Property, Style};

use super::{AlignNode, ColbreakNode, PlaceNode, Spacing, VNode};
use crate::prelude::*;
use crate::text::ParNode;

/// Arrange spacing, paragraphs and block-level nodes into a flow.
///
/// This node is reponsible for layouting both the top-level content flow and
/// the contents of boxes.
#[derive(Hash)]
pub struct FlowNode(pub StyleVec<Content>);

#[node(LayoutBlock)]
impl FlowNode {}

impl LayoutBlock for FlowNode {
    fn layout_block(
        &self,
        world: Tracked<dyn World>,
        regions: &Regions,
        styles: StyleChain,
    ) -> SourceResult<Vec<Frame>> {
        let mut layouter = FlowLayouter::new(regions);

        for (child, map) in self.0.iter() {
            let styles = map.chain(&styles);
            if let Some(&node) = child.downcast::<VNode>() {
                layouter.layout_spacing(node.amount, styles);
            } else if child.has::<dyn LayoutBlock>() {
                layouter.layout_block(world, child, styles)?;
            } else if child.is::<ColbreakNode>() {
                layouter.finish_region();
            } else {
                panic!("unexpected flow child: {child:?}");
            }
        }

        Ok(layouter.finish())
    }
}

impl Debug for FlowNode {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        f.write_str("Flow ")?;
        self.0.fmt(f)
    }
}

/// Performs flow layout.
struct FlowLayouter {
    /// The regions to layout children into.
    regions: Regions,
    /// Whether the flow should expand to fill the region.
    expand: Axes<bool>,
    /// The full size of `regions.size` that was available before we started
    /// subtracting.
    full: Size,
    /// The size used by the frames for the current region.
    used: Size,
    /// The sum of fractions in the current region.
    fr: Fr,
    /// Whether the last block was a paragraph.
    last_block_was_par: bool,
    /// Spacing and layouted blocks.
    items: Vec<FlowItem>,
    /// Finished frames for previous regions.
    finished: Vec<Frame>,
}

/// A prepared item in a flow layout.
enum FlowItem {
    /// Absolute spacing between other items.
    Absolute(Abs),
    /// Fractional spacing between other items.
    Fractional(Fr),
    /// A frame for a layouted block and how to align it.
    Frame(Frame, Axes<Align>),
    /// An absolutely placed frame.
    Placed(Frame),
}

impl FlowLayouter {
    /// Create a new flow layouter.
    fn new(regions: &Regions) -> Self {
        let expand = regions.expand;
        let full = regions.first;

        // Disable vertical expansion for children.
        let mut regions = regions.clone();
        regions.expand.y = false;

        Self {
            regions,
            expand,
            full,
            used: Size::zero(),
            fr: Fr::zero(),
            last_block_was_par: false,
            items: vec![],
            finished: vec![],
        }
    }

    /// Actually layout the spacing.
    fn layout_spacing(&mut self, spacing: Spacing, styles: StyleChain) {
        match spacing {
            Spacing::Relative(v) => {
                // Resolve the spacing and limit it to the remaining space.
                let resolved = v.resolve(styles).relative_to(self.full.y);
                let limited = resolved.min(self.regions.first.y);
                self.regions.first.y -= limited;
                self.used.y += limited;
                self.items.push(FlowItem::Absolute(resolved));
            }
            Spacing::Fractional(v) => {
                self.items.push(FlowItem::Fractional(v));
                self.fr += v;
            }
        }
    }

    /// Layout a block.
    fn layout_block(
        &mut self,
        world: Tracked<dyn World>,
        block: &Content,
        styles: StyleChain,
    ) -> SourceResult<()> {
        // Don't even try layouting into a full region.
        if self.regions.is_full() {
            self.finish_region();
        }

        // Placed nodes that are out of flow produce placed items which aren't
        // aligned later.
        if let Some(placed) = block.downcast::<PlaceNode>() {
            if placed.out_of_flow() {
                let frame = block.layout_block(world, &self.regions, styles)?.remove(0);
                self.items.push(FlowItem::Placed(frame));
                return Ok(());
            }
        }

        // How to align the block.
        let aligns = Axes::new(
            // For non-expanding paragraphs it is crucial that we align the
            // whole paragraph as it is itself aligned.
            styles.get(ParNode::ALIGN),
            // Vertical align node alignment is respected by the flow.
            block
                .downcast::<AlignNode>()
                .and_then(|aligned| aligned.aligns.y)
                .map(|align| align.resolve(styles))
                .unwrap_or(Align::Top),
        );

        // Disable paragraph indent if this is not a consecutive paragraph.
        let reset;
        let is_par = block.is::<ParNode>();
        let mut chained = styles;
        if !self.last_block_was_par && is_par && !styles.get(ParNode::INDENT).is_zero() {
            let property = Property::new(ParNode::INDENT, Length::zero());
            reset = Style::Property(property);
            chained = reset.chain(&styles);
        }

        // Layout the block itself.
        let frames = block.layout_block(world, &self.regions, chained)?;
        let len = frames.len();
        for (i, frame) in frames.into_iter().enumerate() {
            // Grow our size, shrink the region and save the frame for later.
            let size = frame.size();
            self.used.y += size.y;
            self.used.x.set_max(size.x);
            self.regions.first.y -= size.y;
            self.items.push(FlowItem::Frame(frame, aligns));

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

        self.last_block_was_par = is_par;

        Ok(())
    }

    /// Finish the frame for one region.
    fn finish_region(&mut self) {
        // Determine the size of the flow in this region dependening on whether
        // the region expands.
        let mut size = self.expand.select(self.full, self.used);

        // Account for fractional spacing in the size calculation.
        let remaining = self.full.y - self.used.y;
        if self.fr.get() > 0.0 && self.full.y.is_finite() {
            self.used.y = self.full.y;
            size.y = self.full.y;
        }

        let mut output = Frame::new(size);
        let mut offset = Abs::zero();
        let mut ruler = Align::Top;

        // Place all frames.
        for item in self.items.drain(..) {
            match item {
                FlowItem::Absolute(v) => {
                    offset += v;
                }
                FlowItem::Fractional(v) => {
                    offset += v.share(self.fr, remaining);
                }
                FlowItem::Frame(frame, aligns) => {
                    ruler = ruler.max(aligns.y);
                    let x = aligns.x.position(size.x - frame.width());
                    let y = offset + ruler.position(size.y - self.used.y);
                    let pos = Point::new(x, y);
                    offset += frame.height();
                    output.push_frame(pos, frame);
                }
                FlowItem::Placed(frame) => {
                    output.push_frame(Point::zero(), frame);
                }
            }
        }

        // Advance to the next region.
        self.regions.next();
        self.full = self.regions.first;
        self.used = Size::zero();
        self.fr = Fr::zero();
        self.finished.push(output);
    }

    /// Finish layouting and return the resulting frames.
    fn finish(mut self) -> Vec<Frame> {
        if self.expand.y {
            while !self.regions.backlog.is_empty() {
                self.finish_region();
            }
        }

        self.finish_region();
        self.finished
    }
}