typst/crates/typst-pdf/src/outline.rs

use std::num::NonZeroUsize;

use pdf_writer::{Finish, Ref, TextStr};
use typst::foundations::{Content, NativeElement, Smart};
use typst::layout::Abs;
use typst::model::HeadingElem;

use crate::{AbsExt, PdfContext};

/// Construct the outline for the document.
#[tracing::instrument(skip_all)]
pub(crate) fn write_outline(ctx: &mut PdfContext) -> Option<Ref> {
    let mut tree: Vec<HeadingNode> = vec![];

    // Stores the level of the topmost skipped ancestor of the next bookmarked
    // heading. A skipped heading is a heading with 'bookmarked: false', that
    // is, it is not added to the PDF outline, and so is not in the tree.
    // Therefore, its next descendant must be added at its level, which is
    // enforced in the manner shown below.
    let mut last_skipped_level = None;
    for heading in ctx.document.introspector.query(&HeadingElem::elem().select()).iter() {
        let leaf = HeadingNode::leaf((**heading).clone());

        if leaf.bookmarked {
            let mut children = &mut tree;

            // Descend the tree through the latest bookmarked heading of each
            // level until either:
            // - you reach a node whose children would be brothers of this
            // heading (=> add the current heading as a child of this node);
            // - you reach a node with no children (=> this heading probably
            // skipped a few nesting levels in Typst, or one or more ancestors
            // of this heading weren't bookmarked, so add it as a child of this
            // node, which is its deepest bookmarked ancestor);
            // - or, if the latest heading(s) was(/were) skipped
            // ('bookmarked: false'), then stop if you reach a node whose
            // children would be brothers of the latest skipped heading
            // of lowest level (=> those skipped headings would be ancestors
            // of the current heading, so add it as a 'brother' of the least
            // deep skipped ancestor among them, as those ancestors weren't
            // added to the bookmark tree, and the current heading should not
            // be mistakenly added as a descendant of a brother of that
            // ancestor.)
            //
            // That is, if you had a bookmarked heading of level N, a skipped
            // heading of level N, a skipped heading of level N + 1, and then
            // a bookmarked heading of level N + 2, that last one is bookmarked
            // as a level N heading (taking the place of its topmost skipped
            // ancestor), so that it is not mistakenly added as a descendant of
            // the previous level N heading.
            //
            // In other words, a heading can be added to the bookmark tree
            // at most as deep as its topmost skipped direct ancestor (if it
            // exists), or at most as deep as its actual nesting level in Typst
            // (not exceeding whichever is the most restrictive depth limit
            // of those two).
            while children.last().map_or(false, |last| {
                last_skipped_level.map_or(true, |l| last.level < l)
                    && last.level < leaf.level
            }) {
                children = &mut children.last_mut().unwrap().children;
            }

            // Since this heading was bookmarked, the next heading, if it is a
            // child of this one, won't have a skipped direct ancestor (indeed,
            // this heading would be its most direct ancestor, and wasn't
            // skipped). Therefore, it can be added as a child of this one, if
            // needed, following the usual rules listed above.
            last_skipped_level = None;
            children.push(leaf);
        } else if last_skipped_level.map_or(true, |l| leaf.level < l) {
            // Only the topmost / lowest-level skipped heading matters when you
            // have consecutive skipped headings (since none of them are being
            // added to the bookmark tree), hence the condition above.
            // This ensures the next bookmarked heading will be placed
            // at most as deep as its topmost skipped ancestors. Deeper
            // ancestors do not matter as the nesting structure they create
            // won't be visible in the PDF outline.
            last_skipped_level = Some(leaf.level);
        }
    }

    if tree.is_empty() {
        return None;
    }

    let root_id = ctx.alloc.bump();
    let start_ref = ctx.alloc;
    let len = tree.len();

    let mut prev_ref = None;
    for (i, node) in tree.iter().enumerate() {
        prev_ref = Some(write_outline_item(ctx, node, root_id, prev_ref, i + 1 == len));
    }

    ctx.pdf
        .outline(root_id)
        .first(start_ref)
        .last(Ref::new(ctx.alloc.get() - 1))
        .count(tree.len() as i32);

    Some(root_id)
}

/// A heading in the outline panel.
#[derive(Debug, Clone)]
struct HeadingNode {
    element: Content,
    level: NonZeroUsize,
    bookmarked: bool,
    children: Vec<HeadingNode>,
}

impl HeadingNode {
    fn leaf(element: Content) -> Self {
        HeadingNode {
            level: element.expect_field_by_name::<NonZeroUsize>("level"),
            // 'bookmarked' set to 'auto' falls back to the value of 'outlined'.
            bookmarked: element
                .expect_field_by_name::<Smart<bool>>("bookmarked")
                .unwrap_or_else(|| element.expect_field_by_name::<bool>("outlined")),
            element,
            children: Vec::new(),
        }
    }

    fn len(&self) -> usize {
        1 + self.children.iter().map(Self::len).sum::<usize>()
    }
}

/// Write an outline item and all its children.
#[tracing::instrument(skip_all)]
fn write_outline_item(
    ctx: &mut PdfContext,
    node: &HeadingNode,
    parent_ref: Ref,
    prev_ref: Option<Ref>,
    is_last: bool,
) -> Ref {
    let id = ctx.alloc.bump();
    let next_ref = Ref::new(id.get() + node.len() as i32);

    let mut outline = ctx.pdf.outline_item(id);
    outline.parent(parent_ref);

    if !is_last {
        outline.next(next_ref);
    }

    if let Some(prev_rev) = prev_ref {
        outline.prev(prev_rev);
    }

    if !node.children.is_empty() {
        let current_child = Ref::new(id.get() + 1);
        outline.first(current_child);
        outline.last(Ref::new(next_ref.get() - 1));
        outline.count(-(node.children.len() as i32));
    }

    let body = node.element.expect_field_by_name::<Content>("body");
    outline.title(TextStr(body.plain_text().trim()));

    let loc = node.element.location().unwrap();
    let pos = ctx.document.introspector.position(loc);
    let index = pos.page.get() - 1;
    if let Some(page) = ctx.pages.get(index) {
        let y = (pos.point.y - Abs::pt(10.0)).max(Abs::zero());
        outline.dest().page(ctx.page_refs[index]).xyz(
            pos.point.x.to_f32(),
            (page.size.y - y).to_f32(),
            None,
        );
    }

    outline.finish();

    let mut prev_ref = None;
    for (i, child) in node.children.iter().enumerate() {
        prev_ref = Some(write_outline_item(
            ctx,
            child,
            id,
            prev_ref,
            i + 1 == node.children.len(),
        ));
    }

    id
}