fix(a11y): read the enclosing marked content in one step per operator

Marked-text extraction and figure bounds searched the whole marked-content
stack before every operator, so deeply nested unmarked BMC blocks made a page
quadratic. The text stack now holds each level's effective id (its own, else
the enclosing one's), and the bounds pass counts the open sequences that
match.

crates/a11y/src/alt.rs

@@ -183,16 +183,19 @@ fn union(a: Option<[f64; 4]>, b: [f64; 4]) -> [f64; 4] {
 }
 
 /// Where the marked content `ids` is drawn on a page (images, forms and paths).
-fn content_bbox(doc: &Document, page: &Dict, ids: &HashSet<i64>) -> Option<[f64; 4]> {
+pub(crate) fn content_bbox(doc: &Document, page: &Dict, ids: &HashSet<i64>) -> Option<[f64; 4]> {
     let res = page.get(b"Resources").map(|r| doc.resolve(r)).and_then(|r| r.as_dict().cloned()).unwrap_or_default();
     let xobjects = res.get(b"XObject").map(|x| doc.resolve(x)).and_then(|x| x.as_dict().cloned()).unwrap_or_default();
     let props = page_properties(doc, page);
     let mut ctm = vec![Matrix([1.0, 0.0, 0.0, 1.0, 0.0, 0.0])];
+    // Whether each open marked-content sequence is one of `ids`, and how many of them are.
     let mut marks: Vec<bool> = Vec::new();
+    let mut matching = 0usize;
     let mut bbox = None;
     let mut path: Vec<(f64, f64)> = Vec::new();
     for op in pdfcraft_content::parse(&page_content(doc, page)).ops {
-        let inside = marks.iter().any(|m| *m);
+        crate::count_mark_reads(1);
+        let inside = matching > 0;
         let top = ctm.last().copied().unwrap_or(Matrix([1.0, 0.0, 0.0, 1.0, 0.0, 0.0]));
         match op.op.as_slice() {
             b"q" => ctm.push(top),
@@ -207,9 +210,15 @@ fn content_bbox(doc: &Document, page: &Dict, ids: &HashSet<i64>) -> Option<[f64;
                 }
             }
             b"BMC" => marks.push(false),
-            b"BDC" => marks.push(bdc_mcid(doc, &props, &op).is_some_and(|n| ids.contains(&n))),
+            b"BDC" => {
+                let wanted = bdc_mcid(doc, &props, &op).is_some_and(|n| ids.contains(&n));
+                matching += usize::from(wanted);
+                marks.push(wanted);
+            }
             b"EMC" => {
-                marks.pop();
+                if marks.pop() == Some(true) {
+                    matching = matching.saturating_sub(1);
+                }
             }
             b"m" | b"l" => {
                 if let Some([x, y]) = op.nums::<2>() {

crates/a11y/src/headings.rs

@@ -182,6 +182,7 @@ pub(crate) fn mcid_texts(doc: &Document, page: &Dict) -> HashMap<i64, String> {
     let res = sub(doc, page, b"Resources");
     let (fonts, props) = (sub(doc, &res, b"Font"), sub(doc, &res, b"Properties"));
     let mut metrics: HashMap<Vec<u8>, Metrics> = HashMap::new();
+    // The id each open marked-content sequence is in: its own, else the enclosing one's.
     let mut stack: Vec<Option<i64>> = Vec::new();
     let mut out: HashMap<i64, String> = HashMap::new();
     // Where each id's last glyph ended, in user space.
@@ -191,16 +192,20 @@ pub(crate) fn mcid_texts(doc: &Document, page: &Dict) -> HashMap<i64, String> {
     let mut unsaved = 0usize;
     let (mut tm, mut tlm) = (Matrix::IDENTITY, Matrix::IDENTITY);
     for op in pdfcraft_content::parse(&page_content(doc, page)).ops {
-        let id = stack.iter().rev().find_map(|m| *m);
+        crate::count_mark_reads(1);
+        let id = stack.last().copied().flatten();
         match op.op.as_slice() {
-            b"BMC" => stack.push(None),
-            b"BDC" => stack.push(match op.operands.get(1) {
-                Some(Object::Dict(d)) => d.get(b"MCID").and_then(Object::as_int),
-                Some(Object::Name(n)) => {
-                    props.get(n).map(|x| doc.resolve(x)).and_then(|x| x.as_dict().and_then(|d| d.get(b"MCID").and_then(Object::as_int)))
+            b"BMC" => stack.push(id),
+            b"BDC" => stack.push(
+                match op.operands.get(1) {
+                    Some(Object::Dict(d)) => d.get(b"MCID").and_then(Object::as_int),
+                    Some(Object::Name(n)) => {
+                        props.get(n).map(|x| doc.resolve(x)).and_then(|x| x.as_dict().and_then(|d| d.get(b"MCID").and_then(Object::as_int)))
+                    }
+                    _ => None,
                 }
-                _ => None,
-            }),
+                .or(id),
+            ),
             b"EMC" => {
                 stack.pop();
             }

crates/a11y/src/lib.rs

@@ -21,6 +21,20 @@ mod report;
 mod structure;
 mod tags;
 
+#[cfg(test)]
+thread_local! {
+    /// Marked-content stack entries read to decide which marks enclose an operator, for the tests
+    /// that keep that constant per operator.
+    static MARK_READS: std::cell::Cell<usize> = const { std::cell::Cell::new(0) };
+}
+
+/// Counts `n` marked-content stack reads (tests only).
+#[inline]
+fn count_mark_reads(_n: usize) {
+    #[cfg(test)]
+    MARK_READS.with(|c| c.set(c.get().saturating_add(_n)));
+}
+
 pub use alt::{AltError, ArtifactError, Figure, figures, mark_decorative, mark_tag_artifact, set_alt};
 pub use autotag::{AutoTagError, AutoTagResult, autotag_text};
 pub use autotag_fields::tag_form_fields;

crates/a11y/src/tests.rs

@@ -1023,6 +1023,46 @@ fn glyph_by_tm(text: &str, size: f64, y: f64) -> String {
     ops + " ET"
 }
 
+/// Marked-content stack reads while `f` runs.
+fn mark_reads(f: impl FnOnce()) -> usize {
+    MARK_READS.with(|c| c.set(0));
+    f();
+    MARK_READS.with(|c| c.get())
+}
+
+/// Page content nesting `depth` unmarked `BMC` blocks around a marked one, with `ops` operators inside.
+fn deeply_nested_page(doc: &mut Document, depth: usize, ops: usize) -> pdfcraft_cos::Dict {
+    use pdfcraft_cos::{Dict, Stream};
+    let mut data = "/Figure << /MCID 0 >> BDC ".repeat(1);
+    data.push_str(&"/X BMC ".repeat(depth));
+    data.push_str(&"0 0 1 1 re f ".repeat(ops));
+    data.push_str(&"EMC ".repeat(depth + 1));
+    let content = doc.add(Stream::flate(Dict::new(), data.as_bytes()));
+    [(b"Contents".to_vec(), Object::Ref(content))].into_iter().collect()
+}
+
+#[test]
+fn marked_text_reads_its_enclosing_id_in_constant_steps() {
+    let mut doc = Document::new_empty();
+    let page = deeply_nested_page(&mut doc, 5_000, 5_000);
+    let reads = mark_reads(|| {
+        crate::headings::mcid_texts(&doc, &page);
+    });
+    // About 15 000 operators; a scan of the stack per operator reads millions of entries.
+    assert!(reads <= 4 * 15_000, "{reads} stack reads");
+}
+
+#[test]
+fn figure_bounds_read_the_marked_content_state_in_constant_steps() {
+    let mut doc = Document::new_empty();
+    let page = deeply_nested_page(&mut doc, 5_000, 5_000);
+    // No mark encloses figure 7, so a scan of the stack per operator reads all of it.
+    let reads = mark_reads(|| {
+        crate::alt::content_bbox(&doc, &page, &std::collections::HashSet::from([7]));
+    });
+    assert!(reads <= 4 * 15_000, "{reads} stack reads");
+}
+
 /// One tagged page whose H1 headings show `contents` (one marked-content sequence each).
 fn heading_page(contents: &[String]) -> Document {
     let marked: Vec<String> = contents.iter().enumerate().map(|(i, c)| format!("/H1 << /MCID {i} >> BDC {c} EMC")).collect();