fix(a11y): mark_decorative no longer panics on an unpaired ParentTree key (#1809) (#1862)

clear_parent_tree walked /Nums with chunks_mut(2) and indexed pair[1], so a
trailing key without a value that matched the page's StructParents panicked
after the page content had already been replaced. Walk whole pairs only and
leave an unpaired key as it is: it maps nothing, so there is nothing to clear.


Claude-Session: https://claude.ai/code/session_01RUNQSPVXx3dDBabRZHAnjz

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

crates/a11y/src/alt.rs

@@ -414,12 +414,11 @@ fn clear_parent_tree(doc: &mut Document, key: i64, obj: ObjRef) -> Result<(), Al
     let Some(pt) = tree_root(doc).and_then(|r| r.get(b"ParentTree").and_then(Object::as_ref)) else { return Ok(()) };
     let Some(nums) = doc.get(pt).as_dict().and_then(|d| d.get(b"Nums").and_then(|n| n.as_array().cloned())) else { return Ok(()) };
     let mut nums = nums;
-    for pair in nums.chunks_mut(2) {
-        if pair[0].as_int() == Some(key) {
-            match &pair[1] {
-                Object::Array(a) => {
-                    pair[1] = Object::Array(a.iter().map(|x| if x.as_ref() == Some(obj) { Object::Null } else { x.clone() }).collect())
-                }
+    // Whole pairs only: a trailing key without a value maps nothing, so it is left as it is.
+    for [k, v] in nums.as_chunks_mut::<2>().0 {
+        if k.as_int() == Some(key) {
+            match &*v {
+                Object::Array(a) => *v = Object::Array(a.iter().map(|x| if x.as_ref() == Some(obj) { Object::Null } else { x.clone() }).collect()),
                 Object::Ref(r) => {
                     if let Some(a) = doc.get(*r).as_array().cloned() {
                         let a: Vec<Object> = a.iter().map(|x| if x.as_ref() == Some(obj) { Object::Null } else { x.clone() }).collect();

crates/a11y/src/tests.rs

@@ -607,6 +607,23 @@ fn marking_a_leaf_tag_as_an_artifact_preserves_appearance_and_structure_integrit
     assert_eq!(mark_tag_artifact(&mut doc, pdfcraft_cos::ObjRef::new(7, 0)), Err(ArtifactError::NotATag(7)));
 }
 
+#[test]
+fn marking_decorative_survives_a_parent_tree_with_an_unpaired_key() {
+    // A trailing key without a value, matching the page's StructParents (#1809), and the same
+    // with no valid pair at all.
+    let tree = |nums: &str| pdf(&[format!("<< /Nums {nums} >>")], "").get(pdfcraft_cos::ObjRef::new(1, 0)).as_ref().clone();
+    for (nums, cleared) in [("[0 [7 0 R 8 0 R] 0]", "[0 [null 8 0 R] 0]"), ("[0]", "[0]")] {
+        let mut doc = figures_doc();
+        let pt = pdfcraft_cos::ObjRef::new(9, 0);
+        doc.set(pt, tree(nums));
+        let f = figures(&doc);
+        mark_decorative(&mut doc, f[0].obj).unwrap();
+        assert_eq!(figures(&doc).len(), 1, "{nums}");
+        // The pair is cleared and the unpaired key is kept as it was.
+        assert_eq!(*doc.get(pt), tree(cleared), "{nums}");
+    }
+}
+
 fn ref_(n: u32) -> Option<pdfcraft_cos::ObjRef> {
     Some(pdfcraft_cos::ObjRef::new(n, 0))
 }