Each location step accumulated its context nodes' axis results into a
Set<Node> to drop nodes reachable from more than one context, hashing and
copying every node. The wrapped TreeNode is reference-counted, so each copy
is ARC traffic, and the profile showed the Set insert, hashing, and Node
copies dominating evaluation.
De-duplication is needed only when two distinct context nodes can reach the
same node. Skip it on a single context and on the axes whose results are
disjoint per context (child, attribute, namespace, self): those produce the
identical node sequence the Set would, since every insertion succeeds, with
no hashing. Keep the Set on the axes that genuinely overlap (descendant,
ancestor, parent, the siblings, following/preceding). Predicates stay applied
per context and the path result still passes through the final
order-and-unique pass, so results are identical.
Measured xpath ~13x to ~10x slower than libxml2 on the 20k-item generated
corpus via a same-state A/B, ~28% faster, same result. XPathNodeSetDedupTests
pins that a node shared by many contexts through ancestor/parent/descendant
is still counted once, the disjoint axes return every distinct result, and an
overlapping union still de-duplicates. The full XPath suite and the Apache
Xalan XSLT gold-output corpus stay green.
Mihaela Mihaljevic
2eb3741848