siblingPosition(of:) located the context node with
parent.children.firstIndex { $0 === tree } on every call: a linear scan of the
parent's child list, independent of the result. Over a wide parent evaluated
from many context siblings (//x/following-sibling::y) that scan was quadratic,
and the axes then materialized every sibling before the node test filtered it.
Look the index up through the shared per-evaluation SiblingIndexCache (O(1)
after the per-parent table is built), and fuse the step's node test into the
walk so a rejected sibling is never wrapped (the same fusion the descendant and
following/preceding axes use). The selected siblings and their order are
unchanged.
The gratuitous per-context scan and the non-match wrapping are gone (the
remaining per-context iteration over the siblings is inherent to the axis); a
wide-parent following-sibling query is ~1.2-1.6x faster. The full suite, the
W3C XPath/XSLT and Apache Xalan corpora, and the WASM build stay green;
SiblingAxisTests pins the fused node test and document order.
Mihaela Mihaljevic
b104723fed