walkPreceding visits a node and everything before it in document order. It found
each preceding sibling with precedingSibling(of:), which rescans the parent's
whole child list, so visiting k preceding siblings was O(k^2) and numbering n
nodes under a wide parent was O(n^3): 1000 items took ~4.7s and 4000 did not
finish in minutes.
Find the candidate's index in the child list once, then walk the children before
it directly (parent.children[..<index].reversed()), visiting each preceding
subtree in reverse. The walk is now linear in the number of preceding nodes, so
the numbering is quadratic (the residual is inherent to level="any", which
counts the matching preceding nodes per numbered node). The numbers are
identical.
Numbering 1000 items drops ~4.7s to ~0.7s (~6x); cubic becomes quadratic. The
full suite, the W3C XSLT and Apache Xalan corpora, and the WASM build stay
green; XSLTNumberAnyTests pins document-order numbering across nesting.
Mihaela Mihaljevic
971a7e9f56