Computing a node's string-value walked its subtree recursively, so it
overflowed the stack on a deeply-nested node (around 2000 deep on a Swift
Task stack): `XPath.Query("/a/a").strings(over:)` over a 4000-deep chain
crashed. `TreeNode.textContent` and the value-Node string-value behind
`Selection.stringValue` now walk an explicit stack, pushing children reversed
to concatenate in document order.
The value-Node path also wrongly folded descendant comment and
processing-instruction data into an element's string-value, disagreeing with
the tree-node path and XPath 1.0 (5.1/5.5: only text and CDATA contribute to
an element's string-value; a comment or PI node's own string-value, 5.6/5.7,
is still its data). It now matches. No test depended on the old behaviour:
1789 pass, including the XPath and XSLT value corpora.
Closes the string-value sites of #350; the transform walks (canonicalize,
DTD defaults, XSLT result tree) remain tracked there.
Add PureXML.XPath: a compiled location-path query engine reimplemented from the
XPath 1.0 specification; no upstream code copied.
- Compiler parses a path into steps; Evaluator walks the tree returning a
Selection node-set (node or attribute) in document order.
- Forward axes only (child, descendant //, self ., attribute @), since the value
model has no parent pointers. Node tests: name, *, text(), node(), comment().
Predicates: [n], [@a], [@a='v'], [child], [child='v'].
- Public surface: XPath.Query (init compiles, evaluate/elements/strings), the
Selection enum (element / stringValue), QueryError, and the one-shot
PureXML.xpath(_:over:). Upward/sibling axes and the full expression language
are explicitly out of scope.
- 12 XPath tests; 61 total.
