Module eval 

XPath 1.0 Evaluation Engine (§25).

Evaluates compiled XPath expressions against XML trees using the internal Rust representation.

UPSTREAM-PARITY

Full XPath 1.0 evaluation semantics: location paths, axes, node tests, predicates, functions, operators, type conversions, comparison semantics.

Courts

XPATH-EVAL-*

Upstream contract

Mirrors the evaluation core of upstream xpath.c (SRC-LIBXML2-2.15.0-XPATH-C, parity target libxml2 2.15.3 oracle): xmlXPathEvalExpr semantics for location paths, predicates, operators, type conversions and comparison rules.

Conceptual behavior

Walks the AST with an explicit recursion guard (push_recursion / pop_recursion). Absolute paths start at the document node (the _xmlDoc cast to _xmlNode, type 9), NOT at the root element — the R-000102 fix that aligns / with XPath 1.0. Predicates are evaluated per-step with live context position/size; node-sets are deduplicated in document order.

Ownership & safety invariants

Evaluation borrows the tree and the context: node-sets hold borrowed _xmlNode pointers that stay valid while the document lives; the engine never frees tree nodes. Values returned to the C ABI are converted in exports.rs and owned by the _xmlXPathObject. Context mutation is confined to the evaluation call (no global state).

Historical quirks & epochs

R-000102: absolute paths originally evaluated from the root element (candidate bug) and were fixed to the document node. R-000159: //book[position() <= 2] matched one extra node until the predicate context-size semantics were aligned with the oracle. Both were sealed during the 11.1 XPath closure against the 2.15.3 oracle.

Deliberate oddities

The engine evaluates predicates eagerly per step (upstream xmlXPathCompOpEval style) rather than collecting then filtering — position()/last() depend on that order, so it is deliberate.

Proving courts

XPATH-EVAL-* and the XPATH differential probes compare evaluation results byte-identical against the oracle; the XSLT courts (CLI-XSLTPROC-*) run compiled templates through this engine.

Tempting simplifications that would break parity

Do not defer predicate evaluation to a post-filter pass: position() / last() are order-sensitive (R-000159). Do not drop the recursion guard, and do not evaluate absolute paths from the root element (R-000102) — both are observable through xmlXPathEval results.

Functions

eval

Evaluate an XPath expression in the given context.

eval_xpath

Evaluate an XPath expression string against a document.