Module xpath 

XPath 1.0 implementation (§25, §85 Phase 5).

Complete XPath 1.0 engine: location paths, axes, node tests, predicates, functions, variables, namespaces, node sets, boolean/number/string conversion, comparison semantics, NaN/infinity/negative zero, document order, context position/size, extension functions, compiled expressions.

Modules

• ast — expression AST types (axes, node tests, steps, operators)
• lexer — tokenizer
• parser — recursive descent parser
• types — XPath value types (node sets, numbers, strings, booleans)
• context — evaluation context (variables, namespaces, functions)
• axes — axis traversal
• functions — core function library (25 XPath 1.0 functions)
• eval — evaluation engine

Upstream contract

Mirrors upstream xpath.c / xpathInternals.h (SRC-LIBXML2-2.15.0-XPATH-C, parity target libxml2 2.15.3 oracle): expression compilation, the 13 axes, node tests, predicates, the core function library, value stack semantics and the xmlXPathObject / xmlXPathCompExpr / xmlXPathContext C types (R-000126 restored the full xpathInternals.h header surface; R-000128 fixed the opLimit/ opCount field types).

Conceptual behavior

The engine implements XPath 1.0 (W3C-XPATH-1.0): the lexer/parser build an AST, eval walks it with document-order node-set semantics, IEEE 754 number handling (NaN/infinity/negative zero), boolean/string conversion per §4, context position/size, and the extension-function registry bridged to C callbacks (R-000162).

Ownership & safety invariants

xmlXPathCompile results are freed with xmlXPathFreeCompExpr; XPath objects with xmlXPathFreeObject (object owns its node-set/string/ number storage). Node-set entries are borrowed tree pointers — the tree outlives evaluation (types.rs XPathNode SAFETY note). The parser context bridge owns the value stack and the popped-string copy (R-000169 fixed xml_strdup on a non-NUL-terminated Rust String).

Historical quirks & epochs

R-000102 (absolute paths evaluate from the document root node), R-000159 (predicate position() semantics) and R-000166 (number formatting, 1e9/1e-5 threshold, DBL_DIG=15) are upstream behaviors locked by residuals. The node-set dump became newline-separated in the 2.9.10 epoch (E-001, commit da35eeae, an upstream-documented breaking change); the empty-node-set exit-code epochs (E-003) sit at the xmllint layer this engine feeds.

Deliberate oddities

xmlXPathCastNumberToString trailing-zero trimming, the integer shortcut and the e+NN/e-NN exponent form are reproduced exactly (R-000166) rather than delegating to Rust float formatting, which differs on exponent width and trimming.

Proving courts

XPATH / XPOINTER / XINCLUDE court families, XPATH-001 differential probes (courts/suites/data-abi/*), the 967/967 number() corpus and CLI-XSLTPROC-0014/0015/0017 (format-number) require byte-identical output vs the oracle; cargo test runs the engine suites.

Tempting simplifications that would break parity

Do not swap number formatting to format!("{}"): the 1e9/1e-5 scientific threshold, 15-digit fraction computation and exponent padding are oracle-observable (R-000166). Do not deduplicate node-sets by value instead of pointer, and do not drop the borrowed-node invariant — the C ABI hands out raw node pointers.

Modules

ast

XPath 1.0 Expression AST (§25).

axes

XPath 1.0 Axis Implementation (§25).

context

XPath 1.0 Evaluation Context (§25).

eval

XPath 1.0 Evaluation Engine (§25).

exports

XPath / XPointer C export bridge (§25, 11.1-I XPath family closure).

functions

XPath 1.0 Core Function Library (§25).

lexer

XPath 1.0 Expression Lexer/Tokenizer (§25).

parser

XPath 1.0 Expression Parser (§25).

parser_context

XPath parser context and value stack (§25, 11.1-I XPath family closure).

types

XPath 1.0 Runtime Types (§25).

Functions

compile

Parse and compile an XPath expression string.

compile_result

Parse and compile an XPath expression string, exposing the parse error (message + byte offset) for upstream-faithful diagnostics (HOSTILE-FAILURE F3).

evaluate

Evaluate a compiled XPath expression.

evaluate_str

Parse and evaluate in one step.