Performance

Design

• Hand-rolled single-pass lexer and recursive-descent parser; an arena-based DOM (flat Vec, index links) on top of quick-xml.
• Rayon parallelism, applied at three levels:
  • Documents referenced by distinct USE sources load and parse in parallel.
  • Blocks targeting different documents execute in parallel; blocks targeting the same document stay sequential.
  • Large FOREACH loops (≥1024 children, no BREAK) evaluate their expressions in parallel before applying mutations sequentially — exact sequential semantics, parallel speed. TAG loops parallelize only when no match is nested inside another.
• Output strings are pre-sized from the DOM (both the serializer and the final output assembly), avoiding repeated reallocation on large documents.

Why parallel execution is safe here

The arena DOM (xml/dom.rs) stores nodes in a flat Vec<Element> linked by plain usize indexes, rather than as a tree of owned/reference-counted nodes. That layout is what makes read-only parallel evaluation over &Document sound: many threads can read the same arena concurrently without any interior mutability or locking, because evaluation never mutates the arena directly — it only produces a plan.

The evaluate-then-apply split (in eval/mod.rs) is the other half: for large loops, expression evaluation (which is read-only) runs in parallel across children, producing a list of mutation plans; those plans are then applied to the document sequentially, in original order. This guarantees the observable result is bit-for-bit identical to running the loop fully sequentially — parallelism only changes wall-clock time, never behavior. Loops containing BREAK skip this entirely and run sequential, since BREAK's early-exit semantics depend on visiting elements in a fixed order.

Measuring it

SET FORMAT = OFF;   compact output, if you don't need to eyeball the XML
ANALYZE;            per-stage timing report on stderr

ANALYZE (or SET ANALYZE = ON) prints lex, parse, stdin/file read, XML parse (per document), per-block execution, serialization, output assembly, stdout write, total DOM memory, and total wall time — see CLI: Diagnostics.