abc2xml Comparison

How croma compares to abc2xml

abc2xml (by Willem Vree) is the long-standing reference ABC → MusicXML converter, and it is croma's correctness baseline: croma is validated against it over the full 10k corpus. Where abc2xml is spec-correct, croma matches it (9,390 / 9,390 structural matches); croma diverges only where abc2xml departs from the ABC 2.1 spec, and every such case is adjudicated and documented.

croma is a from-scratch, library-first reimplementation that improves on the reference in several ways:

	abc2xml	croma
Direction	ABC → MusicXML (the reverse is a separate script, xml2abc)	ABC ↔ MusicXML in one library
Form	a Python script (needs a Python runtime)	a Rust library + native binaries — zero-dependency, embeddable, crates.io-publishable, callable from any language
Speed	interpreted Python	compiled Rust — 7,081 ABC→MusicXML files/s and 43,247 parse files/s over the 10k corpus
Malformed input	permissive — silent best-effort heuristics	strict ABC 2.1 — structured diagnostics (codes + spans); recovers only when the intent is unambiguous, and always warns
Output artifacts	inserts spurious elements as heuristic side effects — e.g. empty leading/section measures, phantom measures	spec-faithful, minimal MusicXML — declines those artifacts; most croma↔abc2xml divergences are exactly such an artifact croma omits
Beyond conversion	converter only	also a formatter (idempotent + lossless), a language server, a reusable tree-sitter grammar, and a Zed extension
Validation	—	corpus-proven over 10k with a documented gate matrix + a reproducible benchmark baseline
Safety	—	memory-safe Rust (unsafe forbidden), pinned toolchain, clippy + fmt gates

In short: abc2xml is a mature one-way converter script; croma is a fast, embeddable, bidirectional ABC toolkit that holds stricter to the spec and emits cleaner, lower-artifact MusicXML, with editor-grade diagnostics — designed to be linked into applications and proven at scale.

Why divergences are a feature, not a regression

Most croma ↔ abc2xml divergences are an artifact that abc2xml inserts and croma declines (an empty leading measure, a phantom measure). croma treats abc2xml as the baseline precisely so it can earn its correctness claims: matching it on the 9,390 spec-correct files, and adjudicating every remaining file as a documented, spec-justified difference rather than a silent guess. The comparator, the whitelist/dropped baseline, the ABC spec knowledge base, and the divergence-triage process all live in the companion croma-test repo (see How-its-Proven).

croma also stands on abc2xml's shoulders: using it as the parity baseline is how croma's correctness is measured in the first place.