rust

falsify-rs — PRML v0.1 fourth reference implementation

A Rust implementation of PRML v0.1 demonstrating that the canonicalization is implementable in a fourth language byte-for-byte against the v0.1 conformance vectors and the v0.2 candidate vectors.

Status: working draft, intended as portability evidence rather than a production tool. The Python reference implementation (falsify, in the repo root) remains the recommended runtime.

Result: 12 / 12 v0.1 vectors pass byte-for-byte. 6 / 6 v0.2 candidate vectors pass byte-for-byte (including TV-018, the small-magnitude-float case that surfaced Finding 4).

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Build & run

Requires Rust 1.70+.

cd impl/rust
cargo build --release

# Conformance suite
./target/release/falsify-rs test-vectors ../../spec/test-vectors/v0.1/test-vectors.json
./target/release/falsify-rs test-vectors ../../spec/v0.2/test-vectors-candidates.json

# Hash a manifest (JSON input)
./target/release/falsify-rs hash my-manifest.json

# Verify a manifest against its sidecar; if --observed given, evaluate predicate
./target/release/falsify-rs verify my-manifest.json --observed 0.876

Exit codes match the spec: 0 PASS, 3 TAMPERED, 10 FAIL, 11 GUARD.

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What this is

About 600 lines of Rust, two runtime dependencies (serde_json for JSON-with-preserved-number-text parsing, sha2 for SHA-256 hashing). The canonicalizer is hand-rolled to match PyYAML's safe_dump output exactly. We do not use any YAML library; all the work happens against the JSON source via serde_json.

Notable design points:

• Plain-scalar predicate — same rules as the JS and Go implementations: indicator-prefix, leading/trailing whitespace, colon-space and hash-space ambiguity, number-resolution regex, boolean/null set, timestamp regex, control-character escape. ~50 lines of hand-coded Rust.
• Float rendering (Finding 4 patch) — for the threshold field, scientific-notation values from serde_json are post-processed: mantissa receives a .0 injection if missing, exponent receives 2-digit zero-padding and an explicit sign. About 15 lines of code.
• Number preservation — serde_json's Number preserves the source text when handed JSON that contains a number, so 64-bit precision is not lost. This is the same advantage Go's json.Number provides over JavaScript's Number.

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What this is not

• Not a production tool. Use the Python reference implementation for running real evaluations.
• Not a complete YAML implementation. Loading .yaml files is not supported here; pass .json files (which the test vectors use).
• Not stable across spec versions. v0.2 will introduce a formal canonicalization grammar; this implementation will be rewritten to match.

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Why it exists

To prove the v0.1 specification is implementable from a fourth language, in a different paradigm (Rust's strict ownership and trait system vs Python's dynamic typing, JavaScript's loose typing, Go's structural typing). With four byte-for-byte conformant implementations across four languages, the spec is robustly portable.

The full portability analysis covering all four implementations is at spec/analysis/canonicalization-portability-v0.1.md.

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License

MIT. Same as the rest of the repository.