Roadmap

go-ruby-format/format is grown test-first, each capability differential-tested against MRI rather than built in isolation. Ruby's format-string engine — the deterministic, interpreter-independent slice extracted from rbgo's internals — is complete.

Stage	What	Status
Conversion directives	Every MRI conversion: integers d/i/u/b/o/x/X, floats f/e/E/g/G/a/A, s string, p inspect, c character, and the %% literal — each matching reference Ruby's rounding, padding and sign rules.	Done
Flags, width & precision	All flags (- left-justify, + and space sign, 0 zero-pad, # alternate form) with width and precision, including the * form that reads width/precision from an argument.	Done
Named & positional refs	Named references %<name>s and %{name} resolved against a hash, and absolute n$ argument indexing — matching MRI's mixing rules and its errors for clashing styles.	Done
Bignum & hex-float	Arbitrary-precision integer output (%d on a Bignum), and IEEE hex-float (%a / %A) emitted exactly as MRI's sprintf.c does, down to the mantissa digits and exponent.	Done
Error taxonomy	MRI's full set of ArgumentError / KeyError / TypeError cases — too few arguments, unknown directive, mixed numbered/unnumbered, missing named key — surfaced as Go errors.	Done
Differential oracle & coverage	A wide format-string corpus rendered both here and by the system ruby, compared byte-for-byte against MRI; 100% coverage, gofmt + go vet clean, green across all six 64-bit Go arches and three OSes.	Done

Documented out-of-scope boundaries

These are deliberate, recorded so the module's surface is unambiguous:

• No interpreter. The library implements the deterministic algorithm; it never runs arbitrary Ruby. Anything that needs a live binding or evaluation is the consumer's job — that is why rbgo binds this module rather than the reverse.
• Reference is reference Ruby (MRI). Byte-for-byte conformance targets MRI's behaviour; differences across MRI releases are matched to the reference used by the differential oracle.
• Standalone & reusable. The module has no dependency on the Ruby runtime; the dependency runs the other way.

See Usage & API for the surface and Why pure Go for the deterministic/interpreter split.