Roadmap

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

Stage	What	Status
Arbitrary-precision arithmetic	Add, subtract and multiply at unbounded precision, with the sign and scale rules of Ruby's BigDecimal.	Done
Division with precision	div/divmod honouring an explicit precision argument, matching MRI's significant-digit semantics rather than truncating.	Done
Seven rounding modes	ROUND_UP, ROUND_DOWN, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN, ROUND_CEILING, ROUND_FLOOR, each matching reference Ruby exactly.	Done
to_s "E" and "F" forms	Both the engineering ("E") and plain ("F") string forms, with MRI's grouping and exponent formatting byte-for-byte.	Done
NaN and infinities	NaN, +Infinity and -Infinity produced and propagated with Ruby's comparison and arithmetic rules.	Done
Differential oracle & coverage	A wide value corpus exercised here and by the system ruby/bigdecimal, compared byte-for-byte; 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.