Roadmap

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

Stage	What	Status
Severity model	DEBUG=0 INFO=1 WARN=2 ERROR=3 FATAL=4 UNKNOWN=5, the SeverityLabel mapping ("DEBUG".."FATAL", then "ANY"), and CoerceSeverity (MRI's Severity.coerce from an int or a "warn"/"FATAL" string).	Done
Default formatter	MRI's "%.1s, [%s #%d] %5s -- %s: %s\n" with the "%Y-%m-%dT%H:%M:%S.%6N" timestamp, the datetime_format override, and the msg2str coercion (String as-is, Exception → "message (Class)\nbacktrace", else the host's inspect) — pure over an injected clock and pid.	Done
Level gating	Add (alias Log) returns true and formats nothing with no sink or severity < level; the Debug/Info/Warn/Error/Fatal/Unknown helpers, the <<-style raw Write, and the DebugQ..FatalQ predicates all mirror MRI.	Done
Rotation by size	For an integer shift_age + shift_size: ShouldRotateBySize and the ShiftAgeSequence rename plan.	Done
Rotation by period	For a calendar shift_age ("daily"/"weekly"/"monthly", plus "now"/"everytime"): NextRotateTime, PreviousPeriodEnd, ShouldRotateByPeriod, and the PeriodAgeFile rotated-name with MRI's .1...99 collision suffix.	Done
Differential oracle & coverage	The line bytes, the datetime_format override, the Exception coercion, end-to-end gating, the full severity range and the rotation schedule compared byte-for-byte against the system ruby; 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 IO device. The library implements the deterministic decisions — the line bytes and the rename plan; it never opens a file, writes bytes or performs a rename. That is the host's job, wired through the Sink.
• No live clock or pid. The wall clock and process id are injected via Now and Pid; the library never reads them itself, which is what makes the output reproducible and testable.
• No interpreter. Anything that needs a live Ruby binding is the consumer's job — that is why rbgo binds this module rather than the reverse.
• Reference is reference Ruby (MRI 4.0.5 / logger-1.7.0). Byte-for-byte conformance targets that behaviour, pinned by the differential oracle.

See Usage & API for the surface and Why pure Go for the compute-core / host-IO split.