go-ruby-format — a pure-Go reimplementation of Ruby's format-string engine, MRI-compatible, no cgo

go-ruby-format is a pure-Go (no cgo) reimplementation of Ruby's format-string engine — Kernel#sprintf, Kernel#format and String#%. It parses an MRI format string and renders it against its arguments, matching reference Ruby byte-for-byte: every conversion (d i f e g s p x o b c %), every flag (- + space 0 #), width and precision including the * argument form, named references (%<name>s and %{name}), absolute n$ argument indexing, arbitrary-precision Bignum and hex-float (%a) output. It was extracted from rbgo's prelude/internals into a reusable standalone library: no dependency on the Ruby runtime, the dependency runs the other way. It is the format-string backend for go-embedded-ruby, bound by rbgo as a native module just like go-ruby-regexp and go-ruby-erb — differential-tested against MRI, 100% coverage, CI green across 6 arches and 3 OSes.

Conversion directives ready

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.

Flags, width & precision ready

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.

Named & positional refs ready

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.

Bignum & hex-float ready

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.

Error taxonomy ready

MRI’s full set of ArgumentError / KeyError / TypeError cases — too few arguments, unknown directive, mixed numbered/unnumbered, missing named key — surfaced as Go errors.

Differential oracle & coverage ready

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.

ready capability complete (exit criteria met, nothing material left)
active in progress (substantial work shipped, not yet finished)
planned not started yet (or downstream by design)

A faithful port of MRI's sprintf.c in pure Go, cgo disabled, so it cross-compiles and embeds anywhere. It implements every conversion and flag, width/precision with the * argument, named references (%<name>s / %{name}), n$ indexing, Bignum and hex-float. Validated differentially against the system ruby binary — rendered output compared byte-for-byte. It is a standalone, reusable module extracted from rbgo's internals, and the format backend for the sibling org github.com/go-embedded-ruby.