Ruby's CMath in pure Go — complex-aware sqrt / exp / log / trig, MRI-compatible, no cgo.
go-ruby-cmath is a pure-Go (no cgo) reimplementation of Ruby's CMath standard library — the complex-aware trigonometric and transcendental functions of MRI 4.0.5's cmath gem. CMath accepts real or complex arguments and returns a real result when the input lies on the real branch (CMath.sqrt(4) is 2.0) and a complex result otherwise (CMath.sqrt(-4) is (0+2i)). This library reproduces that exact real-vs-complex decision and MRI's branch cuts and float formulas — without any Ruby runtime — over a small explicit Number value model. It is a CMath backend for go-embedded-ruby, a standalone sibling of go-ruby-regexp, go-ruby-marshal and go-ruby-yaml — differential-tested against MRI, 100% coverage, CI green across 6 arches and 3 OSes.
A Number is a real-or-complex value, the Go analogue of Ruby’s Float-or-Complex — Real(x) and Complex(re, im) build arguments, and Complex(re, 0) stays complex exactly as Complex(1, 0).real? is false in Ruby.
Each function returns a real result on the real branch (Sqrt(Real(4)) is 2.0) and a complex result otherwise (Sqrt(Real(-4)) is (0+2i)), reproducing CMath’s exact real-vs-complex decision.
Sqrt, Cbrt, Exp, Log (with optional base), Log2, Log10 — mirroring CMath.sqrt / cbrt / exp / log / log2 / log10, on MRI’s principal branch cuts.
Sin / Cos / Tan, Sinh / Cosh / Tanh, and the inverses Asin / Acos / Atan / Atan2 / Asinh / Acosh / Atanh — the inverses built from this package’s own Log and Sqrt, exactly as the cmath gem builds them.
Outside the real branch, each function uses the same closed-form expression CMath’s Ruby source uses, so the principal branch cuts and float results agree with ruby -rcmath to floating-point rounding.
Golden values captured from MRI 4.0.5 hold coverage at 100%; an oracle suite drives ruby -rcmath across a broad real / negative / out-of-domain / complex corpus. Green across all six 64-bit Go arches and three OSes, cgo disabled.
A faithful port of Ruby's CMath module in pure Go, cgo disabled, so it cross-compiles and embeds anywhere. It reproduces CMath's real-vs-complex branch test, principal branch cuts and closed-form complex expressions over a small Number value model. Validated differentially against ruby -rcmath across a real / negative / out-of-domain / complex corpus. It is a standalone, reusable module and a CMath backend for the sibling org github.com/go-embedded-ruby.