Roadmap

go-ruby-pathname/pathname is grown test-first, each capability differential-tested against MRI rather than built in isolation. The path-algebra slice of Ruby's Pathname — the deterministic, interpreter-independent, filesystem-free core — is complete.

Stage	What	Status
Basename / dirname / extname	Basename/BasenameSuffix, Dirname/Parent, Extname/SubExt — MRI's File.extname rule byte-for-byte, including trailing-dot ("foo." → ".") and dotfile (".foo" → "") cases.	Done
Cleanpath	Collapse ., .. and redundant separators exactly as MRI does, with the absolute-root-drop / relative-escape-keep distinction.	Done
RelativePathFrom	MRI's lexical relative_path_from, with its two ArgumentError cases and message text matching MRI.	Done
Join / split / traversal	Plus/Join (absolute component resets to root); Split/EachFilename/Filenames; Ascend/Descend via Go-callback blocks.	Done
Predicates & comparison	Absolute/Relative/Root; Cmp (<=>), Eql (==/eql?), stable FNV Hash.	Done
Differential oracle & coverage	Every method checked against the system ruby/Pathname over a corpus of edge cases; 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 filesystem. The library is the lexical path algebra only; read, write, exist?, children and the other File/Dir/IO delegations stay host-side in go-embedded-ruby, where they forward to the host's File class. The library never touches disk.
• Lexical, "/"-based, every platform. Like MRI's Pathname, the separator is always "/"; there is no OS-specific path code, and the behaviour is identical on Linux, macOS and Windows.
• Reference is reference Ruby (MRI). Byte-for-byte conformance targets MRI's behaviour, pinned 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 / filesystem split.