PackedParselets.jl

A parser compiler for bit-packed primitive types from declarative patterns.

PackedParselets takes a pattern specification describing a structured string format and compiles it into a Julia primitive type with minimal bit-width storage. The generated code parses directly from byte arrays, prints to IO and memory buffers, and provides named property access, all without heap allocation or regular expressions.

What it does

Given a pattern like:

("PFX-", :id(digits(4)), optional(".v", :ver(digits(max=255))))

PackedParselets generates:

• A primitive type using ⌈{}total bits⌉{} rounded to the next byte
• A parsebytes function that validates and packs input bytes
• print, string, show, and isless methods
• Named property access (.id, .ver)
• A positional constructor (T(id, ver))
• Segment introspection (segments(T), segments(instance))

Design goals

• Zero allocation on parse: no intermediate strings, no exception objects on the hot path
• Minimal storage: fields packed into the fewest bits, not bytes
• Compile-time optimisation: SWAR digit parsing, perfect hashing for choices, word-sized string matching, and length-check elision, all selected at macro expansion time
• Extensibility: custom segment types via the SegmentDef interface

See Usage for the API, Segments for the pattern language, and Optimisations for internals.