msrtc-rans-core

Deterministic, no_std-capable rANS primitives for the msrtc_rans entropy coder.

This crate implements the raw rANS encoding and decoding primitives used by Microsoft MLVC's msrtc_rans package. The arithmetic (reciprocal preparation, Mul64Hi, fast division) is structurally faithful to the C++ implementation.

Status — Raw Engine Sealed ✅

Both RansByteEncoder / RansByteDecoder (u32 state, u8 unit) and Rans64Encoder / Rans64Decoder (u64 state, u32 unit) have been verified byte-for-byte against the pinned Microsoft C++ oracle. All 30 core tests pass.

Features

• #![no_std] — suitable for embedded and WASM targets
• #![forbid(unsafe_code)] — pure safe Rust
• Macro-generated variants from a single generate_rans_impl! macro
• VecSink (reverse-order, matching C++ ResizableBufferSink behavior)
• SliceSource (zero-copy span-like source) with seek(pos) for continuation decoding
• Decoder::from_state(source, state) — construct a decoder with a saved state (persistent stream decoding)
• Checked API: try_new(), try_put_raw(), try_get(), try_advance()
• Transactional decoder — state not committed on failed advance

Usage

use msrtc_rans_core::{RansByteEncoder, RansByteDecoder};
use msrtc_rans_core::sink::VecSink;
use msrtc_rans_core::source::SliceSource;

let sink = VecSink::<u8>::new(64);
let mut encoder = RansByteEncoder::new(sink);
encoder.put_raw(0, 128, 8);
encoder.flush();
let encoded = encoder.into_sink().encoded().to_vec();

let source = SliceSource::new(&encoded);
let mut decoder = RansByteDecoder::new(source);
assert!(decoder.init());
assert!(decoder.advance(0, 128, 8));
assert!(decoder.check_eof());

Persistent continuation (stream mode)

use msrtc_rans_core::{RansByteDecoder, RansByteEncoder};
use msrtc_rans_core::sink::VecSink;
use msrtc_rans_core::source::SliceSource;

let sink = VecSink::<u8>::new(64);
let mut encoder = RansByteEncoder::new(sink);
encoder.put_raw(0, 128, 8);
encoder.flush();
let units = encoder.into_sink().encoded().to_vec();

// Save a decode cursor (position + state), then continue later.
let mut source = SliceSource::new(&units);
let mut decoder = RansByteDecoder::new(source);
assert!(decoder.init());
let pos = decoder.source().position();
let state = decoder.state();
// ... later, from a saved (pos, state):
let mut source2 = SliceSource::new(&units);
source2.seek(pos);
let mut decoder2 = RansByteDecoder::from_state(source2, state);

Repository

Full project: github.com/infinityabundance/msrtc-rans-rs

License

MIT — see LICENSE and NOTICE for attribution notices.