Files

T

enricobuehler 72f8c05aa3 feat: M2 P1.5 (FEC) — nanors-exact Reed-Solomon recovery for the video stream

Moonlight now reconstructs lost video shards from our parity (verified live:
under induced packet loss the picture recovers cleanly instead of failing with
"network connection too bad"; 0% added loss in normal operation).

The decisive finding: Moonlight's nanors uses a CAUCHY generator matrix
(M[j][i] = inv[(m+i)^j], GF(2^8) poly 0x1d), while reed-solomon-erasure is
Vandermonde — so its parity was NOT Moonlight-decodable, despite the old
gf8.rs comment claiming equivalence.

lumen-core:
- Swap the GF(2^8) backend from reed-solomon-erasure to a vendored fec-rs
  (vendor/fec-rs, BSD-2), which builds the byte-identical Cauchy matrix. Pure
  Rust, no FFI — keeps the "one core" hot path. This makes both lumen's own
  protocol and the GameStream parity nanors-compatible.
- Lock it with a regression test against real nanors vectors
  (k=4,m=2 [10,20,30,40] -> parity [136,0]) + an independent matrix-derived
  cross-check + an erase/recover round-trip. Existing FEC/loopback tests stay
  green, so lumen's own protocol is unaffected.

lumen-host video.rs:
- Generate m = ceil(k*pct/100) parity shards per FEC block via Gf8Coder; stamp
  fecInfo with the recomputed wire pct (100*m/k) so the client derives the same
  count; cap per-block data to 255*100/(100+pct) so k+m <= 255.
- CRITICAL byte-exactness: RS runs over the whole `blocksize` shard (Moonlight
  decodes packetSize+16 bytes from the datagram start and PACKET_RECOVERY_FAILUREs
  on a bad reconstructed `flags` byte). So the NV header fields RS must reproduce
  (streamPacketIndex/frameIndex/flags/multiFec*) are written into data shards
  BEFORE encode, and only the transport fields (RTP header/seq/timestamp +
  fecInfo) are stamped AFTER — leaving the flags byte RS-covered. Matches
  Sunshine stream.cpp. Unit-tested incl. flags recovery.
- fec_percentage wired from stream.rs (Sunshine default 20, LUMEN_FEC_PCT
  override; 0 = data-only). LUMEN_VIDEO_DROP injects loss to test recovery.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

2026-06-09 11:34:27 +00:00

2.4 KiB

Raw Blame History

fec-rs

A pure Rust Reed-Solomon erasure coding library with runtime SIMD acceleration.

Features

Pure Rust — No C/C++ dependencies or FFI. Everything is implemented in safe Rust (with targeted unsafe for SIMD intrinsics).
Runtime SIMD detection — Automatically uses the fastest available instruction set via std::is_x86_feature_detected!. A single binary works on all x86_64 systems.
GF(2^8) — Operates over the Galois field GF(2^8) with generating polynomial 29 (0x1D), compatible with the Moonlight streaming protocol.
Shard-by-shard encoding — Incremental encoding via ShardByShard for streaming use cases.
Reconstruction — Reconstruct missing data and/or parity shards from any sufficient subset.

SIMD Acceleration

On x86_64, the library automatically detects CPU features at runtime and uses the best available instruction set:

GFNI + AVX2 — Single-instruction GF multiply on 32 bytes (Intel Alder Lake+, AMD Zen 4+)
AVX2 — VPSHUFB split-table nibble lookup on 32 bytes
GFNI + SSE — Single-instruction GF multiply on 16 bytes
SSSE3 — VPSHUFB split-table nibble lookup on 16 bytes
Scalar — Lookup table fallback

Parallel Encoding

Enable the parallel feature for optional rayon-based parallel encoding:

fec-rs = { version = "0.1", features = ["parallel"] }

When enabled, large encode workloads automatically distribute parity shard computation across threads. Small workloads use the sequential path to avoid overhead.

Usage

use fec_rs::ReedSolomon;

let rs = ReedSolomon::new(4, 2).unwrap();

let mut shards: Vec<Vec<u8>> = vec![
    vec![0, 1, 2, 3],
    vec![4, 5, 6, 7],
    vec![8, 9, 10, 11],
    vec![12, 13, 14, 15],
    vec![0, 0, 0, 0], // parity shard 1
    vec![0, 0, 0, 0], // parity shard 2
];

// Encode parity
rs.encode(&mut shards).unwrap();

// Verify
assert!(rs.verify(&shards).unwrap());

// Simulate loss of shard 0
let mut recovery: Vec<Option<Vec<u8>>> = shards.into_iter().map(Some).collect();
recovery[0] = None;

// Reconstruct
rs.reconstruct(&mut recovery).unwrap();

License: BSD-2-Clause

2.4 KiB Raw Blame History