Crate rans

source ·
Expand description

Ranged Asymmetric Numeral Systems (rANS) encoder and decoder. Under the hood, this is a high-level wrapper over ryg-rans-sys.

ANS is a family of modern entropy coding methods introduced by Jarek Duda from Jagiellonian University. It serves as an alternative to arithmetic and Huffman coding, combining the performance and compression ratio of both. Many recent compression algorithms, such as Facebook’s Zstandard, Apple’s LZFSE, or JPEG XL, use ANS under the hood.

§Types of encoders/decoders in this crate

  • byte_encoder/byte_decoder has a byte-aligned rANS encoder/decoder.
  • b64_encoder/b64_decoder is a 64-bit version that emits entire 32-bit words at a time. It might be faster than byte-aligned variant on 64-bit architectures, and also makes for a very precise arithmetic coder (i.e. it gets quite close to entropy). The trade-off is that this version will be slower on 32-bit machines, and the output bitstream is not endian-neutral.

See the ryg_rans repository for more details.

§See also


use rans::byte_decoder::{ByteRansDecSymbol, ByteRansDecoder};
use rans::byte_encoder::{ByteRansEncSymbol, ByteRansEncoder};
use rans::{RansDecSymbol, RansDecoder, RansEncSymbol, RansEncoder, RansEncoderMulti};

const SCALE_BITS: u32 = 2;

// Encode two symbols
let mut encoder = ByteRansEncoder::new(1024);
let symbol1 = ByteRansEncSymbol::new(0, 2, SCALE_BITS);
let symbol2 = ByteRansEncSymbol::new(2, 2, SCALE_BITS);


let mut data =;

// Decode the encoded data
let mut decoder = ByteRansDecoder::new(data);
let symbol1 = ByteRansDecSymbol::new(0, 2);
let symbol2 = ByteRansDecSymbol::new(2, 2);

// Please note that the data is being decoded in reverse
assert_eq!(decoder.get(SCALE_BITS), 2); // Decoder returns cumulative frequency
decoder.advance(&symbol2, SCALE_BITS);
assert_eq!(decoder.get(SCALE_BITS), 0);
decoder.advance(&symbol1, SCALE_BITS);