rANS Codec

The rANS (range-ANS) codec is a high-precision entropy coder that acts as a peer to the Huffman coder in the PTWM dispatcher.

Theory

Asymmetric Numeral Systems (ANS) offers a compression ratio close to arithmetic coding with a decoding speed comparable to Huffman coding. PTWM's implementation uses 11-bit precision and features four interleaved 32-bit streams.

graph LR
    subgraph State [4 Interleaved 32-bit States]
        S0[State 0]
        S1[State 1]
        S2[State 2]
        S3[State 3]
    end
    
    subgraph ByteStream [Encoded Byte Stream]
        B0[Byte 0]
        B1[Byte 1]
        B2[Byte 2]
        B3[Byte 3]
        B4[...]
    end
    
    S0 -.->|Consumes| B0
    S1 -.->|Consumes| B1
    S2 -.->|Consumes| B2
    S3 -.->|Consumes| B3

rANS is particularly effective on planes with very low entropy or highly skewed distributions (e.g., long runs of zeros after a Move-To-Front transform), where Huffman's 1-bit-per-symbol minimum becomes a bottleneck. The compressor's dispatcher will trial-encode every plane and select rANS if it produces a smaller payload than Huffman.

Usage

This codec accepts any plane descriptor. It includes a raw fallback mechanism to ensure it never expands incompressible data.

References

• Jarek Duda, "Asymmetric numeral systems: entropy coding combining speed of Huffman coding with compression rate of arithmetic coding" (2013). Link