Aymmetric Numeral Systems coders (ANS) are a new approach to entropy coding that allow close to entropy compression at high bandwidths. This is a custom implementation of rANS, one of the variants of ANS that copes well with large alphabets. An evaluation of rANS for ALICE can be found here

The rANS public API is at an early stage and will be evolving over time. Currently the unittests can be used as a reference.