ZPAQ, first released on March 12, 2009, is one of the many offshoots of the PAQ series of lossless compression and archive tools. PAQ uses a context mixing algorithm. Context mixing is related to prediction by partial matching (PPM) in that the compressor is divided into a predictor and an arithmetic coder, but differs in that the next-symbol prediction is computed using a weighted combination of probability estimates from a large number of models conditioned on different contexts. Unlike PPM, a context doesn't need to be contiguous. Many of it's versions are developed by Matt Mahoney.
It is currently being expanded upon by Franco Corbelli as zpaqfranz, which adds a buttload of random features.
