PPMd
PPMd is a data compression algorithm in the Prediction by Partial Matching (PPM) family. It models the probability distribution of the next symbol conditioned on a context formed by preceding symbols, organizing these statistics in a context tree. Each node in the tree stores counts for possible following symbols. When encoding, PPMd uses an escape mechanism: if the current context has no observed count for the next symbol, an escape symbol is emitted and the model backs off to shorter contexts until a symbol with a known probability is found. The resulting probabilities are encoded with an arithmetic or range coder to produce the compressed bitstream.
The strength of PPMd lies in its use of high-order contexts while maintaining practical decoding for data
Historically, PPMd is one of the practical realizations of the PPM concept and has been implemented in