UNBOUNDED LENGTH CONTEXTS FOR PPM

The PPM data compression scheme has set the performance standard in lo ssless compression of text throughout the past decade, PPM is a finite context statistical modelling technique that can be viewed as blendin g together several fixed-order context models to predict the next char acter in the input sequence, This paper gives a brief introduction to PPM, and describes a variant of the algorithm, called PPM, which expl oits contexts of unbounded length. Although requiring considerably gre ater computational resources (in both time and space), this reliably a chieves compression superior to the benchmark PPMC version, Its major contribution is that it shows that the full. information available by considering all substrings of the input string can be used effectively to generate high-quality predictions, Hence, it provides a useful too l for exploring the bounds of compression.