ppmz

PPMZ is a file to file (like gzip) compressor based on the PPM method. PPM stands for Prediction by Partial Match. What this means is: PPM learns the statistics of the file - i.e. it learns how to look at the data and predict what will happen next. When PPM is predicting successfully, it can use arithmetic coding to transmit the data in a much more compact way (i.e. you only need 1 bit to write a correct-guess flag, instead of 1 byte for the character; with arithmetic coding this kind of tradeoff can be adaptively adjusted to the estimated likely-hood of the guess being right.

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