PPMd
PPMd is the context-modeling statistical compression algorithm developed by Dmitry Shkarin, using variable-order PPM models with arithmetic/range coding.
Purpose
It provides high compression ratios on text and structured data (above bzip2), serving as the high-ratio back end of 7z and RAR.
Mechanism
It maintains multi-order context models (PPM*), predicts the next symbol's probability from the context already seen, and outputs through an arithmetic/range coder. The model falls back between orders via an escape mechanism and updates adaptively.
Use cases
- 7z high-ratio mode
- RAR compression
- Text archive compression