ctwz

ctwz is a lossless compressor based on the Context Tree Weighting method¹. It takes byte-level contexts and makes binary predictions using an ASCII decomposition tree². It is mainly good for large files.

Build

To build use cmake

$ cmake .
$ make

or simply compile with C++17.

Usage

$ ./ctwz -h
ctwz:
	Context tree weighting compressor
	author: Meijke Balay <mysatellite99@gmail.com>
usage:
	encode: ctwz [-d depth] file
	decode: ctwz -x file

-d Specifies the depth of the context trees (default=8). Greater depths can improve compression for large files but require more memory and computation.

Benchmarks

Some results on the Canterbury Corpus using depth 12 ctwz, with gzip (Lempel-Ziv) for comparison

file	size(bytes)	ctwz	gzip
E.coli	4638690	1209950	1342009
bible.txt	4047392	898767	1177372
world192.txt	2473400	561751	724995
kennedy.xls	1029744	167460	204016
ptt5	513216	55562	56482
plrabn.txt	481861	151034	194357
alice29.txt	152089	47817	54428
asyoulik.txt	125179	42720	48922

Todo

• Parallelize context tree computations
• Lower memory requirements with pruning

References

1: Willems, F., Shtarkov, Y., & Tjalkens, T. (1995). The context-tree weighting method: basic properties. IEEE Trans. Inf. Theory, 41, 653-664.

2: Volf, P. (2002). Weighting Techniques in Data Compression Theory and Algorithms. Ph.D. thesis, Technische Universiteit Eindhoven.