Introduction

XWRT (XML-WRT) is an efficient XML/HTML compressor (actually it works well with all textual files). It transforms XML to more compressible form and uses zlib (default), LZMA, PPMd, or lpaq6 as back-end compressor. This idea is based on well-known XML compressor: XMill. Moreover XML-WRT creates a semi-dynamic dictionary and replaces frequently used words with shorter codes. There are additional techniques to improve compression ratio:

• word alphabet can consist of start tags (like <tag>), urls, e-mails
• special model for numbers encoding
• input XML file is split into containers
• there are special containers for dates, time, pages and fractional numbers
• end tags (</tag>) are replaced with a single char
• end tags + EOL symbols can also be replaced with a single char
• spaceless words model
• very effective methods for white-space preserving
• quotes modeling (=" and "> replaced with a single char)

Comparision to other XML compressors

All files used for comparision can be downloaded from Wratislavia XML Corpus. Results are given in bpc (bits ber character). Tested with XWRT 3.1:

file	gzip	XMill 0.9 zip	XWRT -l2 (zlib)	LZMA -a1	XWRT -l6 (LZMA)	PPMdJ -o8 -m64	XMill 0.9 PPMd	XMLPPM -l 9	SCMPPM -l 9	XWRT -l9 (PPM)	FastPAQ8 74 MB	XWRT -l11 (FastPAQ8)
dblp	1.463	1.250	0.865	0.943	0.747	0.724	0.940	0.802	0.693	0.690	0.659	0.597
enwikibooks	2.339	2.295	1.742	1.686	1.504	1.565	1.838	1.621	1.621	1.481	1.357	1.269
enwikinews	2.248	2.198	1.597	1.462	1.301	1.291	1.746	1.379	1.398	1.202	1.172	1.090
lineitem	0.721	0.380	0.276	0.421	0.243	0.359	0.270	0.261	0.242	0.243	0.236	0.226
Shakespeare	2.182	2.044	1.481	1.646	1.349	1.245	1.584	1.295	1.293	1.204	1.220	1.185
SwissProt	0.985	0.619	0.475	0.478	0.388	0.490	0.477	0.416	0.417	0.363	0.395	0.313
uwm	0.553	0.382	0.315	0.368	0.278	0.426	0.310	0.259	0.274	0.240	0.254	0.228
average	1.499	1.310	0.964	1.001	0.830	0.871	1.024	0.862	0.848	0.775	0.756	0.701

Usage

Usage: XWRT.exe [options] <file1> [file2] [file3] ...
 where <file> is a XML file or a XWRT compressed file (it's auto-detected)
             you can also use wildcards (e.g., "*.xml")

GENERAL OPTIONS (which also set default additional options):
  -l0 = no compression (memory usage up to 16 MB)
  -l1 = zlib fast (memory usage 16+1 MB)
  -l2 = zlib normal (default, memory usage 16+1 MB)
  -l3 = zlib best (memory usage 16+1 MB)
  -l4 = LZMA dict size 64 KB (memory usage 16+9 MB for compression and 16+3 MB for decompression)
  -l5 = LZMA dict size 1 MB (memory usage 16+18 MB for compression and 16+3 MB for decompression)
  -l6 = LZMA dict size 8 MB (memory usage 16+84 MB for compression and 16+10 MB for decompression)
  -l7 = PPMd model size 16 MB (memory usage 16+20 MB)
  -l8 = PPMd model size 32 MB (memory usage 16+36 MB)
  -l9 = PPMd model size 64 MB (memory usage 16+70 MB)
  -l10 = lpaq6 model size 120 MB (memory usage 16+104 MB)
  -l11 = lpaq6 model size 214 MB (memory usage 16+198 MB)
  -l12 = lpaq6 model size 406 MB (memory usage 16+390 MB)
  -l13 = lpaq6 model size 790 MB (memory usage 16+774 MB)
  -l14 = lpaq6 model size 1560 MB (memory usage 16+1542 MB)
  -0 = preprocessed and uncompressed output optimized for further LZ77 compression
  -1 = preprocessed and uncompressed output optimized for further LZMA compression
  -2 = preprocessed and uncompressed output optimized for further PPM compression
  -3 = preprocessed and uncompressed output optimized for further PAQ compression

ADDITIONAL OPTIONS:
  -bX = Set maximum buffer size while creating dynamic dictionary to X MB
  -c = Turn off containers (without number and word containers)
  +d = Turn on usage of the static dictionary (requires wrt-eng.dic,
       which is available at http://pskibinski.pl/research)
  -eX = Set maximum dictionary size to X words
  -fX = Set minimal word frequency to X
  -i = Delete input files
  -mX = Set maximum memory buffer size to X MB (default=8)
  -n = Turn off number containers
  -o = Force overwrite of output files
  -pX = Preprocess only (file_size/X) bytes in a first pass
  -s = Turn off spaces modeling option
  -t = Turn off "try shorter word" option
  -w = Turn off word containers

Compilation

For Linux/Unix:

make BUILD_SYSTEM=linux

For Windows (MinGW)

make

Used libraries

zlib  (C) 1995-2005 Jean-loup Gailly and Mark Adler 
LZMA  (C) 1999-2006 Igor Pavlov
PPMd (C) 1997-2006 Dmitry Shkarin
lpaq6  (C) 2007 Matt Mahoney and Alexander Ratushnyak