PAQ - History

History

The following lists the major enhancements to the PAQ algorithm. In addition, there have been a large number of incremental improvements, which are omitted.

• PAQ1 was released on January 6, 2002 by Matt Mahoney. It used fixed weights and did not include an analog or sparse model.

• PAQ1SSE/PAQ2 was released on May 11, 2003 by Serge Osnach. It significantly improved compression by adding a SSE stage between the predictor and encoder. SSE (secondary symbol estimation) inputs a short context and the current prediction and outputs a new prediction from a table. The table entry is then adjusted to reflect the actual bit value.

• PAQ3N, released October 9, 2003 added a sparse model.

• PAQ4, released November 15, 2003 by Matt Mahoney used adaptive weighting. PAQ5 (December 18, 2003) and PAQ6 (December 30, 2003) were minor improvements, including a new analog model. At this point, PAQ was competitive with the best PPM compressors and attracted the attention of the data compression community, which resulted in a large number of incremental improvements through April 2004. Berto Destasio tuned the models and adjusted the bit count discounting schedule. Johan de Bock made improvements to the user interface. David A. Scott made improvements to the arithmetic coder. Fabio Buffoni made speed improvements.

• During the period May 20, 2004 through July 27, 2004, Alexander Ratushnyak released seven versions of PAQAR, which made significant compression improvements by adding many new models, multiple mixers with weights selected by context, adding an SSE stage to each mixer output, and adding a preprocessor to improve the compression of Intel executable files. PAQAR stood as the top-ranked compressor through the end of 2004 but was significantly slower than prior PAQ versions.

• During the period January 18, 2005 through February 7, 2005, Przemyslaw Skibinski released four versions of PASqDa, based on PAQ6 and PAQAR with the addition of an English dictionary preprocessor. It achieved the top ranking on the Calgary corpus but not on most other benchmarks.

• A modified version of PAQ6 won the Calgary Challenge on January 10, 2004 by Matt Mahoney. This was bettered by ten subsequent versions of PAQAR by Alexander Ratushnyak. The most recent was submitted on June 5, 2006, consisting of compressed data and program source code totaling 589,862 bytes.

• PAQ7 was released December 2005 by Matt Mahoney. PAQ7 is a complete rewrite of PAQ6 and variants (PAQAR, PAsQDa). Compression ratio was similar to PAQAR but 3 times faster. However it lacked x86 and a dictionary, so it did not compress Windows executables and English text files as well as PAsQDa. It does include models for color BMP, TIFF and JPEG files, so compresses these files better. The primary difference from PAQ6 is it uses a neural network to combine models rather than a gradient descent mixer. Another feature is PAQ7's ability to compress embedded jpeg and bitmap images in Excel-, Word- and pdf-files.

• PAQ8A was released on January 27, 2006, PAQ8C on February 13, 2006. These were experimental pre-release of anticipated PAQ8. It fixed several issues in PAQ7 (poor compression in some cases). PAQ8A also included model for compressing (x86) executables.

• PAQ8F was released on February 28, 2006. PAQ8F had 3 improvements over PAQ8A: a more memory efficient context model, a new indirect context model to improve compression, and a new user interface to support drag and drop in Windows. It does not use an English dictionary like the PAQ8B/C/D/E variants.

• PAQ8G was released March 3, 2006 by Przemyslaw Skibinski. PAQ8G is PAQ8F with dictionaries added and some other improvements as a redesigned TextFilter (which does not decrease compression performance on non-textual files)

• PAQ8H was released on March 22, 2006 by Alexander Ratushnyak and updated on March 24, 2006. PAQ8H is based on PAQ8G with some improvements to the model.

• PAQ8I was released on August 18, 2006 by Pavel L. Holoborodko, with bug fixes on August 24, September 4, and September 13. It added a grayscale image model for PGM files.

• PAQ8J was released on November 13, 2006 by Bill Pettis. It was based on PAQ8F with some text model improvements taken from PAQ8HP5. Thus, it did not include the text dictionaries from PAQ8G or PGM model from PAQ8I.

• Serge Osnach released a series of modeling improvements: PAQ8JA on November 16, 2006, PAQ8JB on November 21, and PAQ8JC on November 28.

• PAQ8JD was released on December 30, 2006 by Bill Pettis. This version has since been ported to 32 bit Windows for several processors, and 32 and 64 bit Linux.

• PAQ8K was released on February 13, 2007 by Bill Pettis. It includes additional models for binary files.

• PAQ8L was released on March 8, 2007 by Matt Mahoney. It is based on PAQ8JD and adds a DMC model.

• PAQ8O was released on August 24, 2007 by Andreas Morphis. Contains improved BMP and JPEG models over PAQ8L. Can be optionally compiled with SSE2 support and for 64-bit Linux. The algorithm has notable performance benefits under 64-bit OS.

• PAQ8P was released on August 25, 2008 by Andreas Morphis. Contains improved BMP model and adds a WAV model.

• PAQ8PX was released on April 25, 2009 by Jan Ondrus. It contains various improvements like better WAV compression and EXE compression.

• PAQ8KX was released on July 15, 2009 by Jan Ondrus. It is a combination of PAQ8K with PAQ8PX.

• PAQ8PF was released on September 9, 2009 by LovePimple without source code (which the GPL license requires). It compresses 7% worse, but is 7 times faster compared to PAQ8PX v66 (measured with 1 MB English text)

• PAQ9A was released on December 31, 2007 by Matt Mahoney. A new experimental version. It does not include models for specific file types, has an LZP preprocessor and supports files over 2 GB.

• ZPAQ was released on March 12, 2009 by Matt Mahoney. It uses a new archive format designed so that future ZPAQ variants may preserve the ability to decompress existing archives (the various PAQ variants listed above are not backwards compatible in this fashion). It achieves this by specifying the decompression algorithm in a bytecode program that is stored in each created archive file.