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 the current ZPAQ program will be able to decompress archives created by future ZPAQ versions (the various PAQ variants listed above are not forward compatible in this fashion). It achieves this by specifying the decompression algorithm in a bytecode program that is stored in each created archive file. Hutter Prizes The series PAQ8HP1 through PAQ8HP8 were released by Alexander Ratushnyak from August 21, 2006 through January 18, 2007 as Hutter Prize submissions. The Hutter Prize is a text compression contest using a 100 MB English and XML data set derived from Wikipedia's source. The PAQ8HP series was forked from PAQ8H. The programs include text preprocessing dictionaries and models tuned specifically to the benchmark. All non-text models were removed. The dictionaries were organized to group syntactically and semantically related words and to group words by common suffix. The former strategy improves compression because related words (which are likely to appear in similar context) can be modeled on the high order bits of their dictionary codes. The latter strategy makes the dictionary easier to compress. The size of the decompression program and compressed dictionary is included in the contest ranking. On October 27, 2006, it was announced that PAQ8HP5 won a Hutter Prize for Lossless Compression of Human Knowledge of €3,416. On June 30, 2007, Ratushnyak's PAQ8HP12 was awarded a second Hutter prize of €1732, improving upon his previous record by 3.46%. PAQ derivations Being free software, PAQ can be modified and redistributed by anyone who has a copy. This has allowed other authors to fork the PAQ compression engine and add new features such as a graphical user interface or better speed (at the expense of compression ratio).