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What are PAR and PAR2 Files?
What are .PAR2 files and how do I use them?
For a technical definition of what PAR2 files are, please visit http://en.wikipedia.org/wiki/Parchive

To handle PAR2 files, download QuickPAR at http://www.quickpar.org.uk/

See the official PARchive home page at http://parchive.sourceforge.net/

PAR2 files are parity data that can be used to recover missing or damaged files in a set, such as a set of RAR files or images. PAR2 files use blocks instead of files. You simply need enough blocks to recover and replace all of the missing or damaged blocks. Having enough blocks of data from the PAR2 files, allows you to recover incomplete files or rebuild entirely missing files. Block sizes vary from set to set, but can be anywhere from a few KB for small sets to several MB for larger file sets (videos, for example).

The file name pattern for .PAR2 files is: FileSetName.vol###+###.par2

The first ### is the number of the first block in the PAR2 set. The second ### is the number of blocks in that file. The first number is only significant in the case that you want to find a full set of PAR2 files. For example, let's say you have the following files:

Filename.vol00+01.par2
Filename.vol01+02.par2
Filename.vol03+04.par2
Filename.vol15+08.par2

The first file starts at block 0 and contains 1 block. The second file starts at block 1 and contains 2 blocks. The third starts with block 3 and contains 4 blocks. The fourth file starts with block 15 and contains 8 blocks. In total, you have 15 (1+2+4+8) blocks of recovery data. You can also conclude from the pattern in the filenames that a PAR2 file is missing, possibly named Filename.vol07+08.par2. Because the number of recover blocks are in the file name, you can actually download the file set, see how many blocks are needed, and then download at least that many recover blocks worth of PAR2 files.
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