About a month ago I started using easynews.com (not cheap, but works well) to download, ahem, back-up copies of media. The company is one of many to provide access to usenet. In any event, I’ve noticed many binary newsgroup posts contain a set of files known as ‘par’ or parity files. The binary files generally found on usenet are divided up into ‘chunks’, ie if there is a 100 meg film being posted, there are ten 10meg zip files uploaded along with say, 20 megs of parity files. The parity files have a magical quality which works like this: If for some reason one of the 10 meg zip files is missing (it happens sometimes), one can use a ‘generic’ par file from the set to replace any of the ten zip files that is missing. Using a special program, the par file will generate a copy of the missing zip.
I hope that makes sense, for more info google par2 or par files or visit http://www.warezfaq.com/allaboutpar.htm
Apparently par files are based on an important CS algorithem known as “Reed-Solomon”, which self-correcting RAID arrays use. Anyways I can’t for the life of me wrap my head around how a ‘generic’ 10 meg par file can replace a specific zip file (ie. zip file #5 of 10). From my original example, how can 20 megs of data be used to kind of represent 100 megs?!? Now you can only replace one or two zips with that amount of pars, but I still don’t get how its interchangeable?
Neat Board You have here btw.