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  #1  
03-20-2011, 10:07 AM
manthing manthing is offline
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i know a little about par2 files, but have a question...

imagine a video, split in 100 rar files. and say the 50th rar file is spoilt.
now, usually par2 files come in the form xxx.vol000+01.par2,
xxx.vol001+02.par2, xxx.vol003+04.par2 etc (or similar).

so the question to you is, in order to repair the 50th rar file, do i
download the xxx.vol050+10.par2 file?
ie any par2 file that is part of the 50th rar file?

or can i just download the first few par2 files, regardless of what
"number" they are? as long as quickpar's functionality can kick in and repair the 50th rar file.

currently i just download N number of par2 files until quickpar has enough to rebuild the spoilt files.

but it would be nice to know the proper thing to do.
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  #2  
03-21-2011, 05:24 AM
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lordsmurf lordsmurf is offline
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If the PAR set is available from multiple locations -- i.e, a mirrored file set -- then you should be able to re-download the bad part from one of the mirrors, and it will be fine.

This assumes the mirror is not corrupted.

If the server or transit corrupted the file, the new PAR download will be fine. If the source distributor mucked up the file before uploading it to all the servers, then you're basically screwed until that person fixes his/her mistake.


Side rant: I'd like to mention that most people split files for no good reason.
However, others are forced by limits of file size allowed by the server. This server, for example, currently enforces a 32MB cap on forum attachments, which is also combined with a timeout of a couple of minutes.

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  #3  
03-21-2011, 06:43 AM
manthing manthing is offline
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ok, i don't think you got the gist of my question.

say a file, "F", was split into 100 parts: F.001, F.002, F.003 and so on until F.100. ok?

and say there are a number of PAR2 files that can rebuild the "F" file if need be.

now, say "F.050" was corrupted.

the question i'm asking is: how does PAR2 work? can i download any of the PAR2 files, or do i specifically need a PAR2 file that "covers" the spoilt "F.050" split file?

i think i already know the answer: it can be any PAR2 file as long as the app, like "QuickPAR", has enough information to rebuild the spoilt file. this is what i've found from experience / observation.

am i correct about this?
or is there some specific method to rebuilding damaged files?

cheers
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  #4  
03-21-2011, 07:45 AM
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lordsmurf lordsmurf is offline
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It sounds like standard error correction. This is built into DVDs, for example. There's a great example of ECC in the DVD Demystified book (not the site, the book). I may be butchering it some, but going off memory, an ECC example looks like this:

1 1 1 1 = 4
1 1 1 1 = 4
1 1 1 1 = 4
1 1 1 1 = 4
4 4 4 4 = X
X = 16

If the read back of the DVD is bad, it may give off a chart like this

1 1 1 1 = 4
1 2 1 1 = 5
1 1 1 1 = 4
1 1 2 1 = 5
4 5 5 4 = X
X = 16
Error, X != 16

Actually, I'm pretty well certain I've screwed up the example. But anyway...

Row and columns create sums that are verifiable. If at any time math is off, the system is able to correct itself, and it understand where the problem is -- unless it's so bad that the verification sums were corrupted, too.

I'd pull my book and look at it, but it's not on the shelf. I have no idea where I put it.

The system will repair itself, unless it's too damaged. There's nothing you, as user, can do to assist or "start" this process. It either works, or it doesn't.

At best, some programs can extract data from broken archives, but it would not help to extract a single file that was broken up across multi-part archives. That would only help if the multi-part was spanning files. For example, if you RAR'd up a whole folder full of files. DiskInternals makes a good recovery tool, and there's a free 30-day functional trial.

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