How do .rar files and .par files work?

Discussion in 'Archived Threads 2001-2004' started by Todd K, Jul 30, 2002.

  1. Todd K

    Todd K Second Unit

    Joined:
    Oct 21, 2001
    Messages:
    477
    Likes Received:
    0
    Hello everyone,

    I am simply looking for a little assurance here. I have a movie that was broken down into .rar files, for which I would normally use winrar to extract. However, two files were missing, so I had to use SmartPar and .par files to recover the missing .rars.

    But my question is, if two files were missing, can .par files recreate the original file exactly? To me it is like replacing a missing volume of an encyclopedia set with a random book from the library. How can this be??

    Thanks for any input,

    Todd
     
  2. Nigel McN

    Nigel McN Supporting Actor

    Joined:
    Oct 23, 2000
    Messages:
    848
    Likes Received:
    0
    but it isn't a random book from the library, it is book from the dictionary publisher explaining what should be in each volume relative to each other.
     
  3. Joe Hsu

    Joe Hsu Supporting Actor

    Joined:
    Jul 2, 2001
    Messages:
    812
    Likes Received:
    0
    That's really odd, as when that happens to me, usually that means a chunk of the movie is cut out. The movie might still run and everything, but you might miss a part of the movie. dunno.
     
  4. Danny R

    Danny R Supporting Actor

    Joined:
    May 23, 2000
    Messages:
    871
    Likes Received:
    0
    If the movie was cut out, then you are somehow recombining the rar files without the missing pieces. THAT is what is odd to me.

    I've rejoined tons of rar files, and its very obvious if the PAR doesn't work. When a par does work, it restores the files EXACTLY as they once were. All you need to have is the same number of PAR files as missing data. The idea of parity bits to correct possibly missing data is old technology thats been around forever.
     
  5. John_Berger

    John_Berger Cinematographer

    Joined:
    Nov 1, 2001
    Messages:
    2,489
    Likes Received:
    0
     
  6. NickSo

    NickSo Producer

    Joined:
    Jul 2, 2000
    Messages:
    4,260
    Likes Received:
    0
    Real Name:
    Nick So
    Wow, i've never heard of the SmartPAR program before...

    So say your rars are supposda have like 10 in total... you have downloded 8. You can use smartpar to build the other two files you're missing?
     
  7. MichaelG

    MichaelG Second Unit

    Joined:
    Jul 10, 2000
    Messages:
    322
    Likes Received:
    0
    I agree John, the individuals that came up with that scheme are pretty smart. The only way to explain how this stuff works is to be technical, I think that you explained it very well. To me stuff like this is so cool because I remember the days of tape backups to recorver data, and that was just back in the '94 time range.

    I haven't really put any thought into it, but I wonder how complex the algorithm must be for the parity bit. I guess that if I was still in college it would make a great project.
     
  8. John_Berger

    John_Berger Cinematographer

    Joined:
    Nov 1, 2001
    Messages:
    2,489
    Likes Received:
    0
     
  9. Danny R

    Danny R Supporting Actor

    Joined:
    May 23, 2000
    Messages:
    871
    Likes Received:
    0
    So say your rars are supposda have like 10 in total... you have downloded 8. You can use smartpar to build the other two files you're missing?

    Yup, provided you have two PAR files to replace the two RARs you are missing.

    A lot of people thus ask, "Well, if you need the same number of files, whats the advantage?" The advantage is that it doesn't matter which par file of the set you use. Thus if someone publishes 5 or so pars, and you only get 4 of them, you don't have to worry about the 5th as long as you aren't missing more than 4 files of the .rar set.

    And it doesn't matter which RAR is missing. Likewise it doesn't matter which PARS you use to correct it, just so long as the number of pars is equal to the number of missing/damaged files.
     
  10. John_Berger

    John_Berger Cinematographer

    Joined:
    Nov 1, 2001
    Messages:
    2,489
    Likes Received:
    0
     
  11. Todd K

    Todd K Second Unit

    Joined:
    Oct 21, 2001
    Messages:
    477
    Likes Received:
    0
    Thanks for your time and effort. Even though I don't quite understand it, I will rest easier knowing my repaired rars are just as good as the originals.

    Todd
     
  12. John_Berger

    John_Berger Cinematographer

    Joined:
    Nov 1, 2001
    Messages:
    2,489
    Likes Received:
    0
    What don't you understand? If you don't understand something, then we really didn't answer your question. As a UNIX trainer, I don't like having my students' questions unanswered. [​IMG]
     
  13. Todd K

    Todd K Second Unit

    Joined:
    Oct 21, 2001
    Messages:
    477
    Likes Received:
    0
    I meant that I don't understand the magic. I understand generally how the process works, but I still am mystified on how computers handle data in general.

    Believe me, you were quite successful in your lesson. It was written in easily understandable terms.
     
  14. John_Berger

    John_Berger Cinematographer

    Joined:
    Nov 1, 2001
    Messages:
    2,489
    Likes Received:
    0
     

Share This Page