this post was submitted on 02 Jun 2024
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I still to this day don't know how it worked, but I remember back when I would pirate games and often there would be like 20 different compressed archives, but somehow you only need to decompress one of them and the game would install. Was like magic.
Multipart archives still exist. They're now used for file sharing websites that have a maximum file size. Before that they were for unreliable p2p networks, so you didn't lose the parts you'd already downloaded when your peer goes offline. Originally it was to fit something big on multiple cd-roms or floppies.
Opening somthing.rar also reads the data in somthing.r01 through somthing.r15 etc
Oh so it's just kinda a part of the rar specification then? How did that work on CDs or floppies, if presumably you'd have had to swap out to insert the next part?
Yes, it asks for the next part if it's not in the same folder with the same name, doesn't really make a difference what it's stored on. Multipart zip and tar also exist.
So the first file acts as a sort of index? From the earlier comment I thought it was autodetecting the presence of the numbered files and expanding what it found.
It's going to have some metadata to that effect yes, like a file index or number of parts or total extracted file size. I don't know the details, I've used them I haven't read the spec. rar is Rarlab's proprietary format so there might not even be a public spec.
They're normally all the same size except for the last part, so it's not that file 1 is just an index.
Most compression programs offer a way to separate your thing in multiple parts, I know 7zip and Peazip do.
I've recently had to properly rename the latter part of a multipart zip because the source I got it from probably just renamed the parts it stole from elsewhere, which broke the whole "extract part 1, everything else comes along!"