How to edit qBittorrent source code?

[kurbads]

I would like to modify the way qBittorrent does sequential download for multipart RAR torrents by adding the following lines:

If sequential is checked and files in torrent are *.rar, *.r00, *.01, ... *.99 then
    normal (non-sequental) download file *.rar
    when finished
    for n = r00 to r99
        normal (non-sequental) download file *.n
        when finished
    next
endif

How do I do that?

Where do I open the source code? Could you share the whole project folder with tree?

[ciaobaby]

Where do I open the source code?

https://github.com/qbittorrent/qBittorrent

and the actual piece transferring etc. is handled by libtorrent (http://www.libtorrent.org/)