Optimize and parallelize the downloads

[Pikrass]

Hi!
First of all, thank you for your script, it really helped me!

I wanted to download a 350000-message group. As you can imagine, it was taking a while.
I realized the script was creating a wget process, and therefore a single connection to Google, for each and every forum page, thread page, and message. Which is really inefficient.

So I improved the generated script so that

1. a bunch of messages were retrieved in one connection, thanks to curl and its -o $output1 $url1 -o $output2 $url2 syntax
2. multiple processes ran at the same time

I even made a nice text UI to monitor the jobs.

Here's my script. I license it under the same terms as your code.

I'm not doing a PR because:

• there's still room for improvement (eg my script only handles the actual messages, not the thread list and message list)
• I didn't look that much into the code, only into the generated script, so there's probably a few tweaks needed
• there are a few decisions involved: my version requires curl, there are parameters for the number of processes and the number of URLs per batch, the cookie file is its own parameter instead of WGET_OPTIONS, etc.

Hope this proves useful. :)

[icy]

That's excellent @Pikrass . I will update the README to mention your script, and I think that's fine enough at the moment.

My intention was to slowly query google server so the script won't be locked down; that's why I didn't have parallel support out-of-the-box. Once you have the generated script, you have a few options to continue, e.g, your way, or you can also feed the gnu parallel tool.

[icy]

I mention your script here https://github.com/icy/google-group-crawler#contributions . Thanks a lot.