How should this function be achieved using MVC

[eluser]pawz[/eluser]
It seems to me that there's at least three possible locations for your general purpose functions:

1. private functions in the controller
2. functions in a library
3. functions in a model

I thought that more people would recommend option 1, as I've seen code that works like this, but so far this seems unpopular.
Option 2 is what I've been doing currently but it's a little messy because it requires the use of get_instance to get the codeigniter object.
Option 3 is one I hadn't really considered, but the comments so far make it sound more attractive. This involves using the model for general processing functions instead of just database functions which is something I hadn't considered.

After the posts here so far, I'm leaning towards option 3. Comments appreciated