Create a calculated field

Hello,

What would be the best approach to calculate a field automatically ?

For instance I want a user to be able to subscribe to my site and it to generate a unique ID based on :

Initials of first and last name + number of other people with same initials + 1 site ID (to differentiate a user who subscribed on english version or french version) unique 5 digit number.

So I would have for instance :

JS6-GBSITE-67364

JS : John Smith 6 : There are already 5 other users with initials JS GBSITE : Site ID 67364 : random number

Why do I need this ?

I need this to make an affiliate system.

I have already created this system with codeigniter but would like to use Expression Engine members system to be able to easily add on the forum / wiki module.

Another application of the calculated field is that I need to create a reference number for each product in my product channel.

Should I convert this to a hook to achieve this ? or can Expression Engine already do this ?