Unique Username Routine

Hello,
I was wondering if you have one or have seen a routine to generate unique
username from first, middle and last names. Here is my requirement -
by default first initial and last name of a user is their username.
But if the expected or default username is already taken then if the
user have middle initial, then the username for this user would be
first initial, middle initial and lastname. If the user does not have
middle name then the username for the user would be first two letters
of first name and lastname and if that's already taken then first
three letters of first name and last name and so on.

Any help or pointer is greatly appriciated.