RE: have to create username programmatically

hi
users has to register to access the site.
registration is a 3 step process.
So when user fills in and clicks on submit button of step1 page.
if page is valid ( data entered in that page is valid) then I have to create username(programmatically) and then redirect him to step2.

rules to create username
1) username should be in this format "M1001" - string(M/F) and then int value(this value is taken from identity column + 1000).
2) step1 page has field "gender" M/F, if user selects "M" then string in username should be "M" else "F"
as this is a website , multiple users would be trying to register at the same time.
how can i do this? how can i create username?
please guide me what steps i have to follow.
there would be n number of cases i have to keep in mind to create username programmatically. what are they?
any help would be highly appreciated.

thanks
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