autonumber field with condition

Hello All,

I have a CRM need that I want a auto-number field to be generated with a condition like this:

2016-10001-abc

2016=year

10001=incremental number

abc= is the condition from which user selects in Opportunity form. it's an option set field with 3 values, let's say : abc, def, ghi

So maybe the next number will be 2016-10002-ghi 

How can i accomplish this?

Thanks!