Salesforce : Dynamic Auto Number

Use Case:

We have 3 separate Sales Channel, I would like to generate different unique ID for different channel. For example:

Opportunity: Type=Online – ON-0001
Opportunity: Type=Direct – DR-0002
Opportunity: Type=Reseller – RS-0003
Opportunity: Type=Online – ON-0004

Solution:

1. Create an Auto Number field (AutoNumber__c) at Opportunity level using display format {0000}. This field can be hidden from the layout.
2. Create a Formula field with IF or Case logic:

   Case (Type,
   "Online", "ON-" + AutoNumber__c,
   "Direct", "DR-" + AutoNumber__c,
   "Reseller", "RS-" + AutoNumber__c,

However, this solution will not work if you need to have a separate counter for each type. For example:

Opportunity: Type=Online – ON-0001
Opportunity: Type=Direct – DR-0001
Opportunity: Type=Reseller – RS-0001
Opportunity: Type=Online – ON-0002
Opportunity: Type=Online – ON-0003
Opportunity: Type=Reseller – RS-0002