Need Mapping Help: Generate index if value comes

Hi Experts,

I need mapping help to generate index if value comes form source. we have 4 fields in item level of source and target side we have to pass the these filed values and sequence number.

below given the structures:

Source:              Target:

Item                    Item

   A1                      Text

    A2                      Seq_No

    A3

    A4

my requirement is if A1 filed values comes from source we should pass to text and pass seq_no to constant-1, same as A2,A3 and A4 filed values also we should pass to text and seq_no for A2 is 2 and A3 is 3 and A4 is 4. suppose if A1 value is not coming from source we should pass seq_no for A2 is 1. if A1 and A3 filed values are not coming from source then we should pass seq_no for A2 and A3 are 1 and 2.

can you please help me how to achieve this. appreciate for your support.

Regards,

Sanjay.