Advice on mapping

Hi Experts,

Im a learner and new to PI . I have a file to Idoc scenario, where the source file comes with multiple records with multiple Header and Item details. Example file structure is as below

H,T1,xx,yy,...

I,T1,aa,bb,..

H,T2,gg,hh...

I,T2,cc,dd,...

.

.

.

Where T1 & T2 are key field values, which identifies the related header and Item records.

Need to generate 1 idoc for each related header & Item data.

So, please guide me how the source data type to be created and how to implement mapping logic.

Thanks,

Aish