Trigger to perform average or sum of child records on parent object based on product type

Trigger to perform average or sum of child records on parent object based on product type
Hi there.. I need help with a trigger which averages sub opportunities revenue field when product type is same else it sums the revenues at the parent (opprtunity object).
The trigger which I have written below only sums. Please help me so that based on the similar product types the revenue gets averages else sumed up.