How to get the subset ID given a face ID?

Hello, I'm trying to do a custom exporter for my scene and i need to know how i can get the mesh subset id, given the face ID. for example: face #223 belongs to subset 3 That's right, i want to get material IDs and write them to the file :)

p.s. my mesh already contains assigned subsets, i've set up the loading part successfully and it works fine.

Thank you!