Rebirth and brain death

I am very new to buddhism, so my understanding is very basic. My previous world views were very materialistic and deterministic. It might not be surprising that the most confusing concept I find so far is rebirth. Lets take this theoretical situation: Somebody is declared brain dead. - Does this person gets reborn? - What if medicine improved and we would bring this person back to life? - What if this person would be left in this condition forever by keeping that person alive with medical resources? Thank you for your help!