Trading Houses

Imagine that all students are assigned dorm rooms in some ad-hoc way. After each one has a room, two students might find that they both prefer the room the other one has, thus they could switch rooms and both be happier. Similarly, three students (A,B, and C) might find that A prefer's B's room, B prefers C's room, and C prefers A's room. These three students might also trade rooms in a cycle. This can go on for even larger cycles. But, how do we find these cycles?

A very simple algorithm, proposed back in the seventies, is to have each agent point to its most preferred choice. If the resulting graph has any loops (it will) then all the agents in the loops exchange rooms and drop out of the game. The remaining agents then point to their most preferred room and we repeat the process until there are no more agents left.

I have implemented a simple demonstration of this top trading cycle algorithm using a simple distributed cycle detection protocol. Its fun to watch!