Need help coming up with a state diagram and circuit design

Need help coming up with a state diagram and circuit design for the following:

Design a system that will turn on the light as the first person enters a room and turn off the light as the last person leaves. Assume that there is a single door and only one person can enter or leave the room at a time. Use photocells (or similar sensors) to detect a person entering or leaving the room.

Solution

http://www.ijictrd.net/papers/IJICTRDV1I4005.pdf

http://www.enggroom.com/Electrical/AUTOMATIC%20ROOM%20LIGHT%20CONTROLLER.pdf