- Elevator starts at ground floor
- Elevator can be in one of the states: "going up", "going down", "waiting", "requires maintenance"
- After pressing the button, elevator closes doors, and once they are closed, it starts the engine
- Once requested floor is reached, elevator stops the engine and opens the door
- Elevator visits floors in order (if pressed buttons are 4,2,5, it will stop in order at 2,4,5)
- Elevator does not change direction (if it's at 2 floor and going to 4, and at 3 we press 1 it still goes up to 4)
- If engine breaks, elevator stops and goes into "requires maintenance" state.
- Elevator in "requires maintenance" state does not react for pressing button
- If elevator discovers sensor malfunction (e.g. visits floor 3 and then 5 without 4 in the middle) it goes into "requires maintenance" state.