Elevator as a State Machine

Code an elevator as a state machine. Asked in the final round technical interview stage. OOD/implementation.

Implement a single-elevator simulator that processes events one at a time and updates the elevator's state. The elevator starts at floor 0 with doors closed and can move one floor per tick.

Function contract

def step_elevator(state, event):

• state is a dict with keys: floor, direction, doors_open, targets, moving.
• event is one of: {"type": "call", "floor": int}, {"type": "select", "floor": int}, or {"type": "tick"}.
• Return the updated state dict.

Rules

1. call and select add a target floor if it is not already present.
2. On tick, if doors are open, close them first and do not move.
3. If doors are closed and there is a target above or below, move one floor toward the nearest target in the current direction, or choose a direction if idle.
4. When the elevator reaches a target floor, open the doors and remove that floor from targets.
5. If no targets remain, the elevator becomes idle with direction = "idle".

Return the full updated state after each event.