Elevator System

Design the control system for a bank of elevators in a building: accept hall calls and in-car requests, dispatch elevators efficiently, and move cars safely. It's hard because it mixes a per-car state machine, a dispatching strategy, and real-time concurrency.

Requirements

Functional

• N elevators serving floors 0..F.
• Hall calls: a person on a floor presses UP or DOWN.
• Car calls: a person inside presses a destination floor.
• Dispatcher assigns each hall call to the best elevator.
• Each car moves floor by floor, stops at requested floors, opens and closes doors.
• Display current floor and direction; capacity/overload detection; maintenance mode; emergency stop.

Non-functional

• Minimise average wait time and avoid starvation (every request is eventually served).
• Thread-safe: button presses arrive concurrently with car movement.
• Scheduling policy must be swappable (SCAN/LOOK, nearest car, zoning for peak hours).
• Safety: doors never open while moving.