Low Level Design: Elevator System with State Pattern & LOOK Dispatch Algorithm

1. Problem Requirements & Functional Scope

An elevator system controls multiple elevator cars in a building with $N$ floors. The system must process external hall calls (floor up/down buttons) and internal car requests (destination floor buttons).

Key goals: Minimize average wait time, prevent starvation, handle emergency stops, enforce weight limits, and maintain thread safety under concurrent requests from multiple floors.

• Elevator States: MOVING_UP, MOVING_DOWN, IDLE, MAINTENANCE
• Door States: OPEN, OPENING, CLOSING, CLOSED
• Sensors: WeightSensor, ObstacleSensor
• Dispatching: LOOK algorithm (sweep in current direction until no more requests)

2. Class Diagram & Object-Oriented Modeling

Core classes include `ElevatorCar`, `ElevatorController`, `Dispatcher`, `HallButton`, `CarButton`, and the `ElevatorState` interface.

Using the State Pattern decouples motion state transitions from the elevator car logic. When moving UP, pressing a floor below the car is queued for the return journey rather than causing erratic directional shifts.

3. The LOOK Dispatching Algorithm

The LOOK algorithm mimics a disk drive head: an elevator continues traveling in its current direction servicing requests until there are no further requests ahead. Only then does it reverse direction.

Requests are stored in two priority queues: `upQueue` (min-heap for floors above) and `downQueue` (max-heap for floors below).

4. Complete Implementation & Code Walkthrough

Below is the core object-oriented implementation showing the State pattern and thread-safe request dispatching:

enum Direction { UP, DOWN, IDLE }
enum DoorState { OPEN, CLOSED }

interface Request {
  floor: number;
  direction: Direction;
}

class ElevatorCar {
  public id: number;
  public currentFloor: number = 0;
  public direction: Direction = Direction.IDLE;
  public doorState: DoorState = DoorState.CLOSED;
  public upRequests = new Set<number>();
  public downRequests = new Set<number>();

  constructor(id: number) { this.id = id; }

  public addRequest(floor: number) {
    if (floor > this.currentFloor) {
      this.upRequests.add(floor);
    } else if (floor < this.currentFloor) {
      this.downRequests.add(floor);
    }
  }

  public step() {
    if (this.direction === Direction.UP) {
      if (this.upRequests.size > 0) {
        this.currentFloor++;
        if (this.upRequests.has(this.currentFloor)) {
          this.upRequests.delete(this.currentFloor);
          this.openDoor();
        }
      } else if (this.downRequests.size > 0) {
        this.direction = Direction.DOWN;
      } else {
        this.direction = Direction.IDLE;
      }
    }
  }

  private openDoor() { this.doorState = DoorState.OPEN; }
}