18. Machine Coding Case Study 2: High-Concurrency Elevator Management System • Low-Level Design (LLD) & Object-Oriented Machine Coding • Rishabh Ki Kaksha

Design a high-concurrency building Elevator Controller:
*   Multiple Elevators in a building.
*   External requests (`floor`, `direction: UP/DOWN`).
*   Internal requests (`destination_floor`).
*   Optimal Dispatcher Algorithm (**LOOK / SCAN Algorithm** - The Disk Scheduling Pattern).

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### 2. Core State & Dispatcher Architecture

```typescript
enum Direction { UP, DOWN, IDLE }
enum ElevatorStatus { MOVING, STOPPED, MAINTENANCE }

class Elevator {
    public currentFloor: number = 0;
    public direction: Direction = Direction.IDLE;
    public upRequests: Set<number> = new Set();
    public downRequests: Set<number> = new Set();

constructor(public id: number, public capacity: number) {}

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

public moveStep() {
        if (this.direction === Direction.UP) {
            this.currentFloor++;
            if (this.upRequests.has(this.currentFloor)) {
                this.upRequests.delete(this.currentFloor);
                console.log(`🛗 Elevator ${this.id} stopped at Floor ${this.currentFloor} (UP)`);
            }
            if (this.upRequests.size === 0) this.direction = this.downRequests.size > 0 ? Direction.DOWN : Direction.IDLE;
        } else if (this.direction === Direction.DOWN) {
            this.currentFloor--;
            if (this.downRequests.has(this.currentFloor)) {
                this.downRequests.delete(this.currentFloor);
                console.log(`🛗 Elevator ${this.id} stopped at Floor ${this.currentFloor} (DOWN)`);
            }
            if (this.downRequests.size === 0) this.direction = this.upRequests.size > 0 ? Direction.UP : Direction.IDLE;
        }
    }
}
```

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### 3. Master Comparison Matrix

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### 4. Interview Ke 3 Sawal Is Topic Se

1. **Multiple elevators me se incoming request kis elevator ko assign karenge?** -> Proximity + Direction match score calculate karke (Elevator moving towards the requested floor in the same direction gets highest score).
2. **Elevator door safety sensor integration kaise model karenge?** -> State Pattern (`DoorState: OPEN, CLOSING, CLOSED`) + Observer event on obstruction.
3. **Peak Morning Rush Hour (saare log ground floor se upar ja rahe hain) optimization kaise karenge?** -> Morning mode policy: idle elevators automatically Ground floor par park ho jaati hain.

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**Next Topic:** 19. Machine Coding Case Study 3: Rate Limiter (Token Bucket, Leaky Bucket, Sliding Window)

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