17. Machine Coding Case Study 1: Multi-Level Parking Lot System (Vehicles, Slots, Payments) • Low-Level Design (LLD) & Object-Oriented Machine Coding • Rishabh Ki Kaksha

Design an extensible, thread-safe Multi-Level Parking Lot:
*   Support multiple vehicle types: `BIKE`, `CAR`, `TRUCK`.
*   Support multiple slot types matching vehicle sizes.
*   Parking strategy (Nearest to entrance / Floor-first).
*   Dynamic Ticket Generation & Fee Calculation on Exit.

---

### 2. Class Diagram & Architecture Design 📐

```
[ Vehicle (Abstract) ] ◄── [ Bike, Car, Truck ]
        │
[ ParkingSlot ] ◄── [ CompactSlot, LargeSlot, BikeSlot ]
        │
[ ParkingFloor ] ── contains ── [ ParkingSlots ]
        │
[ ParkingLot (Singleton) ] ── contains ── [ ParkingFloors ]
        ├── ParkingStrategy (Strategy Pattern)
        └── FeeCalculator (Strategy Pattern)
```

---

### 3. Core Implementation (Clean Object-Oriented TS)

```typescript
enum VehicleType { BIKE, CAR, TRUCK }

abstract class Vehicle {
    constructor(public licensePlate: string, public type: VehicleType) {}
}

class ParkingSlot {
    public isOccupied: boolean = false;
    public parkedVehicle: Vehicle | null = null;

constructor(public slotId: string, public floorNumber: number, public supportedType: VehicleType) {}

public park(vehicle: Vehicle) {
        this.parkedVehicle = vehicle;
        this.isOccupied = true;
    }
    public unpark() {
        this.parkedVehicle = null;
        this.isOccupied = false;
    }
}

class Ticket {
    public entryTime: Date = new Date();
    constructor(public ticketId: string, public vehicle: Vehicle, public slot: ParkingSlot) {}
}

class FeeCalculator {
    calculateFee(ticket: Ticket, exitTime: Date): number {
        const hours = Math.ceil((exitTime.getTime() - ticket.entryTime.getTime()) / (1000 * 60 * 60));
        const ratePerHour = ticket.vehicle.type === VehicleType.TRUCK ? 100 : 50;
        return hours * ratePerHour;
    }
}
```

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### 4. Master Design Patterns Used

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

1. **Multi-threaded environment me slot allocation race conditions kaise prevent karenge?** -> Har Floor level par synchronized lock ya ConcurrentLinkedQueue maintain karke.
2. **EV (Electric Vehicle) Charging Slots support kaise add karenge?** -> `ElectricParkingSlot` class inherit karke additional charging rate strategy inject karke (Open-Closed Principle).
3. **High traffic me Nearest Slot lookup $O(1)$ me kaise karenge?** -> Har vehicle type ke liye Min-Heap maintain karke jisme available slots distance-wise sorted hon.

---
**Next Topic:** 18. Machine Coding Case Study 2: High-Concurrency Elevator Management System

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Rishabh Gehlot

PHP 8+ & Laravel 11 Master Track