Low Level Design: Parking Lot System with Factory & Strategy Patterns

1. Requirements & Entity Modeling

A parking lot consists of multiple floors, each equipped with dedicated spot types: Motorcycle (Small), Compact (Medium), Large (Bus/Truck), and EV Charging spots. The system handles ticketing at entry gates and fee calculation at exit gates.

• Vehicles: Motorcycle, Car, Van, Truck, EV Car
• Spot Types: TwoWheeler, Compact, Large, Electric
• Pricing Models: Hourly, Flat Rate, Time-of-Day Surge
• Concurrency: Thread-safe spot allocation preventing double-parking

2. Applying Design Patterns: Strategy & Factory

We use the Factory Method Pattern to instantiate vehicle and spot objects based on sensor telemetry. The Strategy Pattern decouples fee calculation (HourlyFeeStrategy, SurgeFeeStrategy, WeekendFeeStrategy) from ticket checkout.

For spot allocation, a `ParkingStrategy` interface allows swapping between 'Nearest to Entrance' (min-heap) and 'Even Floor Distribution'.

3. Concurrency & Thread-Safe Spot Allocation

When multiple gates issue tickets simultaneously, two cars might be assigned the exact same parking slot. We protect slot allocation using fine-grained locks (e.g. `ReentrantLock` per floor or atomic compare-and-swap on slot status).

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

class Car extends Vehicle {
  constructor(licensePlate: string) { super(licensePlate, 'CAR'); }
}

interface PricingStrategy {
  calculateFee(durationHours: number): number;
}

class HourlyPricingStrategy implements PricingStrategy {
  constructor(private hourlyRate: number) {}
  calculateFee(hours: number): number {
    return Math.ceil(hours) * this.hourlyRate;
  }
}

class ParkingSpot {
  private isOccupied: boolean = false;
  constructor(public id: string, public floor: number, public type: string) {}
  public occupy(): boolean {
    if (this.isOccupied) return false;
    this.isOccupied = true;
    return true;
  }
  public vacate() { this.isOccupied = false; }
}