Problem 4.1 — Parking Lot

A complete, runnable slice: lot, floors, spots, tickets, pricing and a thread-safe entry flow.

public enum SpotType { BIKE(1), CAR(2), TRUCK(4), EV(2);
  public final int size;
  SpotType(int size) { this.size = size; }
}

public class ParkingSpot {
  private final String id;
  private final SpotType type;
  private final int floor;
  private volatile boolean free = true;      // visible across threads

  public ParkingSpot(String id, SpotType type, int floor) {
    this.id = id; this.type = type; this.floor = floor;
  }
  public synchronized boolean tryOccupy() {   // atomic claim
    if (!free) return false;
    free = false;
    return true;
  }
  public synchronized void release() { free = true; }
  public String id() { return id; }
  public SpotType type() { return type; }
  public int floor() { return floor; }
}

public record Ticket(String id, String plate, SpotType type, long entryTime, ParkingSpot spot) {}

// --- pricing as a STRATEGY ---
public interface PricingPolicy {
  double cost(Ticket t, long exitTime);
}
public class FlatPricing implements PricingPolicy {
  private final double perHour;
  public FlatPricing(double perHour) { this.perHour = perHour; }
  public double cost(Ticket t, long exitTime) {
    long mins = (exitTime - t.entryTime()) / 60_000L;
    return Math.max(1, (mins + 59) / 60) * perHour * t.type().size;  // ceil hours
  }
}

// --- the lot ---
public class ParkingLot {
  private final Map<SpotType, List<ParkingSpot>> freeByType = new ConcurrentHashMap<>();

  public ParkingLot(int floors, int spotsPerFloor) {
    for (SpotType st : SpotType.values())
      freeByType.put(st, new CopyOnWriteArrayList<>());
    for (int f = 1; f <= floors; f++)
      for (int s = 1; s <= spotsPerFloor; s++)
        for (SpotType st : SpotType.values())
          freeByType.get(st).add(new ParkingSpot("F" + f + "-" + s, st, f));
  }

  // called from many entry-gate threads — thread-safe by construction
  public Ticket enter(String plate, SpotType type) {
    ParkingSpot spot = freeByType.get(type).stream()
        .filter(ParkingSpot::tryOccupy)
        .findFirst()
        .orElseThrow(() -> new IllegalStateException("lot full for " + type));
    return new Ticket("T" + System.nanoTime(), plate, type, System.currentTimeMillis(), spot);
  }

  public double exit(Ticket t, PricingPolicy pricing) {
    long now = System.currentTimeMillis();
    t.spot().release();                        // free the spot
    return pricing.cost(t, now);
  }
}

// Demo: ParkingLot lot = new ParkingLot(2, 4);
//       Ticket t = lot.enter("MH12AB1234", SpotType.CAR);
//       double bill = lot.exit(t, new FlatPricing(50));   // ₹50/hr per size unit