5. Design a library management system

public sealed class Book(string isbn, string title, string author) { public string Isbn = isbn, Title = title, Author = author; }
public enum CopyStatus { Available, Loaned, Reserved, Lost }
public sealed class BookCopy(string barcode, Book book) { public string Barcode = barcode; public Book Book = book; public CopyStatus Status = CopyStatus.Available; }
public sealed class Member(int id, string name) { public int Id = id; public string Name = name; public List<Loan> Loans { get; } = []; }

public sealed class Loan(BookCopy copy, Member member, DateOnly borrowed)
{
    public BookCopy Copy { get; } = copy;
    public Member Member { get; } = member;
    public DateOnly Borrowed { get; } = borrowed;
    public DateOnly Due => Borrowed.AddDays(14);
    public DateOnly? Returned { get; private set; }
    public int OverdueDays(DateOnly today) => Math.Max(0, (Returned ?? today).DayNumber - Due.DayNumber);
    public void Close(DateOnly date) => Returned = date;
}

public interface IFinePolicy { decimal Fine(Loan loan, DateOnly today); }
public sealed class PerDayFine(decimal perDay) : IFinePolicy { public decimal Fine(Loan l, DateOnly t) => l.OverdueDays(t) * perDay; }

public sealed class LibraryService(IFinePolicy fines, INotifier notifier)
{
    private const int MaxLoans = 5;
    public Loan Borrow(Member m, BookCopy c, DateOnly today)
    {
        if (m.Loans.Count(l => l.Returned is null) >= MaxLoans) throw new InvalidOperationException("Loan limit reached");
        if (c.Status != CopyStatus.Available) throw new InvalidOperationException("Copy not available");
        c.Status = CopyStatus.Loaned;
        var loan = new Loan(c, m, today);
        m.Loans.Add(loan);
        return loan;
    }
    public decimal Return(Loan loan, DateOnly today)
    {
        loan.Close(today);
        loan.Copy.Status = CopyStatus.Available;
        notifier.NotifyNextReservation(loan.Copy.Book);      // Observer-style hook for reservations
        return fines.Fine(loan, today);
    }
}
public interface INotifier { void NotifyNextReservation(Book book); }