using System;
using System.Collections.Generic;
using System.Linq;

namespace BoundaryLab.Domain;

public interface IDiscountPolicy { int Amount(int subtotal); }
public sealed class Invoice(int subtotal)
{
    public int Total(IDiscountPolicy policy)
    {
        if (subtotal < 0) throw new ArgumentOutOfRangeException(nameof(subtotal));
        int discount = policy.Amount(subtotal);
        if (discount < 0 || discount > subtotal) throw new InvalidOperationException("Invalid discount");
        return subtotal - discount;
    }
}