public decimal GetCost(IEnumerable <IHarryPotterBook> books)
{
decimal cost = 0;
int paymentSetCount = 0;
while (books.Any(x => x.count > paymentSetCount))
{
IEnumerable <IHarryPotterBook> bookSets = books.Where(x => x.count > paymentSetCount);
int bookCount = bookSets.Count();
IPayment payment = GetPayment(bookCount);
int setCount = bookSets.Min(x => x.count) - paymentSetCount;
cost += payment.GetCost(bookSets, setCount);
paymentSetCount += setCount;
}
return(cost);
}