/// <summary>
/// Расчитывать выплаты.
/// X = (S*p) / (1-(1+p)^(1-m))
/// X - размер аннуитетного платежа;
/// S - сумма кредита;
/// p - 1/12 годовой процентной ставки, выраженная в сотых долях;
/// ^ - означает в степени;
/// m - срок кредита, в месяцах.
/// </summary>
/// <param name="amount">Объем долга.</param>
/// <param name="interestRate">Процентная ставка.</param>
/// <param name="loanTerm">Срок кредита(в месяцах).</param>
/// <returns>Выплаты по кредиту.</returns>
public IEnumerable<Payment> Calculate(
Amount amount,
InterestRate interestRate,
LoanTerm loanTerm)
{
if (amount == null || interestRate == null || loanTerm == null)
{
throw new ArgumentException("Bad params for calculation");
}
var payments = new List<Payment>();
var totalBalanceOwed = amount.Value;
int number = 0;
while (totalBalanceOwed > 0)
{
var loanPayment = totalBalanceOwed / (loanTerm.Value - number);
var percentPayment = totalBalanceOwed * interestRate.Value / (100 * 12);
totalBalanceOwed -= loanPayment;
var payment = new Payment(number, percentPayment, loanPayment, totalBalanceOwed);
payments.Add(payment);
number++;
}
return payments;
}
public void TestForOneMonth()
{
var amount = new Amount() { Value = 100 };
var interestRate = new InterestRate() { Value = 12 };
var loanTerm = new LoanTerm() { Value = 1 };
var payments = this.calculator.Calculate(amount, interestRate, loanTerm);
Assert.AreEqual(1, payments.Count());
var payment = payments.FirstOrDefault();
Assert.IsNotNull(payment);
Assert.AreEqual(0, payment.Number);
Assert.AreEqual(1, payment.PercentPayment);
Assert.AreEqual(100, payment.LoanPayment);
Assert.AreEqual(0, payment.BalanceOwed);
}
public void TestForTwoMonths()
{
var amount = new Amount() { Value = 100 };
var interestRate = new InterestRate() { Value = 12 };
var loanTerm = new LoanTerm() { Value = 2 };
var payments = this.calculator.Calculate(amount, interestRate, loanTerm);
Assert.AreEqual(2, payments.Count());
var firstPayment = payments.FirstOrDefault(item => item.Number == 0);
Assert.IsNotNull(firstPayment);
Assert.AreEqual(1, firstPayment.PercentPayment);
Assert.AreEqual(50, firstPayment.LoanPayment);
Assert.AreEqual(50, firstPayment.BalanceOwed);
var secondPayment = payments.FirstOrDefault(item => item.Number == 1);
Assert.IsNotNull(secondPayment);
Assert.AreEqual(0.5m, secondPayment.PercentPayment);
Assert.AreEqual(50, secondPayment.LoanPayment);
Assert.AreEqual(0, secondPayment.BalanceOwed);
}
