public List <Payment> GetPayments(LoanCalculationParameters loanCalculationParameters)
{
var ret = new List <Payment>();
var monthlyPayment = GetMonthlyPayment(loanCalculationParameters);
double montlyInterestRate = loanCalculationParameters.AnnualInterestRate / MagicNumbers.MonthsPerYear; //J
var currentPrincipal = loanCalculationParameters.Principal;
while (currentPrincipal > 0)
{
var currentMonthlyInterest = currentPrincipal * montlyInterestRate; //H
var principalForCurrentMonth = monthlyPayment - currentMonthlyInterest; //C
var endingBalance = currentPrincipal - principalForCurrentMonth; //Q
ret.Add(new Payment
{
Principal = Round(principalForCurrentMonth),
Interest = Round(currentMonthlyInterest),
Balance = Round(endingBalance)
});
currentPrincipal = endingBalance;
}
return(ret);
}
public void CanGenerateSchedule_MonthlyPaymentsShouldBeCorrect()
{
var sut = new FixedMonthlyRateCalculationStrategy();
var parameters = new LoanCalculationParameters
{
Principal = 100000,
AnnualInterestRate = 0.06D,
PaybackTimeInYears = 10
};
var ret = sut.GetPayments(parameters);
ExpectMonthlyPayment(new Payment
{
Interest = 500,
Principal = 610.21D,
Balance = 99389.79D
}, ret[0]);
ExpectMonthlyPayment(new Payment
{
Interest = 471.99D,
Principal = 638.22D,
Balance = 93758.81D
}, ret[9]);
ExpectMonthlyPayment(new Payment
{
Interest = 5.52D,
Principal = 1104.68D,
Balance = 0
}, ret[119]);
}
private void CanGetMonthlyPaymentTest(LoanCalculationParameters parameters, double expected)
{
var sut = new FixedMonthlyRateCalculationStrategy();
var ret = sut.GetMonthlyPayment(parameters);
Assert.AreEqual(expected, Math.Round(ret, MagicNumbers.DecimalPonitPrecision));
}
public double GetMonthlyPayment(LoanCalculationParameters loanCalculationParameters)
{
double montlyInterestRate = loanCalculationParameters.AnnualInterestRate / MagicNumbers.MonthsPerYear; //J
int durationInMonths = loanCalculationParameters.PaybackTimeInYears * MagicNumbers.MonthsPerYear; //N
double j = montlyInterestRate;
double n = durationInMonths;
double p = loanCalculationParameters.Principal;
return(p * (j / (1 - Math.Pow(1 + j, -n))));
}
public void CanGenerateSchedule_ShouldGenerateProperNumberOfPayments()
{
var sut = new FixedMonthlyRateCalculationStrategy();
var parameters = new LoanCalculationParameters
{
Principal = 100000,
AnnualInterestRate = 0.06D,
PaybackTimeInYears = 10
};
var ret = sut.GetPayments(parameters);
Assert.AreEqual(120, ret.Count);
}