/// <summary>
/// Calculates loan payment based on information in <see cref="LoanCalculatorInput"/>.
/// </summary>
/// <param name="input">Serves as an input for payment calculation.</param>
/// <returns>The payment wrapped in a <see cref="CalculationResult{TResult}"/> instance.</returns>
public CalculationResult <decimal> Calculate(LoanCalculatorInput input)
{
if (input == null)
{
throw new ArgumentNullException(nameof(input));
}
if (input.TermMonths <= 0)
{
return(CalculationResult <decimal> .Fail(CalculationFailureReason.InvalidInput));
}
if (input.PrincipalAmount < 0)
{
return(CalculationResult <decimal> .Fail(CalculationFailureReason.InvalidInput));
}
if (input.InterestRate < 0)
{
return(CalculationResult <decimal> .Fail(CalculationFailureReason.InvalidInput));
}
decimal calculationOutput;
if (input.InterestRate == 0)
{
calculationOutput = input.PrincipalAmount / input.TermMonths;
}
else
{
double presentValue = (double)input.PrincipalAmount;
double rateOverPeriods = (double)input.InterestRate / 12;
int numberOfPeriods = input.TermMonths;
double payment = presentValue * rateOverPeriods / (1 - Math.Pow(1 + rateOverPeriods, -numberOfPeriods));
calculationOutput = Convert.ToDecimal(payment);
}
return(CalculationResult <decimal> .Success(Math.Round(calculationOutput, 2)));
}