private void ValdateInput(LoanCalcInput input)
{
if (input.Amount < 0)
{
throw new ArgumentOutOfRangeException(ParamLabelInputAmount, input.Amount, NegativeNumberNotAllowedMessage);
}
if (input.DownPayment < 0)
{
throw new ArgumentOutOfRangeException(ParamLabelInputDownPayment, input.DownPayment, NegativeNumberNotAllowedMessage);
}
if (input.Term < 0)
{
throw new ArgumentOutOfRangeException(ParamLabelInputTerm, input.Term, NegativeNumberNotAllowedMessage);
}
if (input.Interest < 0)
{
throw new ArgumentOutOfRangeException(ParamLabelInputInterest, input.Interest, NegativeNumberNotAllowedMessage);
}
if (input.DownPayment > input.Amount)
{
throw new ArgumentOutOfRangeException(ParamLabelInputDownPayment, input.DownPayment, DownPaymentMoreThanLoanAmountMessage);
}
}
/// <summary>
/// Calculate the loan payment based on given input data.
/// </summary>
/// <param name="input">Loan amount, down payment, rate, and term</param>
/// <returns>JSON</returns>
public string DoTheMath(LoanCalcInput input)
{
// Check some typical error cases and throw.
this.ValdateInput(input);
// The following formula is implemented here:
//
// M = payment amount
// P = principal (amount of money borrowed)
// J = effective interest rate
// N = total number of payments
//
// P = amount - downpayment
// M = P * (J / (1 - (1 + J)^-N))
// TotalPayments = M * N
// TotalInterest = TotalPayments - P
var principal = input.Amount - input.DownPayment;
var numMonthlyPayments = input.Term * 12;
var negNumPayments = numMonthlyPayments * -1.0;
// Calculate the effective interest rate by converting to a fraction
// and then dividing by 12 (number of months in a year).
var effectiveInterest = (input.Interest / 100.0) / 12;
var monthlyPayment = principal * (effectiveInterest / (1 - Math.Pow(1 + effectiveInterest, negNumPayments)));
var totalPayments = monthlyPayment * numMonthlyPayments;
var totalInterest = totalPayments - principal;
// Initialize a temporary object with properly formatted strings
// before we convert it to JSON. The LoanCalcOutput class uses
// specific property name attributes as required in the problem
// statement.
var loanCalcOutput = new LoanCalcOutput
{
MonthlyPayment = string.Format("{0:0.00}", monthlyPayment),
TotalInterest = string.Format("{0:0.00}", totalInterest),
TotalPayment = string.Format("{0:0.00}", totalPayments)
};
// Use JSON.net to create a JSON-formatted string of the above
// temporary object.
return(JsonConvert.SerializeObject(loanCalcOutput));
}
