private decimal GetExtraPayment(ExtraPayment extraPayment, DateTime currentServicingDate)
{
if (extraPayment != null && // do we need to return 0 if extraPayment.DateApplied < historicPayments.Last().PaymentDate ?
extraPayment.IsExtraPaymentApplicable(currentServicingDate))
{
// check if extra payment amount is valid relative to current date. if yes, return value. if not, return 0.
return(extraPayment.Amount);
}
return(0);
}
public AmortizationSchedule Calculate(decimal loanAmount, decimal interestRate, int termInMonths, DateTime startDate, decimal?monthlyPayment = null, ExtraPayment extraPayment = null)
{
if (termInMonths <= 0)
{
throw new Exception($"Loan term in months must be greater than zero. Term received was {termInMonths}.");
}
var monthlyInterestRate = interestRate / MonthsInYear;
// calculate estimated monthly payment if calling code does not send an amount or valid amount
if (monthlyPayment <= 0m || monthlyPayment == null)
{
monthlyPayment = loanAmount * (decimal)((double)monthlyInterestRate / (1 - Math.Pow(1 + (double)monthlyInterestRate, -termInMonths)));
}
var remainingBalance = loanAmount;
var interestPaidToDate = 0m;
DateTime maturityDate = startDate.AddMonths(termInMonths);
var currentServicingDate = startDate;
var amortizationPayments = new List <AmortizationPayment>();
var monthCount = 0; // month count will result in remaining balance > 0 by end of term if we account for missed payments
while (monthCount < termInMonths || Math.Round(remainingBalance, 2) > MinimumBalanceLeft)
{
if (monthCount > 0) // added if to prevent adding of month if it is the first month
{
currentServicingDate = currentServicingDate.AddMonths(1); // assuming monthly intervals for payments
}
monthCount++;
// while loop will always continue to end of term so that we can calculate relative total interest if actual balance is completed before end of term
if (Math.Round(remainingBalance, 2) <= MinimumBalanceLeft) // can this remaining balance <= 0 be checked against 0.00 e.g. Math.Round(remainingBalance, 2) <= 0 currently there are scenarios where the remaining balance may be 0.00000001
{
continue;
}
// set base interest and principal for month
var interestForMonth = remainingBalance * monthlyInterestRate;
var principalForMonth = monthlyPayment.Value - interestForMonth;
// checks if current term will contain last payment
if (remainingBalance < monthlyPayment.Value)
{
// updates principal for month since that will be the only value affected by the last payment
principalForMonth = (monthlyInterestRate + 1) * remainingBalance - interestForMonth; // above is equal to principalForMonth = remainingBalance when you simplify.
}
// get any additional or historical payments
var extraPaymentForMonth = GetExtraPayment(extraPayment, currentServicingDate);
// calculate any left over extra payment. currently just stored in local variable
if (remainingBalance < principalForMonth + extraPaymentForMonth) // moved remaining balance to the front for readibility
{
var leftoverExtraPayment = (principalForMonth + extraPaymentForMonth) - remainingBalance; // this equation works for both when principal < remaining balance and principal > remaining balance
extraPaymentForMonth -= leftoverExtraPayment;
}
remainingBalance -= principalForMonth + extraPaymentForMonth;
interestPaidToDate += interestForMonth;
// apply payments saved and maturity date current iteration is last payment
if (Math.Round(remainingBalance, 2) <= MinimumBalanceLeft)
{
// moved this setting of data out of previous if blocks because there are multiple ways to end a loan
// remaining balance < monthly payment or remaining balance < principal for month + extra payment for month (if an extra payment is made on the last term)
maturityDate = currentServicingDate;
}
// create amort payment and add to schedule (no rounding)
amortizationPayments.Add(new AmortizationPayment()
{
PaymentDate = currentServicingDate,
PaymentInterest = interestForMonth,
PaymentPrincipal = principalForMonth,
PaymentAmount = principalForMonth + interestForMonth,
AdditionalPrincipal = extraPaymentForMonth,
InterestPaidToDate = interestPaidToDate,
BalanceToDate = remainingBalance
});
}
var amortizationSchedule = new AmortizationSchedule()
{
InterestPaid = interestPaidToDate,
ExtraPrincipalPaid = amortizationPayments.Sum(x => x.AdditionalPrincipal),
MonthlyPayment = monthlyPayment.Value,
MaturityDate = maturityDate,
Schedule = amortizationPayments
};
return(amortizationSchedule);
}