public Loan(LoanSpecification specification, decimal installment, decimal administrationFee, LoanIndicators indicators)
{
Specification = specification;
Installment = installment;
AdministrationFee = administrationFee;
Indicators = indicators;
}
private decimal CalculateEffectiveApr(LoanSpecification specification, decimal apr)
{
var periodicApr = 1 + apr / (specification.NumberOfCompoundPeriodsSimplify * 100);
var effectiveApr = (decimal)Math.Pow((double)periodicApr, specification.NumberOfCompoundPeriodsSimplify) - 1;
return(effectiveApr * 100);
}
private decimal CalculateAdministrationFee(LoanSpecification loanSpecification)
{
var percentageAmount = loanSpecification.OriginalPrincipal / 100 * _bankInterestProvider.AdministrationFeePercentage;
return(percentageAmount < _bankInterestProvider.AdministrationFeeMaximumAmount
? percentageAmount
: _bankInterestProvider.AdministrationFeeMaximumAmount);
}
public Loan CreateLoan(LoanSpecification loanSpecification)
{
var administrationFee = CalculateAdministrationFee(loanSpecification);
var installment = _installmentCalculator.CalculateInstallment(loanSpecification);
var indicators = _loanIndicatorsGenerator.GenerateLoanIndicators(loanSpecification, administrationFee);
return(new Loan(loanSpecification, installment, administrationFee, indicators));
}
private decimal CalculateEffectiveAnnualRate(LoanSpecification loanSpecification)
{
var interestRate = loanSpecification.AnnualInterestRate / 100;
var periodicRate = (double)interestRate / loanSpecification.NumberOfCompoundPeriodsSimplify;
var effectiveAnnualRate = (decimal)(Math.Pow(1 + periodicRate, loanSpecification.NumberOfCompoundPeriodsSimplify) - 1) * 100;
return(effectiveAnnualRate);
}
public decimal CalculateInstallment(LoanSpecification loanSpecification)
{
var interestRate = loanSpecification.AnnualInterestRate / 100;
var periodicRate = (double)interestRate / loanSpecification.NumberOfCompoundPeriodsSimplify;
var qPowN = Math.Pow(1 + periodicRate, loanSpecification.DurationInMonths);
var installment = (decimal)((double)loanSpecification.OriginalPrincipal * qPowN * (periodicRate / (qPowN - 1)));
return(installment);
}
public LoanIndicators GenerateLoanIndicators(LoanSpecification loanSpecification, decimal administrationFee)
{
var ear = CalculateEffectiveAnnualRate(loanSpecification);
var apr = CalculateApr(loanSpecification, administrationFee);
var eapr = CalculateEffectiveApr(loanSpecification, apr);
//effective continuous compound interest
//var continuousEffectiveAnnualRate = Math.Pow(Math.E,
// ((double) (annualInterestRate / 100) * months / 12));
return(new LoanIndicators
{
EffectiveAnnualRate = ear,
AnnualPercentageRate = apr,
EffectiveAnnualPercentageRate = eapr
});
}
private decimal CalculateApr(LoanSpecification loanSpecification, decimal administrationFee)
{
var approximation = loanSpecification.AnnualInterestRate;
var difference = 1m;
var amountToAdd = 0.0001m;
var originalInstallment = _installmentCalculator.CalculateInstallment(loanSpecification);
var borrowedAmount = loanSpecification.OriginalPrincipal - administrationFee;
while (difference != 0)
{
var newOrigin = _installmentCalculator.CalculateInstallment(new LoanSpecification
{
OriginalPrincipal = borrowedAmount,
DurationInMonths = loanSpecification.DurationInMonths,
CompoundFrequency = loanSpecification.CompoundFrequency,
AnnualInterestRate = approximation
});
difference = originalInstallment - newOrigin;
if (difference is <= ErrorMargin and >= -ErrorMargin)
{
break;
}
if (difference > 0)
{
amountToAdd *= 2;
approximation += amountToAdd;
}
else
{
amountToAdd /= 2;
approximation -= amountToAdd;
}
}
return(approximation);
}
public LoanProposalDto ProposeLoan(ProposeLoanDto dto)
{
if (dto.Principal <= 0 || dto.Principal > _loanInputRequirements.CustomerMaximumPrincipal)
{
throw new LoanException("principal amount is incorrect");
}
if (dto.Months <= 0 || dto.Months > _loanInputRequirements.MaximumLoanPeriodInMonths)
{
throw new LoanException("loan period is incorrect");
}
var loanSpecification = new LoanSpecification
{
OriginalPrincipal = dto.Principal,
DurationInMonths = dto.Months,
CompoundFrequency = CompoundFrequency.Monthly,
AnnualInterestRate = _bankInterestProvider.AnnualInterestRate
};
var loan = _loanFactory.CreateLoan(loanSpecification);
return(_loanMapper.Map(loan));
}