• 计算公式
  调用

double hhh = [self calculatePrincipalAndInterestBidWithInvestInterest:20 amount:@"2000"];

- (double)calculatePrincipalAndInterestBidWithInvestInterest:(double)interest
                                                      amount:(NSString *)investAmount{
    
    NSDecimalNumberHandler *roundBanker = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundBankers
                                                                                                 scale:2.0f
                                                                                      raiseOnExactness:NO
                                                                                       raiseOnOverflow:NO
                                                                                      raiseOnUnderflow:NO
                                                                                   raiseOnDivideByZero:YES];
    NSDecimalNumber *investAmountDec = [[NSDecimalNumber alloc] initWithString:investAmount];//投资金额
    NSDecimalNumber *monthInterestDec = [[NSDecimalNumber decimalNumberWithString:[NSString stringWithFormat:@"%f",interest/100]] decimalNumberByDividingBy:[NSDecimalNumber decimalNumberWithString:[NSString stringWithFormat:@"%f",12.00]]];//月利率
    
    double month = 12;//标的募集期数
//    if (selectBid.creditType == CreditType_DaylyOneOffPayment) {
//        //按天标,转化为月份
//        month = month/30.00;
//    }
    NSDecimalNumber *monthDec = [NSDecimalNumber decimalNumberWithString:[NSString stringWithFormat:@"%f",month]];//还款月数
    //    *  公式： 每月本息还款额=(投资金额*月利率*(1+月利率)^还款月数)/(((1+月利率)^还款月数)-1)
    NSDecimalNumber *decNum1 = [[[[[NSDecimalNumber decimalNumberWithString:[NSString stringWithFormat:@"%d",1]] decimalNumberByAdding:monthInterestDec] decimalNumberByRaisingToPower:month] decimalNumberByMultiplyingBy:monthInterestDec] decimalNumberByMultiplyingBy:investAmountDec];
    NSDecimalNumber *decNum2 = [[[[NSDecimalNumber decimalNumberWithString:[NSString stringWithFormat:@"%d",1]] decimalNumberByAdding:monthInterestDec] decimalNumberByRaisingToPower:month] decimalNumberBySubtracting:[NSDecimalNumber decimalNumberWithString:[NSString stringWithFormat:@"%d",1]]];
    NSDecimalNumber *monthIncomeDec = [decNum1 decimalNumberByDividingBy:decNum2];//月本息还款额
    NSDecimalNumber *totalIncomeDec = [[monthDec decimalNumberByMultiplyingBy:monthIncomeDec] decimalNumberBySubtracting:investAmountDec withBehavior:roundBanker];//总还款额 - 本金 = 总收益
    return [totalIncomeDec doubleValue] > 0 ? [totalIncomeDec doubleValue] : 0;
}