mortage.cpp

#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <iostream>

using namespace std;

enum MonthDay {
    JANUARY = 31,
    FEBRUARY = 28,
    MARCH = 31,
    APRIL = 30,
    MAY = 31,
    JUNE = 30,
    JULY = 31,
    AUGUST = 31,
    SEPTEMBER = 30,
    OCTOBER = 31,
    NOVEMBER = 30,
    DECEMBER = 31
};

static MonthDay monthDay[12] = {
    JANUARY,
    FEBRUARY,
    MARCH,
    APRIL,
    MAY,
    JUNE,
    JULY,
    AUGUST,
    SEPTEMBER,
    OCTOBER,
    NOVEMBER,
    DECEMBER
};

void calDebt(double paidAmount, double borrowAmount, int year,
                 double annualInterestRate, double cashRebateRate,
                 double mortageLinkAmount, double mortageLinkRate) {
    double rebate = borrowAmount * cashRebateRate / 100;

    double curBorrowAmount = borrowAmount;

    double totalMonths = year * 12;

    double effectiveInterestPerMonth = annualInterestRate / 12 / 100;

    double monthlyPayAmount =   borrowAmount
                              * effectiveInterestPerMonth 
                              * pow((1+effectiveInterestPerMonth), totalMonths)
                              / (pow((1+effectiveInterestPerMonth), totalMonths) - 1);


    int monthDayIndex = 11;

    double totalDebt = 0;
    double totalMortageLinkInterest = 0;
    double totalInterest = 0 ;

    for (int i=0; i<totalMonths; ++i) {
        double curMortageLinkInterest =   min(mortageLinkAmount, curBorrowAmount/2) * mortageLinkRate / 100
                                        / 365
                                        * monthDay[monthDayIndex];
        totalMortageLinkInterest += curMortageLinkInterest;

        double curMonthlyInterest =    curBorrowAmount * annualInterestRate / 100 
                                    / 365
                                    * monthDay[monthDayIndex];
        totalInterest += curMonthlyInterest;

        double curMonthlyPrinciple = monthlyPayAmount - curMonthlyInterest;

        if (curMonthlyPrinciple < 0) {
            curMonthlyPrinciple = 0;
        }

        curBorrowAmount = curBorrowAmount - curMonthlyPrinciple; 

        if (curBorrowAmount < 0) {
            curBorrowAmount = 0;
        }

        totalDebt += curMonthlyInterest + curMonthlyPrinciple;

        printf("#%3d %2d interest: %10.3f principal: %10.3f remaining principle: %10.3f mortage interest: %10.3f\n",
                i+1, monthDayIndex, curMonthlyInterest, curMonthlyPrinciple, curBorrowAmount,
                curMortageLinkInterest);

        monthDayIndex = (monthDayIndex + 1 ) % 12;
    }

    printf("\nmonthlyPayAmount: %10.3f\n", monthlyPayAmount);
    printf("\nlost/fixed principal: %10.3f\n", (paidAmount + mortageLinkAmount));
    printf("paid amount: %10.3f\n", paidAmount);
    printf("borrow amount: %10.3f\n", borrowAmount);
    printf("total debt: %10.3f\n", totalDebt);
    printf("cash rebate: %10.3f\n", rebate);
    printf("total interest: %10.3f\n", totalInterest);
    printf("totalMortageLinkInterest: %10.3f\n", totalMortageLinkInterest);
    printf("total debt  - cash rebate: %10.3f\n", (totalDebt - rebate));
    printf("total debt  - cash rebate - totalMortageLinkInterest: %10.3f\n",
            (totalDebt - rebate - totalMortageLinkInterest));
    printf("effective interest: %10.3f\n", (totalInterest - totalMortageLinkInterest));
    printf("effective interest/borrow amount: %10.3f\n", (totalInterest - totalMortageLinkInterest) / borrowAmount);
}

int main(int argc, char** argv) {
    if (argc != 8) {
        printf("\
 usage: %s flatPrice \
 borrowPercentage \
 years \
 borrowRate \
 cashRebateRate \
 mortageLinkAmount \
 mortageLinkRate\n",
        argv[0]);
        exit(1);
    }

    double flatPrice = atof(argv[1]);
    double borrowPercentage= atof(argv[2]);
    double year = atoi(argv[3]);
    double borrowRate = atof(argv[4]);
    double cashRebate = atof(argv[5]);
    double mortageLinkAmount = atof(argv[6]);
    double mortageLinkRate = atof(argv[7]);

    calDebt(flatPrice * (1-borrowPercentage/100), 
              flatPrice * (borrowPercentage/100), 
              year, borrowRate, 
              cashRebate,
              mortageLinkAmount,
              mortageLinkRate);
}