PC World Komputer 1998 November/pcwk_11_98a.iso/Wtestowe/Money99/money99.1/college/logic.js

Text File | 1998-07-25 | 6KB | 193 lines

// This function calculates annual interest rate income on monthly
// compound basis.
function annualDeposit(balance, interest)
{
    var    numMon = 12
    var    retBal = balance
    var    monthlyInt = interest / numMon

    while(numMon > 0) {            // do the monthly compounding
        retBal = retBal * (1 + monthlyInt)
        numMon = numMon - 1
    }
    retBal = retBal - balance    // get net interest rate income
    return retBal
}

// This function calculates annual interest rate income on monthly
// compound and quarterly deposit basis.
function quarterDeposit(balance, interest, amount)
{
    var    retBal = balance
    var    monthlyInt = interest / 12    // monthly interest
    var    i, j

    i = 0
    while(i < 4) {            // quarterly loop
        j = 0
        while(j < 3) {        // monthly loop
            retBal = retBal * (1 + monthlyInt)
            j = j + 1
        }
        i = i + 1
        retBal = retBal + amount    // add the quarterly deposit
    }
    retBal = retBal - (balance + amount * 4)    // net interest rate income
    return retBal
}

// This function calculates annual interest rate income on monthly
// compound and monthly deposit basis.
function monthDeposit(balance, interest, amount)
{
    var retBal = balance
    var    monthlyInt = interest / 12    // monthly interest
    var i

    i = 0
    while(i < 12) {
        retBal = retBal * (1 + monthlyInt)
        retBal = retBal + amount
        i = i + 1
    }
    retBal = retBal - (balance + amount * 12)
    return retBal
}


var sto = parent

var calcType = sto.calcType // Either "contribute" or "spend"

var interestRate = sto.rateOfReturn;
var inflationRate = sto.rateOfCostIncrease;
var taxRate = sto.marginalTaxRate / 100;
var cost = sto.currentCostCollege;
var length = sto.lengthOfEducation;
var yearsUntilCollege = sto.yearsUntilCollege;
var initialBalance = sto.initialBalance;
var depositInterval = sto.depositInterval;
var depositAmount = sto.regularDeposit;
var numKids = sto.numberOfChildren;


if(calcType == "contribute") {
    // Calculate how much the user needs to contribute to his/her college fund to
    // meet the cost of the education at the specified time in the future

    var monthlyInt = interestRate / (12 * 100);
    var    inflation = 1 + inflationRate / 100;

    // calculate total college year cost including inflation
    b0 = 0;
    for(j=0; j<length; j++)
        b0 += Math.pow(inflation, j);
    cost = cost * b0;

    amt = numKids * cost * Math.pow(inflation, yearsUntilCollege);

    if (interestRate != 0) {
        // if no marginal tax, use standard capital gains tax.
        if(sto.bTaxedAnnually == "no") {
            if(sto.bCapitalGains == "yes")
                taxRate = 0.28;
            else
                taxRate = 0;
        }

        // the goal is to save enough until the end of the last college year.
        var b0 = 1 + monthlyInt;
        b0 = (1 - taxRate) * Math.pow(b0, 12) + taxRate;
        bn = b0;
        bn =(1 - Math.pow(b0, yearsUntilCollege)) / (1 - b0)

        // calculate annual deposit amount
        dep = (amt - initialBalance * Math.pow(b0, yearsUntilCollege)) / bn;
    }
    else {
        taxRate = 0;

        dep = (amt - initialBalance) / yearsUntilCollege;
    }
    // calculate total interest earned and tax paid
    prevBal = initialBalance;
    totalDeposits = dep * yearsUntilCollege;
    totalReturn = 0;
    totalTaxes = 0;
    for(j=0; j<yearsUntilCollege; j++) {
        yearlyRet = prevBal * (Math.pow(1+monthlyInt, 12) - 1);
        totalReturn += yearlyRet;
        totalTaxes += yearlyRet * taxRate;
        prevBal += dep + yearlyRet * (1 - taxRate);
    }
    totalCostOfCollege = amt;
    annualPayment = dep;
}
else {
    // Calculate how much education the user will be able to afford for his/her
    // children based on current savings patterns

    var n=0;
    var tmpRate = interestRate / 100.0;
    var totalDeposits = 0;

    // if no marginal tax, use standard capital gains tax.
    if(sto.bTaxedAnnually == "no") {
        if(sto.bCapitalGains == "yes")
            taxRate = 0.28;
        else
            taxRate = 0;
    }

    if(depositInterval == "annually")
        totalDeposits = depositAmount * yearsUntilCollege;
    else
    if(depositInterval == "quarterly")
        totalDeposits = depositAmount * yearsUntilCollege*4;
    else // depositInterval == "monthly"
        totalDeposits = depositAmount * yearsUntilCollege*12;

    var totalReturn = 0.0;
    var totalTaxes = 0.0;
    var totalCostOfCollege = initialBalance;

    for(i=0; i<yearsUntilCollege; i++) {
        if(depositInterval == "annually") {
            yearlyRet = annualDeposit(totalCostOfCollege, tmpRate)
            tmpAnnualPayment = depositAmount;
        }
        else if(depositInterval == "quarterly") {
            yearlyRet = quarterDeposit(totalCostOfCollege, tmpRate, depositAmount);
            tmpAnnualPayment = depositAmount * 4;
        }
        else if(depositInterval == "monthly") {
            yearlyRet = monthDeposit(totalCostOfCollege, tmpRate, depositAmount);
            tmpAnnualPayment = depositAmount * 12;
        }

        totalReturn += yearlyRet;
        totalTaxes += yearlyRet * taxRate;
        totalCostOfCollege +=  yearlyRet * (1 - taxRate);

        totalCostOfCollege += tmpAnnualPayment;
    }

    var tmpInflation = 1.0 + (inflationRate/100.0);
    ftotal = tmpInflation;
    for(n=1; n < yearsUntilCollege; n++)
        ftotal *= tmpInflation;
    var cvalue = (totalCostOfCollege / ftotal) / length / numKids;
    var ctemp = totalCostOfCollege / length / numKids;

    var privateYears = 4 * cvalue / privateCost;
    var publicYears = 4 * cvalue / publicCost;
    var communityYears = 4 * cvalue / communityCost;

    var cost = Math.round(cvalue);
    totalCostOfCollege = Math.round(totalCostOfCollege);
    totalDeposits = Math.round(totalDeposits);
    totalReturn = Math.round(totalReturn);
    totalTaxes = Math.round(totalTaxes);

}