/*

*  Finance Calculator with Adjustable APR

*

* Author: Marlon King <marlon.king@gforces.co.uk>

*

* Notes: For use without APR being adjustable, pass in a fixed interest rate

*/







function updateResultsFromApr( price, rate, adjustableApr ){

    //reset values before starting

    var totalPrice          = price; //passed in from php page

    var deposit             = 0;     //selectable

    var loanPeriod          = 0;     //selectable in years

    var monthlyPayments     = 0;

    var creditAmount        = 0;

    var interestPayable     = 0;

    var totalAmountPayable  = 0;

    var apr                 = 0;

    var totalNumberOfMonths = 0;

    var interestRate        = rate;  //ie 12.2





    //retrieve inputed values



    deposit             = parseInt( $('input_deposit').value );

    loanPeriod          = parseInt( $('input_loanPeriod').value );

    apr_selected        = parseFloat( $('input_apr').value )/100;



    if ( deposit > 0 && loanPeriod > 0 ) {



        //calculate all

        creditAmount        = (totalPrice - deposit);

        creditAmountIni     = (totalPrice - deposit); //without 165 added

        totalNumberOfMonths = loanPeriod * 12;

        interestRate        = interestRate / 100;

        monthlyPayments     = creditAmount / totalNumberOfMonths;





        //creditAmount = creditAmount + 165;



        interestPayable      = ( creditAmount * interestRate) * loanPeriod ;

        totalAmountPayable   = ( creditAmount + interestPayable );

        monthlyPayments      = ( totalAmountPayable / totalNumberOfMonths );



        apr = calculateAPR( creditAmountIni, totalNumberOfMonths, monthlyPayments );



        interestPayable    = ((monthlyPayments * totalNumberOfMonths)) - creditAmountIni;

        totalAmountPayable = ((monthlyPayments * totalNumberOfMonths) + (deposit));







        //display results

        //$('totalPrice').innerHTML         = '<strong>£' + addCommas(Math.round(totalPrice * 100 ) / 100 ) + '</strong>';

        $('creditAmount').innerHTML       = '£' + addCommas(Math.round(creditAmountIni * 100 ) / 100 );

        $('interestPayable').innerHTML    = '£' + addCommas(Math.round( interestPayable  * 100 ) / 100 );

        $('monthlyPayments').innerHTML    = '£' + addCommas(Math.round( monthlyPayments * 100 ) / 100 );

        $('totalAmountPayable').innerHTML = '£' + addCommas(Math.round( totalAmountPayable * 100 ) / 100);



        if ( adjustableApr == true ){



            apr = reverseCalculateAPR( creditAmountIni, totalNumberOfMonths, totalAmountPayable, apr_selected, deposit );



        }else{



            $('apr').innerHTML                =  Math.round( parseFloat( apr ) * 10 ) / 10 + '%';

        }



    }else if ( deposit >= 0 ){



        creditAmount        = (totalPrice - deposit);

        $('creditAmount').innerHTML       = '£' + addCommas(Math.round( creditAmount * 100 ) / 100);



    }

}





function calculateLoanFromAPR( apr, totalNumberOfMonths, monthlyPayments ) {



    var loan = 0;



    for( time = 1; time <= totalNumberOfMonths; time++ ) {



        payment = monthlyPayments;



        //payment += ( time == totalNumberOfMonths ) ? 45 : 0;//final sum added on to final payment - REMOVE AS NECESSARY



        interest = ( payment / ( Math.pow( 1 + apr, time / 12 ) ) );



        loan +=interest;



        //alert('debug ' + time + ' - loan: ' + loan + ', payment: ' + payment + ', period: ' + time  + ' PV: ' + interest );

    }



    return loan;



}







function calculateAPR( creditAmount, totalNumberOfMonths, monthlyPayments ) {



    //alert( creditAmount + ' - ' + totalNumberOfMonths + ' - ' + monthlyPayments );



    minApr = 0;

    minAprCalculatedLoan = calculateLoanFromAPR( minApr, totalNumberOfMonths, monthlyPayments );

    maxApr = 1;

    maxAprCalculatedLoan = calculateLoanFromAPR( maxApr, totalNumberOfMonths, monthlyPayments );



    time_to_live = 200;



    target_amount = creditAmount;



    do {

        midApr = ( ( maxApr - minApr ) / 2 ) + minApr;

        midAprCalculatedLoan = calculateLoanFromAPR( midApr, totalNumberOfMonths, monthlyPayments );



        if( target_amount < midAprCalculatedLoan ) {



            minApr = midApr;



        } else if( target_amount > midAprCalculatedLoan ) {



            maxApr = midApr;

        }



        difference = Math.abs(midAprCalculatedLoan - target_amount);



        //alert('debug - apr: ' + midApr + ', ' + midAprCalculatedLoan + ' < ' + target_amount + ', loan difference: ' + difference + ', loan: ' + creditAmount );



    } while ( (midAprCalculatedLoan < target_amount || difference > 0.001) && time_to_live-- > 0 );



    if( time_to_live < 0 ) {



        return "NAN";



    } else {



        return midApr * 100;



    }



}



//change the month payment

function reverseCalculateAPR( creditAmount, totalNumberOfMonths, monthlyPayments, newApr, deposit ) {



    var loan   = 0;

    //var adjust = 0;



    payment = creditAmount/totalNumberOfMonths;



    for( time = 1; time <= totalNumberOfMonths; time++ ) {



        interest = ( payment * ( Math.pow( 1 + newApr, time / 12 ) ) );





        loan +=interest;

        //alert ( "interest: " +  interest );

        //adjust += ( time / 12 );

    }



    //alert ( 'Adjust: ' + adjust );



    //alert ('loan2: ' + loan);

    //alert ('monthly payment' + ( loan / (totalNumberOfMonths )) );





    monthlyPayments        = ( loan / ( totalNumberOfMonths ) );

    loan                   = monthlyPayments * totalNumberOfMonths;

    var newInterest        = loan - creditAmount;

    var totalAmountPayable = ( loan + deposit);



    //display results

    $('interestPayable').innerHTML    = '£' + addCommas(Math.round( newInterest  * 100 ) / 100 );

    $('monthlyPayments').innerHTML    = '£' + addCommas(Math.round( monthlyPayments * 100 ) / 100 );

    $('totalAmountPayable').innerHTML = '£' + addCommas(Math.round( totalAmountPayable * 100 ) / 100);

    // $('apr').innerHTML                =  Math.round( parseFloat( (newApr*100)) * 10 ) / 10 + '%';





}









function addCommas(nStr)

{

    nStr += '';

    x = nStr.split('.');

    x1 = x[0];

    if ( x.length > 1 ){

        theDec = x[1];

        theDec = ( theDec.length == 1 )? x[1] + '0' : theDec;

        x2 = '.' + theDec;

    }else{

        x2 = '';

    }

    var rgx = /(\d+)(\d{3})/;

    while (rgx.test(x1)) {

        x1 = x1.replace(rgx, '$1' + ',' + '$2');

    }

    return x1 + x2;

}