/**
* Returns the interest and principal payment for a given period for a cash flow with constant
* periodic payments (annuities) and interest rate
*
* @param float Interest rate per period
* @param int Number of periods
* @param float Present Value
* @param float Future Value
* @param int Payment type:
FINANCE_PAY_END (default): at the end of each period
FINANCE_PAY_BEGIN: at the beginning of each period
* @return array
* @static
* @access private
*/
function _interestAndPrincipal($rate, $per, $nper, $pv, $fv, $type)
{
$pmt = Math_Finance::payment($rate, $nper, $pv, $fv, $type);
//echo "pmt: $pmt\n\n";
$capital = $pv;
for ($i = 1; $i <= $per; $i++) {
// in first period of advanced payments no interests are paid
$interest = $type && $i == 1 ? 0 : -$capital * $rate;
$principal = $pmt - $interest;
$capital += $principal;
//echo "$i\t$capital\t$interest\t$principal\n";
}
return array($interest, $principal);
}
function testpayment()
{
// various random calculations
$this->assertTrue(abs(-36.157534 - Math_Finance::payment(0.08, 20, 355)) < FINANCE_PRECISION);
$this->assertTrue(abs(-180.797585 - Math_Finance::payment(0.03, 5, 828)) < FINANCE_PRECISION);
$this->assertTrue(abs(-166.305561 - Math_Finance::payment(0.29, 7, 477)) < FINANCE_PRECISION);
$this->assertTrue(abs(-62.142857 - Math_Finance::payment(0, 7, 435)) < FINANCE_PRECISION);
$this->assertTrue(abs(-258.137498 - Math_Finance::payment(0.1 / 12, 3 * 12, 8000, 0, FINANCE_PAY_END)) < FINANCE_PRECISION);
$this->assertTrue(abs(-256.00413 - Math_Finance::payment(0.1 / 12, 3 * 12, 8000, 0, FINANCE_PAY_BEGIN)) < FINANCE_PRECISION);
// cannot pass negative number of periods
$this->assertType('object', Math_Finance::payment(0.29, -7, 435));
// cannot pass a type differenet from 0 and 1
$this->assertType('object', Math_Finance::payment(0.29, 7, 435, 0, 3));
}
