<?php
class Metier_Util_Luhn
{
    private $sumTable = array(
        array(0,1,2,3,4,5,6,7,8,9),
        array(0,2,4,6,8,1,3,5,7,9)
    );

    /**
     * Calculate check digit according to Luhn's algorithm
     * New method (suggested by H.Johnson),
     * see http://www.phpclasses.org/discuss/package/8471/thread/1/
     *
     * @param string $number
     * @return integer
     */
    public function calculate(string $number)
    {
        $length = strlen($number);
        $sum = 0;
        $flip = 1;
        // Sum digits (last one is check digit, which is not in parameter)
        for($i=$length-1;$i>=0;--$i) {
            $sum += $this->sumTable[$flip++ & 0x1][$number[$i]];
        }
        // Multiply by 9
        $sum *= 9;
        // Last digit of sum is check digit
        return (int)substr($sum,-1,1);
    }

    /**
     * Validate number against check digit
     *
     * @param string $number
     * @param integer $digit
     * @return boolean
     */
    public function validate(string $number, int $digit)
    {
        $calculated = $this->calculate($number);
        if($digit == $calculated) {
            return true;
        }
        else {
            return false;
        }
    }
}