public function similarity(&$A, &$B)
    {
        $alpha = $this->alpha;
        $beta = $this->beta;

        $a = array_fill_keys($A,1);
        $b = array_fill_keys($B,1);

        $min = min(count(array_diff_key($a,$b)),count(array_diff_key($b, $a)));
        $max = max(count(array_diff_key($a,$b)),count(array_diff_key($b, $a)));

        $intersect = count(array_intersect_key($a,$b));

        return $intersect/($intersect + ($beta * ($alpha * $min + $max*(1-$alpha)) ));
    }