private function find_string($string, $start = -1, $end = -1) {
        if (($start == -1) or ($end == -1)) {
            // find_string is called with only one parameter, set start end end
            $start = 0;
            $end = $this->total;
        }
        if (abs($start - $end) <= 1) {
            // We're done, now we either found the string, or it doesn't exist
            $txt = $this->get_original_string($start);
            if ($string == $txt)
                return $start;
            else
                return -1;
        }
        else if ($start > $end) {
            // start > end -> turn around and start over
            return $this->find_string($string, $end, $start);
        }
        else {
            // Divide table in two parts
            $half = (int)(($start + $end) / 2);
            $cmp = strcmp($string, $this->get_original_string($half));
            if ($cmp == 0)
                // string is exactly in the middle => return it
                return $half;
            else if ($cmp < 0)
                // The string is in the upper half
                return $this->find_string($string, $start, $half);
            else
                // The string is in the lower half
                return $this->find_string($string, $half, $end);
        }
    }