public function diff(float $from, float $to, int $precision = 0, bool $format_string = false)
    {
        if (!$from) {
            ++$from;
            ++$to;
        } // Stop division by `0`.

        $precision = max(0, $precision);
        $percent   = ($to - $from) / $from * 100;

        if ($precision) {
            $percent = (float) number_format($percent, $precision, '.', '');
        } else {
            $percent = (int) $percent;
        }
        if ($format_string) { // e.g., add a prefix/suffix?
            $percent = $percent > 0 ? '+'.$percent.'%' : $percent.'%';
        }
        return $percent;
    }