public function spaceLeft(int $usedSpace): int
    {
        if (self::UNLIMITED === $this->amount) {
            return $usedSpace;
        }

        if ($usedSpace >= $this->amount) {
            return 0;
        }

        $spaceLeft = $this->amount - $usedSpace;
        if (0 >= $spaceLeft) {
            return 0;
        }

        return $spaceLeft;
    }