class Quota

class Quota {
    
    /**
     * unlimited quota
     */
    const UNLIMITED = -1;
    
    /**
     * quota in bytes
     *
     * A value of -1 is treated as unlimited.
     *
     * @type  int
     */
    private $amount;
    
    /**
     * constructor
     *
     * @param  int  $amount  quota in bytes
     */
    public function __construct($amount) {
        $this->amount = $amount;
    }
    
    /**
     * create with unlimited space
     *
     * @return  Quota
     */
    public static function unlimited() {
        return new self(self::UNLIMITED);
    }
    
    /**
     * checks if a quota is set
     *
     * @return  bool
     */
    public function isLimited() {
        return self::UNLIMITED < $this->amount;
    }
    
    /**
     * checks if given used space exceeda quota limit
     *
     *
     * @param     int   $usedSpace
     * @return    int
     */
    public function spaceLeft($usedSpace) {
        if (self::UNLIMITED === $this->amount) {
            return $usedSpace;
        }
        if ($usedSpace >= $this->amount) {
            return 0;
        }
        $spaceLeft = $this->amount - $usedSpace;
        if (0 >= $spaceLeft) {
            return 0;
        }
        return $spaceLeft;
    }

}