class CheckSum
  {
  public:
    /**
     * Creates a checksum for algorithm \param type
     * \throws if the checksum is invalid and can't be constructed
     */
    CheckSum( const std::string & type, const std::string & checksum);
    CheckSum( const std::string & type, std::istream & input_r );
    CheckSum();

  public:
    static const std::string & md5Type();
    static const std::string & shaType();
    static const std::string & sha1Type();
    static const std::string & sha256Type();

    static CheckSum md5( const std::string & checksum )
    { return  CheckSum( md5Type(), checksum); }
    static CheckSum sha( const std::string & checksum )
    { return  CheckSum( shaType(), checksum); }
    static CheckSum sha1( const std::string & checksum )
    { return  CheckSum( sha1Type(), checksum); }
    static CheckSum sha256( const std::string & checksum )
    { return  CheckSum( sha256Type(), checksum); }

    static CheckSum md5( std::istream & input_r )
    { return  CheckSum( md5Type(), input_r ); }
    static CheckSum sha( std::istream & input_r )
    { return  CheckSum( sha1Type(), input_r ); }
    static CheckSum sha1( std::istream & input_r )
    { return  CheckSum( sha1Type(), input_r ); }
    static CheckSum sha256( std::istream & input_r )
    { return  CheckSum( sha256Type(), input_r ); }

  public:
    std::string type() const;
    std::string checksum() const;
    bool empty() const;

  private:
    std::string _type;
    std::string _checksum;
  };