code: public function hash(string $algo = 'sha256'): string
      {
          $this->rewind();
  
          $hc = \hash_init($algo);
          \hash_update_stream($hc, $this->handle);
          return \hash_final($hc);
      }
docs: [hash description]
  @param  string $algo [description]
  @return string       [description]