code: func (opts *PUBLISHOptions) validate() error {
  	// Check the QoS.
  	if !mqtt.ValidQoS(opts.QoS) {
  		return ErrInvalidQoS
  	}
  
  	// Check the length of the Topic Name.
  	if len(opts.TopicName) > maxStringsLen {
  		return ErrTopicNameExceedsMaxStringsLen
  	}
  
  	// Check if the Topic Name contains the wildcard characters.
  	if bytes.IndexAny(opts.TopicName, wildcards) != -1 {
  		return ErrTopicNameContainsWildcards
  	}
  
  	// Check the length of the Application Message.
  	if len(opts.Message) > maxStringsLen {
  		return ErrMessageExceedsMaxStringsLen
  	}
  
  	// End the validation if the QoS equals to QoS 0.
  	if opts.QoS == mqtt.QoS0 {
  		return nil
  	}
  
  	// Check the Packet Identifier.
  	if opts.PacketID == 0 {
  		return ErrInvalidPacketID
  	}
  
  	return nil
  }
docs: // validate validates the options.