You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: publish.go
    path of file: ./repos/kubeedge/vendor/github.com/256dpi/gomqtt/packet
    
    the code of the file until where you have to start completion: package packet

import (
	"encoding/binary"
	"fmt"
)

// A Publish packet is sent from a client to a server or from server to a client
// to transport an application message.
type Publish struct {
	// The message to publish.
	Message Message

	// If the Dup flag is set to false, it indicates that this is the first
	// occasion that the client or server has attempted to send this
	// Publish packet. If the dup flag is set to true, it indicates that this
	// might be re-delivery of an earlier attempt to send the packet.
	Dup bool

	// The packet identifier.
	ID ID
}

// NewPublish creates a new Publish packet.
func NewPublish() *Publish {
	return &Publish{}
}

// Type returns the packets type.
func (pp *Publish) Type() Type {
	return PUBLISH
}

// String returns a string representation of the packet.
func (pp *Publish) String() string {
	return fmt.Sprintf("<Publish ID=%d Message=%s Dup=%t>",
		pp.ID, pp.Message.String(), pp.Dup)
}

// Len returns the byte length of the encoded packet.
func (pp *Publish) Len() int {
	ml :
    
func (pp *Publish) Decode(src []byte) (int, error) {
	total := 0

	// decode header
	hl, flags, rl, err := headerDecode(src[total:], PUBLISH)
	total += hl
	if err != nil {
		return total, err
	}

	// read flags
	pp.Dup = ((flags >> 3) & 0x1) == 1
	pp.Message.Retain = (flags & 0x1) == 1
	pp.Message.QOS = QOS((flags >> 1) & 0x3)

	// check qos
	if !pp.Message.QOS.Successful() {
		return total, makeError(pp.Type(), "invalid QOS level (%d)", pp.Message.QOS)
	}

	// check buffer length
	if len(src) < total+2 {
		return total, makeError(pp.Type(), "insufficient buffer size, expected %d, got %d", total+2, len(src))
	}

	n := 0

	// read topic
	pp.Message.Topic, n, err = readLPString(src[total:], pp.Type())
	total += n
	if err != nil {
		return total, err
	}

	if pp.Message.QOS != 0 {
		// check buffer length
		if len(src) < total+2 {
			return total, makeError(pp.Type(), "insufficient buffer size, expected %d, got %d", total+2, len(src))
		}

		// read packet id
		pp.ID = ID(binary.BigEndian.Uint16(src[total:]))
		total += 2

		// check packet id
		if !pp.ID.Valid() {
			return total, makeError(pp.Type(), "packet id must be grater than zero")
		}
	}

	// calculate payload length
	l := int(rl) - (total - hl)

	// read payload
	if l > 0 {
		pp.Message.Payload = make([]byte, l)
		copy(pp.Message.Payload, src[total:total+l])
		total += len(pp.Message.Payload)
	}

	return total, nil
}