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: protocol.go
    path of file: ./repos/gev
    
    the code of the file until where you have to start completion: package gev

import (
	"github.com/Allenxuxu/ringbuffer"
)

var _ Protocol = &DefaultProtocol{}

// Protocol 自定义协议编解码接口
type Protocol interface {
	UnPacket(c *Connection, buf
    
func (d *DefaultProtocol) UnPacket(c *Connection, buffer *ringbuffer.RingBuffer) (interface{}, []byte) {
	s, e := buffer.PeekAll()
	if len(e) > 0 {
		size := len(s) + len(e)
		userBuffer := *c.UserBuffer()
		if size > cap(userBuffer) {
			userBuffer = make([]byte, size)
			*c.UserBuffer() = userBuffer
		}

		copy(userBuffer, s)
		copy(userBuffer[len(s):], e)
		buffer.RetrieveAll()

		return nil, userBuffer[:size]
	} else {
		buffer.RetrieveAll()

		return nil, s
	}
}