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: userid.go
    path of file: ./repos/teller/vendor/github.com/ProtonMail/go-crypto/openpgp/packet
    
    the code of the file until where you have to start completion: // Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package packet

import (
	"io"
	"io/ioutil"
	"strings"
)

// UserId contains text that is intended to represent the name and email
// address of the key holder. See RFC 4880, section 5.11. By convention, this
// takes the form "Full Name (Comment) <email@example.com>"
type UserId struct {
	Id string // By convention, this takes the form "Full Name (Comment) <email@example.com>" which is split out in the fields below.

	Name, Comment, Email string
}

func hasInvalidCharacters(s string) bool {
	for _, c := range s {
		switch c {
		case '(', ')', '<', '>', 0:
			return true
		}
	}
	return false
}

// NewUserId returns a UserId or nil if any of the arguments contain invalid
// charac
    
func parseUserId(id string) (name, comment, email string) {
	var n, c, e struct {
		start, end int
	}
	var state int

	for offset, rune := range id {
		switch state {
		case 0:
			// Entering name
			n.start = offset
			state = 1
			fallthrough
		case 1:
			// In name
			if rune == '(' {
				state = 2
				n.end = offset
			} else if rune == '<' {
				state = 5
				n.end = offset
			}
		case 2:
			// Entering comment
			c.start = offset
			state = 3
			fallthrough
		case 3:
			// In comment
			if rune == ')' {
				state = 4
				c.end = offset
			}
		case 4:
			// Between comment and email
			if rune == '<' {
				state = 5
			}
		case 5:
			// Entering email
			e.start = offset
			state = 6
			fallthrough
		case 6:
			// In email
			if rune == '>' {
				state = 7
				e.end = offset
			}
		default:
			// After email
		}
	}
	switch state {
	case 1:
		// ended in the name
		n.end = len(id)
	case 3:
		// ended in comment
		c.end = len(id)
	case 6:
		// ended in email
		e.end = len(id)
	}

	name = strings.TrimSpace(id[n.start:n.end])
	comment = strings.TrimSpace(id[c.start:c.end])
	email = strings.TrimSpace(id[e.start:e.end])

	// RFC 2822 3.4: alternate simple form of a mailbox
	if email == "" && strings.ContainsRune(name, '@') {
		email = name
		name = ""
	}

	return
}