1package message

2

3import (

4 "fmt"

5 "strings"

6)

7

8// HeaderWriter helps create headers, folding to the next line when it would

9// become too large. Useful for creating Received and DKIM-Signature headers.

10type HeaderWriter struct {

11 b *strings.Builder

12 lineLen int

13 nonfirst bool

14}

15

16// Addf formats the string and calls Add.

17func (w *HeaderWriter) Addf(separator string, format string, args ...any) {

18 w.Add(separator, fmt.Sprintf(format, args...))

19}

20

21// Add adds texts, each separated by separator. Individual elements in text are

22// not wrapped.

23func (w *HeaderWriter) Add(separator string, texts ...string) {

24 if w.b == nil {

25 w.b = &strings.Builder{}

26 }

27 for _, text := range texts {

28 n := len(text)

29 if w.nonfirst && w.lineLen > 1 && w.lineLen+len(separator)+n > 78 {

30 w.b.WriteString("\r\n\t")

31 w.lineLen = 1

32 } else if w.nonfirst && separator != "" {

33 w.b.WriteString(separator)

34 w.lineLen += len(separator)

35 }

36 w.b.WriteString(text)

37 w.lineLen += len(text)

38 w.nonfirst = true

39 }

40}

41

42// AddWrap adds data, folding anywhere in the buffer. E.g. for base64 data.

43func (w *HeaderWriter) AddWrap(buf []byte) {

44 for len(buf) > 0 {

45 line := buf

46 n := 78 - w.lineLen

47 if len(buf) > n {

48 line, buf = buf[:n], buf[n:]

49 } else {

50 buf = nil

51 n = len(buf)

52 }

53 w.b.Write(line)

54 w.lineLen += n

55 if len(buf) > 0 {

56 w.b.WriteString("\r\n\t")

57 w.lineLen = 1

58 }

59 }

60}

61

62// Newline starts a new line.

63func (w *HeaderWriter) Newline() {

64 w.b.WriteString("\r\n\t")

65 w.lineLen = 1

66 w.nonfirst = true

67}

68

69// String returns the header in string form, ending with \r\n.

70func (w *HeaderWriter) String() string {

71 return w.b.String() + "\r\n"

72}

73