电子邮件主题，采用不同字符集（例如ISO-2022-JP，GB-2312等）的标头解码

Question

I am working on a project which needs to deal with email encoding/decoding in different charsets. A python code for this can be shown in the below:

from email.header import Header, decode_header, make_header
from charset import text_to_utf8    

class ....
def decode_header(self, header):
    decoded_header = decode_header(header)

    if decoded_header[0][1] is None:
        return text_to_utf8(decoded_header[0][0]).decode("utf-8", "replace")
    else:
        return decoded_header[0][0].decode(decoded_header[0][1].replace("windows-", "cp"), "replace")

Basically, for the text like "=?iso-2022-jp?b?GyRCRW1CQE86GyhCIDxtb21vQHRhcm8ubmUuanA=?="; the "decode_header" function just tries to find the encoding: 'iso-2022-jp'; then it will use the "decode" function to decode the charset to unicode.

Now, in go, i can do something similar to like:

import "mime"

dec := new(mime.WordDecoder)
text := "=?utf-8?q?=C3=89ric?= <[e-mail]>, =?utf-8?q?Ana=C3=AFs?= <[e-mail]>"
header, err := dec.DecodeHeader(text)

Seems that there mime.WordDecoder allow to put a charset decoder "hook": 
type WordDecoder struct {
   // CharsetReader, if non-nil, defines a function to generate
   // charset-conversion readers, converting from the provided
   // charset into UTF-8.
   // Charsets are always lower-case. utf-8, iso-8859-1 and us-ascii charsets
   // are handled by default.
   // One of the the CharsetReader's result values must be non-nil.
   CharsetReader func(charset string, input io.Reader) (io.Reader, error)
}           

I am wondering is there any library which can allow me to convert arbitrary charset like the "decode" function in python as shown in the above example. I don't want to write a big "switch-case"like the one being used in mime/encodedword.go:

func (d *WordDecoder) convert(buf *bytes.Buffer, charset string, content []byte) error {
   switch {
   case strings.EqualFold("utf-8", charset):
      buf.Write(content)
   case strings.EqualFold("iso-8859-1", charset):
      for _, c := range content {
         buf.WriteRune(rune(c))
      }
....

Any help would be very appreciated.

Thanks.

Answer 1

I'm not sure it is what you are looking for but there is golang.org/x/text package which I'm using to convert Windows-1251 to UTF-8. Code looks like

import (
    "golang.org/x/text/encoding/charmap"
    "golang.org/x/text/transform"
    "io/ioutil"
    "strings"
)

func convert(s string) string {
    sr := strings.NewReader(s)
    tr := transform.NewReader(sr, charmap.Windows1251.NewDecoder())
    buf, err := ioutil.ReadAll(tr)
    if err != nil {
        return ""
    }
    return string(buf)
}

I think in your case if you want to avoid "a big 'switch-case'" you can create kind of map with full list of available encodings and just make something like:

var encodings = map[string]transform.Transformer{
    "win-1251": charmap.Windows1251.NewDecoder(),
}

func convert(s, charset string) string {
    buf, err := ioutil.ReadAll(transform.NewReader(strings.NewReader(s), encodings[charset]))
    if err != nil {
        return ""
    }
    return string(buf)
}