src/internal/mime/header.go

// Copyright 2015 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 mime

import (
	"bytes"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"strconv"
	"strings"
	"unicode"
)

// EncodeWord encodes a string into an RFC 2047 encoded-word.
func EncodeWord(s string) string {
	// UTF-8 "Q" encoding
	b := bytes.NewBufferString("=?utf-8?q?")
	for i := 0; i < len(s); i++ {
		switch c := s[i]; {
		case c == ' ':
			b.WriteByte('_')
		case isVchar(c) && c != '=' && c != '?' && c != '_':
			b.WriteByte(c)
		default:
			fmt.Fprintf(b, "=%02X", c)
		}
	}
	b.WriteString("?=")
	return b.String()
}

// DecodeWord decodes an RFC 2047 encoded-word.
func DecodeWord(s string) (string, error) {
	fields := strings.Split(s, "?")
	if len(fields) != 5 || fields[0] != "=" || fields[4] != "=" {
		return "", errors.New("address not RFC 2047 encoded")
	}
	charset, enc := strings.ToLower(fields[1]), strings.ToLower(fields[2])
	if charset != "us-ascii" && charset != "iso-8859-1" && charset != "utf-8" {
		return "", fmt.Errorf("charset not supported: %q", charset)
	}

	in := bytes.NewBufferString(fields[3])
	var r io.Reader
	switch enc {
	case "b":
		r = base64.NewDecoder(base64.StdEncoding, in)
	case "q":
		r = qDecoder{r: in}
	default:
		return "", fmt.Errorf("RFC 2047 encoding not supported: %q", enc)
	}

	dec, err := ioutil.ReadAll(r)
	if err != nil {
		return "", err
	}

	switch charset {
	case "us-ascii":
		b := new(bytes.Buffer)
		for _, c := range dec {
			if c >= 0x80 {
				b.WriteRune(unicode.ReplacementChar)
			} else {
				b.WriteRune(rune(c))
			}
		}
		return b.String(), nil
	case "iso-8859-1":
		b := new(bytes.Buffer)
		for _, c := range dec {
			b.WriteRune(rune(c))
		}
		return b.String(), nil
	case "utf-8":
		return string(dec), nil
	}
	panic("unreachable")
}

type qDecoder struct {
	r       io.Reader
	scratch [2]byte
}

func (qd qDecoder) Read(p []byte) (n int, err error) {
	// This method writes at most one byte into p.
	if len(p) == 0 {
		return 0, nil
	}
	if _, err := qd.r.Read(qd.scratch[:1]); err != nil {
		return 0, err
	}
	switch c := qd.scratch[0]; {
	case c == '=':
		if _, err := io.ReadFull(qd.r, qd.scratch[:2]); err != nil {
			return 0, err
		}
		x, err := strconv.ParseInt(string(qd.scratch[:2]), 16, 64)
		if err != nil {
			return 0, fmt.Errorf("mime: invalid RFC 2047 encoding: %q", qd.scratch[:2])
		}
		p[0] = byte(x)
	case c == '_':
		p[0] = ' '
	default:
		p[0] = c
	}
	return 1, nil
}

// isVchar returns true if c is an RFC 5322 VCHAR character.
func isVchar(c byte) bool {
	// Visible (printing) characters.
	return '!' <= c && c <= '~'
}