// Copyright 2013 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 charset provides common text encodings for HTML documents.
//
// The mapping from encoding labels to encodings is defined at
// https://encoding.spec.whatwg.org/.
package charset // import "golang.org/x/net/html/charset"
import (
"bytes"
"fmt"
"io"
"mime"
"strings"
"unicode/utf8"
"golang.org/x/net/html"
"golang.org/x/text/encoding"
"golang.org/x/text/encoding/charmap"
"golang.org/x/text/encoding/htmlindex"
"golang.org/x/text/transform"
)
// Lookup returns the encoding with the specified label, and its canonical
// name. It returns nil and the empty string if label is not one of the
// standard encodings for HTML. Matching is case-insensitive and ignores
// leading and trailing whitespace. Encoders will use HTML escape sequences for
// runes that are not supported by the character set.
func Lookup(label string) (e encoding.Encoding, name string) {
e, err := htmlindex.Get(label)
if err != nil {
return nil, ""
}
name, _ = htmlindex.Name(e)
return &htmlEncoding{e}, name
}
type htmlEncoding struct{ encoding.Encoding }
func (h *htmlEncoding) NewEncoder() *encoding.Encoder {
// HTML requires a non-terminating legacy encoder. We use HTML escapes to
// substitute unsupported code points.
return encoding.HTMLEscapeUnsupported(h.Encoding.NewEncoder())
}
// DetermineEncoding determines the encoding of an HTML document by examining
// up to the first 1024 bytes of content and the declared Content-Type.
//
// See http://www.whatwg.org/specs/web-apps/current-work/multipage/parsing.html#determining-the-character-encoding
func DetermineEncoding(content []byte, contentType string) (e encoding.Encoding, name string, certain bool) {
if len(content) > 1024 {
content = content[:1024]
}
for _, b := range boms {
if bytes.HasPrefix(content, b.bom) {
e, name = Lookup(b.enc)
return e, name, true
}
}
if _, params, err := mime.ParseMediaType(contentType); err == nil {
if cs, ok := params["charset"]; ok {
if e, name = Lookup(cs); e != nil {
return e, name, true
}
}
}
if len(content) > 0 {
e, name = prescan(content)
if e != nil {
return e, name, false
}
}
// Try to detect UTF-8.
// First eliminate any partial rune at the end.
for i := len(content) - 1; i >= 0 && i > len(content)-4; i-- {
b := content[i]
if b < 0x80 {
break
}
if utf8.RuneStart(b) {
content = content[:i]
break
}
}
hasHighBit := false
for _, c := range content {
if c >= 0x80 {
hasHighBit = true
break
}
}
if hasHighBit && utf8.Valid(content) {
return encoding.Nop, "utf-8", false
}
// TODO: change default depending on user's locale?
return charmap.Windows1252, "windows-1252", false
}
// NewReader returns an io.Reader that converts the content of r to UTF-8.
// It calls DetermineEncoding to find out what r's encoding is.
func NewReader(r io.Reader, contentType string) (io.Reader, error) {
preview := make([]byte, 1024)
n, err := io.ReadFull(r, preview)
switch {
case err == io.ErrUnexpectedEOF:
preview = preview[:n]
r = bytes.NewReader(preview)
case err != nil:
return nil, err
default:
r = io.MultiReader(bytes.NewReader(preview), r)
}
if e, _, _ := DetermineEncoding(preview, contentType); e != encoding.Nop {
r = transform.NewReader(r, e.NewDecoder())
}
return r, nil
}
// NewReaderLabel returns a reader that converts from the specified charset to
// UTF-8. It uses Lookup to find the encoding that corresponds to label, and
// returns an error if Lookup returns nil. It is suitable for use as
// encoding/xml.Decoder's CharsetReader function.
func NewReaderLabel(label string, input io.Reader) (io.Reader, error) {
e, _ := Lookup(label)
if e == nil {
return nil, fmt.Errorf("unsupported charset: %q", label)
}
return transform.NewReader(input, e.NewDecoder()), nil
}
func prescan(content []byte) (e encoding.Encoding, name string) {
z := html.NewTokenizer(bytes.NewReader(content))
for {
switch z.Next() {
case html.ErrorToken:
return nil, ""
case html.StartTagToken, html.SelfClosingTagToken:
tagName, hasAttr := z.TagName()
if !bytes.Equal(tagName, []byte("meta")) {
continue
}
attrList := make(map[string]bool)
gotPragma := false
const (
dontKnow = iota
doNeedPragma
doNotNeedPragma
)
needPragma := dontKnow
name = ""
e = nil
for hasAttr {
var key, val []byte
key, val, hasAttr = z.TagAttr()
ks := string(key)
if attrList[ks] {
continue
}
attrList[ks] = true
for i, c := range val {
if 'A' <= c && c <= 'Z' {
val[i] = c + 0x20
}
}
switch ks {
case "http-equiv":
if bytes.Equal(val, []byte("content-type")) {
gotPragma = true
}
case "content":
if e == nil {
name = fromMetaElement(string(val))
if name != "" {
e, name = Lookup(name)
if e != nil {
needPragma = doNeedPragma
}
}
}
case "charset":
e, name = Lookup(string(val))
needPragma = doNotNeedPragma
}
}
if needPragma == dontKnow || needPragma == doNeedPragma && !gotPragma {
continue
}
if strings.HasPrefix(name, "utf-16") {
name = "utf-8"
e = encoding.Nop
}
if e != nil {
return e, name
}
}
}
}
func fromMetaElement(s string) string {
for s != "" {
csLoc := strings.Index(s, "charset")
if csLoc == -1 {
return ""
}
s = s[csLoc+len("charset"):]
s = strings.TrimLeft(s, " \t\n\f\r")
if !strings.HasPrefix(s, "=") {
continue
}
s = s[1:]
s = strings.TrimLeft(s, " \t\n\f\r")
if s == "" {
return ""
}
if q := s[0]; q == '"' || q == '\'' {
s = s[1:]
closeQuote := strings.IndexRune(s, rune(q))
if closeQuote == -1 {
return ""
}
return s[:closeQuote]
}
end := strings.IndexAny(s, "; \t\n\f\r")
if end == -1 {
end = len(s)
}
return s[:end]
}
return ""
}
var boms = []struct {
bom []byte
enc string
}{
{[]byte{0xfe, 0xff}, "utf-16be"},
{[]byte{0xff, 0xfe}, "utf-16le"},
{[]byte{0xef, 0xbb, 0xbf}, "utf-8"},
}