// Decode decodes a percent-encoded URL.
// Invalid percent-encoded sequences are left as is.
// Invalid UTF-8 sequences are replaced with U+FFFD.
func Decode(rawurl string) string {
var buf bytes.Buffer
i := 0
const replacement = "\xEF\xBF\xBD"
outer:
for i < len(rawurl) {
r, rlen := utf8.DecodeRuneInString(rawurl[i:])
if r == '%' && i+2 < len(rawurl) &&
byteutil.IsHexDigit(rawurl[i+1]) &&
byteutil.IsHexDigit(rawurl[i+2]) {
b := byteutil.Unhex(rawurl[i+1])<<4 | byteutil.Unhex(rawurl[i+2])
if b < 0x80 {
buf.WriteByte(b)
i += 3
continue
}
var n int
if b&0xe0 == 0xc0 {
n = 1
} else if b&0xf0 == 0xe0 {
n = 2
} else if b&0xf8 == 0xf0 {
n = 3
}
if n == 0 {
buf.WriteString(replacement)
i += 3
continue
}
rb := make([]byte, n+1)
rb[0] = b
j := i + 3
for k := 0; k < n; k++ {
b, j = advance(rawurl, j)
if j > len(rawurl) || b&0xc0 != 0x80 {
buf.WriteString(replacement)
i += 3
continue outer
}
rb[k+1] = b
}
r, _ := utf8.DecodeRune(rb)
buf.WriteRune(r)
i = j
continue
}
buf.WriteRune(r)
i += rlen
}
return buf.String()
}
func advance(s string, pos int) (byte, int) {
if pos >= len(s) {
return 0, len(s) + 1
}
if s[pos] != '%' {
return s[pos], pos + 1
}
if pos+2 < len(s) &&
byteutil.IsHexDigit(s[pos+1]) &&
byteutil.IsHexDigit(s[pos+2]) {
return byteutil.Unhex(s[pos+1])<<4 | byteutil.Unhex(s[pos+2]), pos + 3
}
return '%', pos + 1
}
