utf-8 decoder

utf8decode.c

/* decodes next unicode code point in utf-8-encoded buffer. returns number of bytes                                                                          
   read so stream can be advanced, -1 if invalid utf-8 sequence. */
size_t decode_next_utf8(const unsigned char *str, size_t len, int *cp)
{
  *cp = 0;
  if (*str <= 0x7f) {
    *cp = (int)*str;
    return 1;
  } else if (((*str & 0xe0) == 0xc0) && len > 1) {
    if ((*(str+1) & 0xc0) != 0x80) {
      return -1;
    }
    *cp |= (*str++ & 0x1f) << 6;
    *cp |= (*str   & 0x3f) << 0;
    return 2;
  } else if (((*str & 0xf0) == 0xe0) && len > 2) {
    if (((*(str+1) & 0xc0) != 0x80) || (*(str+2) & 0xc0) != 0x80) {
      return -1;
    }
    *cp |= (*str++ &  0xf) << 12;
    *cp |= (*str++ & 0x1f) << 6;
    *cp |= (*str   & 0x3f) << 0;
    return 3;
  } else if (((*str & 0xf8) == 0xf0) && len > 3) {
    if ((((*(str+1) & 0xc0) != 0x80) || (*(str+2) & 0xc0) != 0x80) || (*(str+3) & 0xc0) != 0x80) {
      return -1;
    }
    *cp |= (*str++ &  0x7) << 18;
    *cp |= (*str++ &  0xf) << 12;
    *cp |= (*str++ & 0x1f) << 6;
    *cp |= (*str   & 0x3f) << 0;
    return 4;
  }
  return -1;
}