判断一个字串是否是GBK编码

isgbk.c

/* http://en.wikipedia.org/wiki/GBK */
size_t fixGBK(const char *str, size_t len)
{
	const unsigned char *string = (const unsigned char *)str;
	size_t idx = 0;
        for (idx = 0; idx < len; idx++) {
                int val = string[idx], val2;
                if (val < 128)
                        continue;
		if (idx + 1 >= len)
			return idx;
                val2 = string[idx + 1];
                if (((val >= 0xA1 && val <= 0xA9) && (val2 >= 0xA1 && val2 <= 0xFE)) || 
                    ((val >= 0xB0 && val <= 0xF7) && (val2 >= 0xA1 && val2 <= 0xFE)) || 
                    ((val >= 0x81 && val <= 0xA0) && (val2 >= 0x40 && val2 <= 0xFE && val2 != 0x7F)) || 
                    ((val >= 0xAA && val <= 0xFE) && (val2 >= 0x40 && val2 <= 0xA0 && val2 != 0x7F)) ||
                    ((val >= 0xA8 && val <= 0xA9) && (val2 >= 0x40 && val2 <= 0xA0 && val2 != 0x7F)) ||
                    ((val >= 0xAA && val <= 0xAF) && (val2 >= 0xA1 && val2 <= 0xFE)) || 
                    ((val >= 0xF8 && val <= 0xFE) && (val2 >= 0xA1 && val2 <= 0xFE)) ||
                    ((val >= 0xA1 && val <= 0xA7) && (val2 >= 0x40 && val2 <= 0xA0 && val2 != 0x7F))) {
			    idx++;
		} else {
			return idx;
		}
	}

	return idx;
}

#define isGBK(str, len) (fixGBK(str, len) == (len))