#define POLY 0xA001
#define POLY 0x8005
unsigned crc16(unsigned crc, unsigned char *buf, size_t len)
{
    while (len--) {
        crc ^= *buf++;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
        crc = crc & 1 ? (crc >> 1) ^ POLY : crc >> 1;
    }
    return crc;
}

int main(void)
{
    unsigned crc;
    unsigned char buf[] = {0x00, 0x19, 0x02 };

    crc = crc16(0x0, buf, 3);
    printf("%04x\n", crc);
    crc = crc16(0xFFFF, buf, 3);
    printf("%04x\n", crc);
    return 0;
}