ax25_crc16.c

uint16_t ax25crc16(unsigned char *data_p, uint16_t lenght) {
    uint16_t crc = 0xFFFF;
    uint32_t data;
    uint16_t crc16_table[] = {
            0x0000, 0x1081, 0x2102, 0x3183,
            0x4204, 0x5285, 0x6306, 0x7387,
            0x8408, 0x9489, 0xa50a, 0xb58b,
            0xc60c, 0xd68d, 0xe70e, 0xf78f
    };

    while(lenght--){
        crc = ( crc >> 4 ) ^ crc16_table[(crc & 0xf) ^ (*data_p & 0xf)];
        crc = ( crc >> 4 ) ^ crc16_table[(crc & 0xf) ^ (*data_p++ >> 4)];
    }

    data = crc;
    crc = (crc << 8) | (data >> 8 & 0xff); // do byte swap here that is needed by AX25 standard
    return (~crc);
}