CRC25 - kuidas ?

[felch]

Sellist CRC-d kasutab MAVlink (kopterite jms mudelite vaheline side). Sain protokollist lõpuks aru aga ma pean sõnumite saatmiseks oskama ise CRC-d arvutada. Ta on 2-baidine ja algväärtus on 0xFFFF. Edasi...ma ei mõista. Kes "tõlgiks" ?
Kood:

Kood:

#ifdef __cplusplus
extern "C" {
#endif

#ifndef _CHECKSUM_H_
#define _CHECKSUM_H_

#include "inttypes.h"


/**
*
*  CALCULATE THE CHECKSUM
*
*/

#define X25_INIT_CRC 0xffff
#define X25_VALIDATE_CRC 0xf0b8

/**
* @brief Accumulate the X.25 CRC by adding one char at a time.
*
* The checksum function adds the hash of one char at a time to the
* 16 bit checksum (uint16_t).
*
* @param data new char to hash
* @param crcAccum the already accumulated checksum
**/
static inline void crc_accumulate(uint8_t data, uint16_t *crcAccum)
{
        /*Accumulate one byte of data into the CRC*/
        uint8_t tmp;

        tmp=data ^ (uint8_t)(*crcAccum &0xff);
        tmp^= (tmp<<4);
        *crcAccum = (*crcAccum>>8) ^ (tmp<<8) ^ (tmp <<3) ^ (tmp>>4);
}

/**
* @brief Initiliaze the buffer for the X.25 CRC
*
* @param crcAccum the 16 bit X.25 CRC
*/
static inline void crc_init(uint16_t* crcAccum)
{
        *crcAccum = X25_INIT_CRC;
}


/**
* @brief Calculates the X.25 checksum on a byte buffer
*
* @param  pBuffer buffer containing the byte array to hash
* @param  length  length of the byte array
* @return the checksum over the buffer bytes
**/
static inline uint16_t crc_calculate(uint8_t* pBuffer, int length)
{

        // For a "message" of length bytes contained in the unsigned char array
        // pointed to by pBuffer, calculate the CRC
        // crcCalculate(unsigned char* pBuffer, int length, unsigned short* checkConst) < not needed

        uint16_t crcTmp;
        //uint16_t tmp;
        uint8_t* pTmp;
        int i;

        pTmp=pBuffer;
        

        /* init crcTmp */
        crc_init(&crcTmp);

        for (i = 0; i < length; i++){
                crc_accumulate(*pTmp++, &crcTmp);
        }

        /* This is currently not needed, as only the checksum over payload should be computed
        tmp = crcTmp;
        crcAccumulate((unsigned char)(~crcTmp & 0xff),&tmp);
        crcAccumulate((unsigned char)((~crcTmp>>8)&0xff),&tmp);
        *checkConst = tmp;
        */
        return(crcTmp);
}




#endif /* _CHECKSUM_H_ */

#ifdef __cplusplus
}
#endif