#ifdef __cplusplus
extern "C" {
#endif

#ifndef _CHECKSUM_H_
#define _CHECKSUM_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(const uint8_t *pBuffer, uint16_t length)
{
    uint16_t crcTmp;

    crc_init(&crcTmp);
    while (length--) {
        crc_accumulate(*pBuffer++, &crcTmp);
    }
    return crcTmp;
}

/**
 * @brief Accumulate the X.25 CRC by adding an array of bytes
 *
 * The checksum function adds the hash of one char at a time to the
 * 16 bit checksum (uint16_t).
 *
 * @param data new bytes to hash
 * @param crcAccum the already accumulated checksum
 **/
static inline void crc_accumulate_buffer(uint16_t *crcAccum, const char *pBuffer, uint8_t length)
{
    const uint8_t *p = (const uint8_t *)pBuffer;

    while (length--) {
        crc_accumulate(*p++, crcAccum);
    }
}


#endif /* _CHECKSUM_H_ */

#ifdef __cplusplus
}
#endif