#include "neo_m8_ubx_checksum.h"

/* Check page 136 of document UBX-13003221 - R15 for more details */

/*
 *  The checksum algorithm used is the > 8-Bit Fletcher Algorithm <, which is used in the TCP standard (RFC 1145).
 *	This algorithm works as follows:
 *	Buffer[N] contains the data over which the checksum is to be calculated.
 *	The two CK_ values are 8-Bit unsigned integers, only! If implementing with larger-sized integer values, make
 *	sure to mask both CK_A and CK_B with 0xFF after both operations in the loop.
 */

/* Straightforward implementation of the Fletcher Algorithm */
uint16_t Fletcher16(uint8_t *data, int count)
{
    uint16_t sum1 = 0;
    uint16_t sum2 = 0;
    int index;

    for (index = 0; index < count; ++index)
    {
        sum1 = (sum1 + data[index]) % 255;
        sum2 = (sum2 + sum1) % 255;
    }

    return (sum2 << 8) | sum1;
}

void UBX_Fletcher(uint8_t *data, int count, uint8_t *cka, uint8_t *ckb)
{
    int i;

    *cka = 0;
    *ckb = 0;

    for (i = 0; i < count; i++)
    {
        *cka = *cka + data[i];
        *ckb = *ckb + *cka;
    }
}