{
           /* Compute Internet Checksum for "count" bytes
            *         beginning at location "addr".
            */
       register long sum = 0;

        while( count > 1 )  {
           /*  This is the inner loop */
               sum += * (unsigned short) addr++;
               count -= 2;
       }

           /*  Add left-over byte, if any */
       if( count > 0 )
               sum += * (unsigned char *) addr;

           /*  Fold 32-bit sum to 16 bits */
       while (sum>>16)
           sum = (sum & 0xffff) + (sum >> 16);

       checksum = ~sum;
   }

unsigned int hwport_rfc1071_checksum(const void *s_data, size_t s_size)
{ /* hwport_internet_checksum (RFC1071) */
    register uint32_t s_result = (uint32_t)0u;

    while(s_size > ((size_t)1u)) {
        s_result += (uint32_t)(*((const uint16_t *)s_data));
        s_data = ((const uint16_t *)s_data) + ((size_t)1u);
        s_size -= (size_t)2u;
    }

    if(s_size > ((size_t)0u)) {
        s_result += (uint32_t)(*((const uint8_t *)s_data));
    }

    /*  Fold 32-bit sum to 16 bits */
    s_result = (s_result >> 16) + (s_result & ((uint32_t)0xffffu));
    s_result += s_result >> 16;

    return((unsigned int)((~s_result) & ((uint32_t)0xffffu)));
}

static inline uint16_t csum16_add(uint16_t csum, /* BE16 */ uint16_t addend)
{
        uint16_t res = (uint16_t)csum;

        res += (uint16_t)addend;
        return (uint16_t)(res + (res < (uint16_t)addend));
}

static inline uint16_t csum16_sub(uint16_t csum, /* BE16 */ uint16_t addend)
{
        return csum16_add(csum, ~addend);
}

static inline void csum_replace2(uint16_t *sum, /* BE16 */ uint16_t old, /* BE16 */ uint16_t new)
{
        *sum = ~csum16_add(csum16_sub(~(*sum), old), new);
}