IPv4 checksum in C/C++ Impl

Impl

inline unsigned short                                                           ip_standard_chksum(void* dataptr, int len) noexcept {
                unsigned int acc;
                unsigned short src;
                unsigned char* octetptr;

                acc = 0;
                /* dataptr may be at odd or even addresses */
                octetptr = (unsigned char*)dataptr;
                while (len > 1) {
                    /* declare first octet as most significant
                       thus assume network order, ignoring host order */
                    src = (unsigned short)((*octetptr) << 8);
                    octetptr++;
                    /* declare second octet as least significant */
                    src |= (*octetptr);
                    octetptr++;
                    acc += src;
                    len -= 2;
                }
                if (len > 0) {
                    /* accumulate remaining octet */
                    src = (unsigned short)((*octetptr) << 8);
                    acc += src;
                }
                /* add deferred carry bits */
                acc = (unsigned int)((acc >> 16) + (acc & 0x0000ffffUL));
                if ((acc & 0xffff0000UL) != 0) {
                    acc = (unsigned int)((acc >> 16) + (acc & 0x0000ffffUL));
                }
                /* This maybe a little confusing: reorder sum using htons()
                   instead of ntohs() since it has a little less call overhead.
                   The caller must invert bits for Internet sum ! */
                return ntohs((unsigned short)acc);
            }

            inline unsigned short                                                           inet_chksum(void* dataptr, int len) noexcept {
                return (unsigned short)~ip_standard_chksum(dataptr, len);
            }

            inline unsigned int                                                             FOLD_U32T(unsigned int u) noexcept {
                return ((unsigned int)(((u) >> 16) + ((u) & 0x0000ffffUL)));
            }

            inline unsigned int                                                             SWAP_BYTES_IN_WORD(unsigned int w) noexcept {
                return (((w) & 0xff) << 8) | (((w) & 0xff00) >> 8);
            }

            inline unsigned short                                                           ine