Ipv4 header checksum calculation

ipv4_checksum.cpp

/** 
 * Compute Internet Checksum for "count" bytes
 * beginning at location "addr".
 * @param addr start of the ip header
 * @param count ip header size
 * @return Taking the ones' complement (flipping every bit) yields 0000, 
 * which indicates that no error is detected.
 */
uint16_t checksum(void *addr, int count) {
    uint32_t sum = 0;

    auto *ptr_16 = static_cast<uint16_t *>(addr);

    for (; count > 1; count -= 2) {
        // This is the inner loop
        sum += *ptr_16++;
    }

    // Add left-over byte, if any
    if (count > 0)
        sum += *static_cast<uint8_t *>(addr);

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

    return ~sum;
}