checksum

checksum.cc

#include <sys/types.h> // for network

static constexpr u_int16_t checksum(const u_int16_t* data, const size_t n)
{
    u_int32_t sum = 0;
    // The inner loop.
    for (size_t i = n; i > 1; i -= 2, data++) {
        sum += *data;
    }
    // Add left-over byte, if any.
    if (n & 0x01) {
        sum += *reinterpret_cast<const u_int8_t*>(data);
    }
    // Fold 32-bit sum to 16bits.
    while (sum >> 16) {
        sum = (sum & 0xffff) + (sum >> 16);
    }

    return static_cast<u_int16_t>(~sum);
}