Internet Checksum

checksum.cpp

#include <cstdint>
#include <ios>
#include <iostream>
#include <vector>

uint16_t checksum(const std::vector<uint16_t> &data) {
    uint32_t checksum = 0;

    for (auto word : data) {
        checksum += word;
        while (checksum >> 16u != 0) {
            checksum = (checksum & 0xffffu) + (checksum >> 16u);
        }
    }

    return ~checksum;
}

uint16_t checksum(const std::vector<uint8_t> &data) {
    std::vector<uint16_t> shorts;
    for (int i = 0; i < data.size(); i += 2) {
        uint16_t j = data[i];
        j <<= 8u;
        if (i + 1 < data.size()) {
            j |= data[i + 1];
        }
        shorts.push_back(j);
    }
    return checksum(shorts);
}


int main() {
    std::vector<uint8_t> data = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A};
    std::cout << std::hex << checksum(data) << std::endl;
}