#include <iostream>
#include <istream>
#include <random>
#include <string>
#include <utility>
#include <map>
#include <sstream>

std::string random_line(std::istream& input)
{
    static auto gen = std::mt19937{std::random_device{}()};
    auto count = 0u;
    auto selected = std::string{};
    std::string line;
    while (std::getline(input, line)) {
        if (std::uniform_int_distribution{0u,count++}(gen) == 0) {
            selected = std::move(line);
        }
    }
    return selected;
}

int main()
{
    auto const lines = "one\n"
                       "two\n"
                       "three\n"
                       "four\n"
                       "five\n"
                       "six\n";
    auto counts = std::map<std::string,unsigned int>{};
    for (auto i = 0;  i < 120000;  ++i) {
        auto is = std::istringstream{lines};
        ++counts[random_line(is)];
    }

    {
        // print the counts
        auto is = std::istringstream{lines};
        std::string line;
        while (std::getline(is, line)) {
            std::cout << line << ": " << counts[line] << '\n';
        }
    }
}