c++ hashmap

// This is a personal academic project. Dear PVS-Studio, please check it.
// PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
#include <numeric>
#include <algorithm>
#include <string>
#include <vector>
#include <chrono>
#include <random>
#include <iostream>
#include <array>
#include <mutex>
#include <thread>
#include <unordered_set>
#include <string_view>
#include <cassert>

using namespace std;

void update_str(string& s)
{
    constexpr array<char, 63> alphabet{ "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789" };
    random_device r;
    default_random_engine gen(r());
    uniform_int_distribution<size_t> dis(0, alphabet.size() - 2); // оба конца диапазона включаются, не учитываем '\0'

    for_each(s.begin(), s.end(),
        [&](auto& c)
        {
            c = alphabet.at(dis(gen));
        }
    );
}

class random_string20 : public string
{
public:
    random_string20()
    {
        resize(20);
        update_str(*this);
    }
};


void update_vector(vector<string>& v)
{
    for_each(
        v.begin(), v.end(),
        [](string& s)
        {
            update_str(s);
        }
    );
}

size_t test(vector<string>& vector_string100)
{
    auto result = size_t(0);
    auto begin = chrono::high_resolution_clock::now();
    update_vector(vector_string100);
    auto end = chrono::high_resolution_clock::now();
    {
        cout << "String generation time: "
            << chrono::duration_cast<chrono::milliseconds>
            (end - begin).count() << '\n';
        // как будут готовы range, заменить на ranges
        array<int, 1000> const range{};
        /////////////////////////////////////////////////////
        begin = chrono::high_resolution_clock::now();
        unordered_set<string_view> hash_set;
        hash_set.reserve(81 * vector_string100.size());
        for (auto&& s : vector_string100)
        {
            assert(s.size() >= 19);
            for (size_t i = 0; i < s.size() - 19; ++i)
            {
                hash_set.insert(string_view(&s.at(i), 20));
            }
        }
        end = chrono::high_resolution_clock::now();
        cout << "Hash generation time: "
            << chrono::duration_cast<chrono::milliseconds>
            (end - begin).count() << '\n';
        /////////////////////////////////////////////////////
        begin = chrono::high_resolution_clock::now();
        result = accumulate(
            range.begin(), range.end(),
            size_t(0),
            [&](size_t acc, size_t)
            {
                random_string20 const s20;
                auto const iter = hash_set.find(s20);
                bool const found = (iter != hash_set.end());
                return found ? acc + 1 : acc;
            }
        );
        end = chrono::high_resolution_clock::now();
        cout << "Substring search time: "
            << chrono::duration_cast<chrono::milliseconds>
            (end - begin).count() << '\n';
        begin = chrono::high_resolution_clock::now();
    }
    end = chrono::high_resolution_clock::now();
    cout << "Hashmap destructor time: "
        << chrono::duration_cast<chrono::milliseconds>
        (end - begin).count() << '\n';
    return result;
}

int main()
{
    for (int i = 0; i < 10; ++i)
    {
        auto const begin = chrono::high_resolution_clock::now();
        vector<string> vector_string100(100000, string(100, ' '));
        size_t const matches = test(vector_string100);
        auto const end = chrono::high_resolution_clock::now();
        cout << matches << " strings matches, time = "
            << chrono::duration_cast<chrono::milliseconds>
            (end - begin).count() << "\n\n\n";
    }
}