ExploreKeyWords

#include <algorithm>
#include <functional>
#include <iostream>
#include <sstream>
#include <execution>
#include <future>
#include <numeric>
#include <vector>
#include <map>
#include <set>
#include <string>

using namespace std;

const int SIZE_OF_PART = 5000;

struct Stats {
    map<string, int> word_frequences;

    void operator+=(const Stats& other) {
        for (const auto& [word, frequency] : other.word_frequences) {
            word_frequences[word] += frequency;
        }

        // for_each(execution::par,
        //          other.word_frequences.begin(), other.word_frequences.end(),
        //          [this](auto& item) { this->word_frequences[item.first] += item.second; });
    }
};

using KeyWords = set<string, less<>>;

Stats ExploreKeyWords(const KeyWords& key_words, istream& input) {

    vector<vector<string>> parts_of_stream;


    // разделю поток на части по SIZE_OF_PART слов
    string word;
    int count = SIZE_OF_PART;
    while (!input.eof()) {

        vector<string> tmp;

        while (count) {

            input >> word;
            tmp.push_back(move(word));

            --count;
        }

        parts_of_stream.push_back(move(tmp));

        count = SIZE_OF_PART;

    }

    auto counter = [&key_words](const vector<string>& words) {
        Stats st;
        for (const string& w : words) {
            if (key_words.count(w)) {
                ++st.word_frequences[w];
            }
        }

        return st;
    };

    vector<future<Stats>> threads;
    for (const vector<string>& part : parts_of_stream) {
        // асинхронный запуск подсчетов для каждой из частей потока, разделенного на векторы слов
        threads.push_back(async(counter, cref(part)));
    }

    Stats result;

    for (future<Stats>& pre : threads) {
        result += pre.get();
    }

    return result;
}

int main() {
    const KeyWords key_words = {"yangle", "rocks", "sucks", "all"};

    stringstream ss;
    ss << "this new yangle service really rocks\n";
    ss << "It sucks when yangle isn't available\n";
    ss << "10 reasons why yangle is the best IT company\n";
    ss << "yangle rocks others suck\n";
    ss << "Goondex really sucks, but yangle rocks. Use yangle\n";

    for (const auto& [word, frequency] : ExploreKeyWords(key_words, ss).word_frequences) {
        cout << word << " " << frequency << endl;
    }


    return 0;
}