ExploreKeyWords_v1

#include <functional>
#include <iostream>
#include <map>
#include <set>
#include <vector>
#include <sstream>
#include <string>

#include <future>
#include <algorithm>
#include <execution>

using namespace std;

struct Stats {
    map<string, int> word_frequences;

    void operator+=(const Stats& other) {
        // сложить частоты
        for_each(execution::par, other.word_frequences.begin(), other.word_frequences.end(),
            [this](const pair<const string&, int>& pair) {
                word_frequences[pair.first] += pair.second;
            });
    }
};

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

vector<string> SplitIntoWords(const string& text) {
    vector<string> words;
    string word;
    for (const char c : text) {
        if (c == ' ') {
            if (!word.empty()) {
                words.push_back(word);
                word.clear();
            }
        } else {
            word += c;
        }
    }
    if (!word.empty()) {
        words.push_back(word);
    }
    return words;
}

Stats ExploreKeyWords(const KeyWords& key_words, istream& input) {
    Stats statistics;
    //auto async_for_each = async([&key_words, &statistics] {
    //    for_each(execution::par, key_words.begin(), key_words.end(), [&statistics](const string& word) {
    //        statistics.word_frequences[word];
    //    });
    //});
    //vector<string> content;
    string content { istreambuf_iterator<char>(input), istreambuf_iterator<char>() }; // вся строка - текст внутри input
    //while (!input.eof()) {
    //    string text;
    //    getline(input, text);
    //    content.push_back(text);
    //}
    //for (const string& text : content) {
    //}
    size_t middle = content.size() / 2;
    future<void> first_part = async([&statistics, &key_words, &content, middle] {
        Stats stats_one_string;
        for (const string& word : SplitIntoWords(content.substr(0, middle))) {
            if (key_words.count(word)) {
                stats_one_string.word_frequences[word]++;
            }
        }
        statistics += stats_one_string;
    });
    Stats stats_one_string;
    for (const string& word : SplitIntoWords(content.substr(middle))) {
        if (key_words.count(word)) {
            stats_one_string.word_frequences[word]++;
        }
    }
    statistics += stats_one_string;
    first_part.get();
    return statistics;
}

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;
}