ExploreKeyWords_v3

Stats CalculateStats(const KeyWords& key_words, vector<string>::iterator begin_range, vector<string>::iterator end_range) {
    Stats stats_one_string;
    for (vector<string>::iterator it_all_string = begin_range; it_all_string != end_range; ++it_all_string) {
        vector<string_view> words = SplitIntoWords(*it_all_string);
        for (vector<string_view>::iterator it = words.begin(); it != words.end(); ++it) {
            if (key_words.count(*it)) {
                stats_one_string.word_frequences[string(*it)]++;
            }
        }
    }
    return stats_one_string;
}

Stats ExploreKeyWords(const KeyWords& key_words, istream& input) {
    Stats statistics;
    vector<string> content;
    while (!input.eof()) {
        string text;
        getline(input, text);
        if (text.empty()) {
            break;
        }
        if (!CheckPrintSymbols(text)) {
            return statistics;
        }
        content.push_back(move(text));
    }

    vector<string>::iterator begin_range = content.begin();
    vector<string>::iterator end_range = content.end();
    const size_t distance = content.end() - content.begin();
    const size_t part_length = max(1, static_cast<int>(distance) / 3);
    vector<string>::iterator middle_left = begin_range + part_length;
    vector<string>::iterator middle_right = end_range - part_length;

    future<Stats> first_part = async([&key_words, begin_range, middle_left] {
        return CalculateStats(key_words, begin_range, middle_left); });
    future<Stats> second_part = async([&key_words, middle_left, middle_right] {
        return CalculateStats(key_words, middle_left, middle_right); });

    statistics += CalculateStats(key_words, middle_right, end_range);
    statistics += first_part.get();
    statistics += second_part.get();

    return statistics;
}