const auto query = ParseQuery(raw_query);
vector<string_view> matched_words;
for (const auto &word: query.plus_words) {
    if (word_to_document_freqs_.at(word).count(document_id)) {
        matched_words.push_back(word);
    }
...
}

//----------------------------------

    struct Query {
        std::set<std::string> plus_words;
        std::set<std::string> minus_words;
    };

    struct QueryVector {
        std::vector<std::string> plus_words;
        std::vector<std::string> minus_words;
    };