const auto words = SplitIntoWordsNoStop(document);
const double inv_word_count = 1 / static_cast<double>(words.size());
set<string> s{words.begin(), words.end()};
map<string_view, double> freqs;
for (const auto &word: s) {
    word_to_document_freqs_[word][document_id] += inv_word_count;
    freqs[word] += inv_word_count;
}