Indexes keys of `counter`.


        Indexes keys of `counter` according to frequency thresholds such as `most_freq_count` and
        `min_freq`.

def _index_counter_keys(self, counter, unknown_token, reserved_tokens, most_freq_count,
                            min_freq):
        """Indexes keys of `counter`.


        Indexes keys of `counter` according to frequency thresholds such as `most_freq_count` and
        `min_freq`.
        """

        assert isinstance(counter, collections.Counter), \
            '`counter` must be an instance of collections.Counter.'

        unknown_and_reserved_tokens = set(reserved_tokens) if reserved_tokens is not None else set()
        unknown_and_reserved_tokens.add(unknown_token)

        token_freqs = sorted(counter.items(), key=lambda x: x[0])
        token_freqs.sort(key=lambda x: x[1], reverse=True)

        token_cap = len(unknown_and_reserved_tokens) + (
            len(counter) if most_freq_count is None else most_freq_count)

        for token, freq in token_freqs:
            if freq < min_freq or len(self._idx_to_token) == token_cap:
                break
            if token not in unknown_and_reserved_tokens:
                self._idx_to_token.append(token)
                self._token_to_idx[token] = len(self._idx_to_token) - 1