class Co2SpeedPolicy:

    def __init__(
        self,
        *,
        base_speed: int,
        thresholds: tuple[
            tuple[int, int],
            ...
        ],
        hysteresis: int,
    ):
        if hysteresis < 0:
            raise ValueError(
                "hysteresis must be >= 0"
            )

        if not thresholds:
            raise ValueError(
                "thresholds cannot be empty"
            )

        previous_ppm = None

        speeds = [base_speed]

        for ppm, speed in thresholds:

            if previous_ppm is not None:
                if ppm <= previous_ppm:
                    raise ValueError(
                        "CO2 thresholds must "
                        "be strictly increasing"
                    )

            if speed in speeds:
                raise ValueError(
                    "AUTO speeds must be unique"
                )

            speeds.append(speed)
            previous_ppm = ppm

        self._base_speed = base_speed
        self._thresholds = thresholds
        self._hysteresis = hysteresis
        self._speeds = tuple(speeds)


    def select_speed(self, co2: int, current_speed: int | None) -> int:

        if co2 < 0:
            raise ValueError(
                "CO2 cannot be negative"
            )

        # --------------------------------------------------
        # Первое решение AUTO.
        #
        # Гистерезис пока применять не к чему:
        # предыдущей AUTO-скорости ещё нет.
        # --------------------------------------------------

        if (
            current_speed is None
            or current_speed not in self._speeds
        ):
            return self._select_initial_speed(co2)

        index = self._speeds.index(current_speed)

        # --------------------------------------------------
        # CO2 растёт.
        #
        # Проверяем пороги перехода вверх.
        # За один вызов можем перепрыгнуть
        # сразу несколько скоростей.
        # --------------------------------------------------

        while index < len(
            self._thresholds
        ):

            ppm, _ = self._thresholds[index]

            if co2 < ppm:
                break

            index += 1

        # --------------------------------------------------
        # CO2 падает.
        #
        # Для перехода вниз используем:
        #
        # threshold - hysteresis
        #
        # Поэтому скорость не будет прыгать
        # туда-сюда около одного порога.
        # --------------------------------------------------

        while index > 0:

            ppm, _ = self._thresholds[index - 1]

            down_threshold = ppm - self._hysteresis

            if co2 > down_threshold:
                break

            index -= 1

        return self._speeds[index]


    def _select_initial_speed(self, co2: int) -> int:

        speed = self._base_speed

        for ppm, candidate_speed in (
            self._thresholds
        ):

            if co2 < ppm:
                break

            speed = candidate_speed

        return speed