How to use the timezonefinder.helpers_numba.coord2shortcut function in timezonefinder

class HelperTestNumba(HelperTest):
    # all Numba compiled functions have to receive their arguments in the proper data type (and numpy array shape)
    try:
        import timezonefinder.helpers_numba as helpers
        fct_dict = {
            "all_the_same": helpers.all_the_same,
            "coord2int": helpers.coord2int,
            "int2coord": helpers.int2coord,
            "distance_to_point_on_equator": helpers.distance_to_point_on_equator,
            "distance_to_polygon": helpers.distance_to_polygon,
            "distance_to_polygon_exact": helpers.distance_to_polygon_exact,
            "haversine": helpers.haversine,
            "inside_polygon": helpers.inside_polygon,
            "coord2shortcut": helpers.coord2shortcut,
            "rectify_coordinates": helpers.rectify_coordinates,
            'convert2coords': helpers.convert2coords,
            'convert2coord_pairs': helpers.convert2coord_pairs,
        }
        print('\nNumba installation found.\ntesting helpers_numba.py functions...')

    except ImportError:
        fct_dict = {
            "all_the_same": None,
            "coord2int": None,
            "int2coord": None,
            "distance_to_point_on_equator": None,
            "distance_to_polygon": None,
            "distance_to_polygon_exact": None,
            "haversine": None,
            "inside_polygon": None,

def exact_routine(polygon_nr):
            coords = self.coords_of(polygon_nr)
            nr_points = len(coords[0])
            empty_array = empty([2, nr_points], dtype=DTYPE_FORMAT_F_NUMPY)
            return distance_to_polygon_exact(lng, lat, nr_points, coords, empty_array)

        def normal_routine(polygon_nr):
            coords = self.coords_of(polygon_nr)
            nr_points = len(coords[0])
            return distance_to_polygon(lng, lat, nr_points, coords)

        lng, lat = rectify_coordinates(lng, lat)

        # transform point X into cartesian coordinates
        current_closest_id = None
        central_x_shortcut, central_y_shortcut = coord2shortcut(lng, lat)

        lng = radians(lng)
        lat = radians(lat)

        possible_polygons = []

        # there are 2 shortcuts per 1 degree lat, so to cover 1 degree two shortcuts (rows) have to be checked
        # the highest shortcut is 0
        top = max(central_y_shortcut - NR_SHORTCUTS_PER_LAT * delta_degree, 0)
        # the lowest shortcut is 359 (= 2 shortcuts per 1 degree lat)
        bottom = min(central_y_shortcut + NR_SHORTCUTS_PER_LAT * delta_degree, 359)

        # the most left shortcut is 0
        left = max(central_x_shortcut - NR_SHORTCUTS_PER_LNG * delta_degree, 0)
        # the most right shortcut is 359 (= 1 shortcuts per 1 degree lng)
        right = min(central_x_shortcut + NR_SHORTCUTS_PER_LAT * delta_degree, 359)

def certain_timezone_at(self, *, lng, lat):
        """
        this function looks up in which polygon the point certainly is included
        this is much slower than 'timezone_at'!
        :param lng: longitude of the point in degree
        :param lat: latitude in degree
        :return: the timezone name of the polygon the point is included in or None
        """

        lng, lat = rectify_coordinates(lng, lat)
        shortcut_id_x, shortcut_id_y = coord2shortcut(lng, lat)
        possible_polygons = self.polygon_ids_of_shortcut(shortcut_id_x, shortcut_id_y)

        # x = longitude  y = latitude  both converted to 8byte int
        x = coord2int(lng)
        y = coord2int(lat)

        # check if the point is actually included in one of the polygons
        for polygon_nr in possible_polygons:

            # get boundaries
            self.poly_max_values.seek(4 * NR_BYTES_I * polygon_nr)
            boundaries = self.fromfile(self.poly_max_values, dtype=DTYPE_FORMAT_SIGNED_I_NUMPY, count=4)
            if not (x > boundaries[0] or x < boundaries[1] or y > boundaries[2] or y < boundaries[3]):

                outside_all_holes = True
                # when the point is within a hole of the polygon this timezone doesn't need to be checked

def timezone_at(self, *, lng, lat):
        """
        this function looks up in which polygons the point could be included in
        to speed things up there are shortcuts being used (stored in a binary file)
        especially for large polygons it is expensive to check if a point is really included,
        so certain simplifications are made and even when you get a hit the point might actually
        not be inside the polygon (for example when there is only one timezone nearby)
        if you want to make sure a point is really inside a timezone use 'certain_timezone_at'
        :param lng: longitude of the point in degree (-180 to 180)
        :param lat: latitude in degree (90 to -90)
        :return: the timezone name of a matching polygon or None
        """
        lng, lat = rectify_coordinates(lng, lat)

        shortcut_id_x, shortcut_id_y = coord2shortcut(lng, lat)
        self.shortcuts_unique_id.seek(
            (180 * NR_SHORTCUTS_PER_LAT * NR_BYTES_H * shortcut_id_x + NR_BYTES_H * shortcut_id_y))
        try:
            # if there is just one possible zone in this shortcut instantly return its name
            return timezone_names[unpack(DTYPE_FORMAT_H, self.shortcuts_unique_id.read(NR_BYTES_H))[0]]
        except IndexError:
            possible_polygons = self.polygon_ids_of_shortcut(shortcut_id_x, shortcut_id_y)
            nr_possible_polygons = len(possible_polygons)
            if nr_possible_polygons == 0:
                return None
            if nr_possible_polygons == 1:
                # there is only one polygon in that area. return its timezone name without further checks
                return timezone_names[self.id_of(possible_polygons[0])]

            # create a list of all the timezone ids of all possible polygons
            ids = self.id_list(possible_polygons, nr_possible_polygons)