How to use the pysurvive.get_bsd_idx function in pysurvive

def record_sweep(self, time, channel, sensor_id, timecode, flag):
        bsd_idx = pysurvive.get_bsd_idx(self.so.contents.ctx, channel)
        if bsd_idx == -1:
            return
        last_sweep = self.so.contents.last_sync_time[bsd_idx]
        time_since_sync = (pysurvive.timecode_difference(timecode, last_sweep) / 48000000.)
        hz = 48000000. / self.so.contents.last_time_between_sync[bsd_idx]
        time_per_rot = 1. / hz

        if time_since_sync > time_per_rot:
            return

        key = (sensor_id, channel)
        raw_angle = time_since_sync / time_per_rot * 2. * math.pi
        self.raw_angles[key].append([time, raw_angle])


# int8_t bsd_idx = survive_get_bsd_idx(ctx, channel);