'calculate the signal strength of access point
:param self: An AccessPointFinder object
:type self: AccessPointFinder
:return: Signal strength of access point
:rtype: int'

def _calculate_signal_strength(self, rssi):

    if (rssi <= (-100)):
        signal_strength = 0
    elif (rssi >= (-50)):
        signal_strength = 100
    else:
        signal_strength = (2 * (rssi + 100))
    return signal_strength
