def good_fft_number(goal):
    """pick a number >= goal that has only factors of 2,3,5. FFT will be much
    faster if I use such a number"""
    assert goal < 1e5
    choices = [2**a * 3**b * 5**c for a in range(17) for b in range(11)
                                  for c in range(8)]
    return min(x for x in choices if x >= goal)