utils.py 文件源码

def nextfastpower(n):
    """Return the next integral power of small factors greater than the given
    number.  Specifically, return m such that
        m >= n
        m == 2**x * 3**y * 5**z
    where x, y, and z are integers.
    This is useful for ensuring fast FFT sizes.

    From https://gist.github.com/bhawkins/4479607 (Brian Hawkins)
    """
    if n < 7:
        return max (n, 1)
    # x, y, and z are all bounded from above by the formula of nextpower.
    # Compute all possible combinations for powers of 3 and 5.
    # (Not too many for reasonable FFT sizes.)
    def power_series (x, base):
        nmax = ceil (log (x) / log (base))
        return np.logspace (0.0, nmax, num=nmax+1, base=base)
    n35 = np.outer (power_series (n, 3.0), power_series (n, 5.0))
    n35 = n35[n35<=n]
    # Lump the powers of 3 and 5 together and solve for the powers of 2.
    n2 = nextpower (n / n35)
    return int (min (n2 * n35))