poissonianlikemethods.py 文件源码

def poisson(self, x, mu):
    """
    Poisson function taken from:
    https://github.com/scipy/scipy/blob/master/scipy/stats/_discrete_distns.py

    For license see documentation/BSDLicense_scipy.md

    Author:  Travis Oliphant  2002-2011 with contributions from
             SciPy Developers 2004-2011
    """
    if len(np.atleast_1d(x)) == 1:
        check_val = x
    else:
        check_val = x[0]

    if check_val > 1e18:
        self.log.warning('The current value in the poissonian distribution '
                         'exceeds 1e18! Due to numerical imprecision a valid '
                         'functional output cannot be guaranteed any more!')

    # According to the central limit theorem, a poissonian distribution becomes
    # a gaussian distribution for large enough x. Since the numerical precision
    # is limited to calculate the logarithmized poissonian and obtain from that
    # the exponential value, a self defined cutoff is introduced and set to
    # 1e12. Beyond that number a gaussian distribution is assumed, which is a
    # completely valid assumption.

    if check_val < 1e12:
        return np.exp(xlogy(x, mu) - gammaln(x + 1) - mu)
    else:
        return np.exp(-((x - mu) ** 2) / (2 * mu)) / (np.sqrt(2 * np.pi * mu))