modeling.py 文件源码

def log_negative_binomial(x, p, log_r, eps = 0.0):
    """
    Compute log pdf of a negative binomial distribution with success probability p and number of failures, r, until the experiment is stopped, at values x.

    A simple variation of Stirling's approximation is used: log x! = x log x - x.
    """

    x = T.clip(x, eps, x)

    p = T.clip(p, eps, 1.0 - eps)

    r = T.exp(log_r)
    r = T.clip(r, eps, r)

    y = T.gammaln(x + r) - T.gammaln(x + 1) - T.gammaln(r) \
        + x * T.log(p) + r * T.log(1 - p)

    return y