A Python Implementation of the Inverse Chi-Square Function

import math
def chi2P(chi, df):
 """Return prob(chisq >= chi, with df degrees of
freedom).
 df must be even.
 """
 assert df & 1 == 0
 # XXX If chi is very large, exp(-m) will underflow to 0.
 m = chi / 2.0
 sum = term = math.exp(-m)
 for i in range(1, df//2):
 term *= m / i
 sum += term
 # With small chi and large df, accumulated
 # roundoff error, plus error in
 # the platform exp(), can cause this to spill
 # a few ULP above 1.0. For
 # example, chi2P(100, 300) on my box
 # has sum == 1.0 + 2.0**-52 at this
 # point. Returning a value even a teensy
 # bit over 1.0 is no good.
 return min(sum, 1.0)