Artemis.Engine.Maths.SpecialFunctions.ComplementedIncompleteGamma C# (CSharp) Method

        public static double ComplementedIncompleteGamma(double a, double x)
        {
            double big = 4.503599627370496e15;
            double biginv = 2.22044604925031308085e-16;
            double ans, ax, c, yc, r, t, y, z;
            double pk, pkm1, pkm2, qk, qkm1, qkm2;

            if (x <= 0 || a <= 0) return 1.0;

            if (x < 1.0 || x < a) return 1.0 - IncompleteGamma(a, x);

            ax = a * Math.Log(x) - x - LogGamma(a);
            if (ax < -MAXLOG) return 0.0;

            ax = Math.Exp(ax);

            /* continued fraction */
            y = 1.0 - a;
            z = x + y + 1.0;
            c = 0.0;
            pkm2 = 1.0;
            qkm2 = x;
            pkm1 = x + 1.0;
            qkm1 = z*x;
            ans = pkm1/qkm1;

            do
            {
                c += 1.0;
                y += 1.0;
                z += 2.0;
                yc = y*c;
                pk = pkm1*z - pkm2*yc;
                qk = qkm1*z - qkm2*yc;
                if (qk != 0)
                {
                    r = pk/qk;
                    t = Math.Abs((ans - r) / r);
                    ans = r;
                }
                else
                    t = 1.0;

                pkm2 = pkm1;
                pkm1 = pk;
                qkm2 = qkm1;
                qkm1 = qk;
                if (Math.Abs(pk) > big)
                {
                    pkm2 *= biginv;
                    pkm1 *= biginv;
                    qkm2 *= biginv;
                    qkm1 *= biginv;
                }
            } while (t > MACHEP);

            return ans*ax;
        }