Microsoft.Scripting.Utils.MathUtils.LogGamma C# (CSharp) Method

        public static double LogGamma(double v0) {
            // Calculate the log of the Gamma function using the Lanczos approximation

            if (double.IsInfinity(v0)) {
                return double.PositiveInfinity;
            }
            double a = Math.Abs(v0);

            // Gamma is undefined on non-positive integers
            if (v0 <= 0.0 && a % 1.0 == 0.0) {
                return double.NaN;
            }

            // lim(LGamma(v0)) = -log|v0| as v0 approaches 0.0
            if (a < 1e-50) {
                return -Math.Log(a);
            }

            double res;
            if (v0 < 0.0) {
                // For negative values, use the reflection formula:
                //     Gamma(x) = pi / sin(pi * x) / Gamma(1 - x)
                // ==> LGamma(x) = log(pi / |sin(pi * x)|) - LGamma(1 - x)
                res = Math.Log(Math.PI / AbsSinPi(v0));
                res -= PositiveLGamma(1.0 - v0);
            } else {
                res = PositiveLGamma(v0);
            }

            return res;
        }