GammaLn.cc

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#include "CommonTools/Statistics/src/GammaLn.h"
#include <cmath>

float GammaLn(float z) {
  const static double coefficients[6] = {76.18009172947146,
                                         -86.50532032941677,
                                         24.01409824083091,
                                         -1.231739572450155,
                                         0.1208650973866179e-2,
                                         -0.5395239384953e-5};

  double temp = z + 5.5;
  temp -= (z + 0.5) * log(temp);
  double y = z;
  double series = 1.000000000190015;
  for (int term = 0; term < 6; term++)
    series += coefficients[term] / ++y;
  return -temp + log(2.5066282746310005 * series / z);
}