//
// ********************************************************************
// * DISCLAIMER                                                       *
// *                                                                  *
// * The following disclaimer summarizes all the specific disclaimers *
// * of contributors to this software. The specific disclaimers,which *
// * govern, are listed with their locations in:                      *
// *   http://cern.ch/geant4/license                                  *
// *                                                                  *
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// * institutes,nor the agencies providing financial support for this *
// * work  make  any representation or  warranty, express or implied, *
// * regarding  this  software system or assume any liability for its *
// * use.                                                             *
// *                                                                  *
// * This  code  implementation is the  intellectual property  of the *
// * GEANT4 collaboration.                                            *
// * By copying,  distributing  or modifying the Program (or any work *
// * based  on  the Program)  you indicate  your  acceptance of  this *
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// ********************************************************************
//
// --------------------------------------------
// Adapted from RNGAMA function of the KERNLIB
// Shoot at random according to the gamma
// distribution.
// --------------------------------------------
#include "RandomGamma.hh"
#include <CLHEP/Random/Randomize.h>

double RandomGamma(double p) {
  double h;
  double stor[15];

  h = 0.;
  if (p > 15) {
    while (h <= 0.) {
      double a;
      a = CLHEP::RandExponential::shoot();
      h = p * (1. - 1. / (9 * p) + a / (3 * sqrt(p)));
      h = h * h * h;
    }
  } else {
    int m = int(p);
    double f = p - m;
    if (m > 0) {
      double x = 1.;
      CLHEP::RandFlat::shootArray(m, stor);
      for (int i = 0; i < m; i++)
        x *= stor[i];
      h = -log(x);
    }
    if (f >= 0.00001) {
      double x = CLHEP::RandFlat::shoot();
      double x1 = -log(x);
      if (f >= 0.9999)
        h += x1;
      else {
        x = CLHEP::RandFlat::shoot();
        double wlog;
        while (1) {
          wlog = log(x) / f;
          if (wlog > -100.) {
            double w1 = exp(wlog);
            x = CLHEP::RandFlat::shoot();
            wlog = log(x) / (1. - f);
            if (wlog < -100.) {
              h += x1;
              break;
            } else {
              double w = w1 + exp(wlog);
              if (w <= 1.) {
                h += w1 / w;
                break;
              }
            }
          }
        }
      }
    }
  }
  return h;
}