Gabriele Cosmo
144 lines
4.6 KiB
C++
144 lines
4.6 KiB
C++
//
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// ********************************************************************
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// * License and Disclaimer *
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// * *
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// * The Geant4 software is copyright of the Copyright Holders of *
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// * the Geant4 Collaboration. It is provided under the terms and *
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// * conditions of the Geant4 Software License, included in the file *
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// * LICENSE and available at http://cern.ch/geant4/license . These *
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// * include a list of copyright holders. *
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// * *
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// * Neither the authors of this software system, nor their employing *
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// * institutes,nor the agencies providing financial support for this *
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// * work make any representation or warranty, express or implied, *
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// * regarding this software system or assume any liability for its *
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// * use. Please see the license in the file LICENSE and URL above *
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// * for the full disclaimer and the limitation of liability. *
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// * *
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// * This code implementation is the result of the scientific and *
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// * technical work of the GEANT4 collaboration. *
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// * By using, copying, modifying or distributing the software (or *
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// * any work based on the software) you agree to acknowledge its *
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// * use in resulting scientific publications, and indicate your *
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// * acceptance of all terms of the Geant4 Software license. *
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// ********************************************************************
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//
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//
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//
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// ------------------------------------------------------------
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// GEANT 4 class implementation
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// ------------------------------------------------------------
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#include <cmath>
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#include <string.h>
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#include "Gamma.hh"
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MyGamma::MyGamma(){}
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MyGamma::~MyGamma(){}
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//____________________________________________________________________________
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double MyGamma::Gamma(double z)
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{
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if (z <= 0)
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return 0;
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return std::tgamma(z);
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}
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//____________________________________________________________________________
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double MyGamma::Gamma(double a,double x)
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{
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// Computation of the incomplete gamma function P(a,x)
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//
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// The algorithm is based on the formulas and code as denoted in
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// Numerical Recipes 2nd ed. on p. 210-212 (W.H.Press et al.).
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//
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//--- Nve 14-nov-1998 UU-SAP Utrecht
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if (a <= 0 || x <= 0) return 0;
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if (x < (a+1)) return GamSer(a,x);
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else return GamCf(a,x);
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}
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//____________________________________________________________________________
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double MyGamma::GamCf(double a,double x)
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{
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// Computation of the incomplete gamma function P(a,x)
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// via its continued fraction representation.
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//
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// The algorithm is based on the formulas and code as denoted in
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// Numerical Recipes 2nd ed. on p. 210-212 (W.H.Press et al.).
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//
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//--- Nve 14-nov-1998 UU-SAP Utrecht
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int itmax = 100; // Maximum number of iterations
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double eps = 3.e-7; // Relative accuracy
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double fpmin = 1.e-30; // Smallest double value allowed here
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if (a <= 0 || x <= 0) return 0;
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double gln = LnGamma(a);
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double b = x+1-a;
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double c = 1/fpmin;
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double d = 1/b;
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double h = d;
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double an,del;
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for (int i=1; i<=itmax; i++) {
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an = double(-i)*(double(i)-a);
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b += 2;
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d = an*d+b;
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if (Abs(d) < fpmin) d = fpmin;
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c = b+an/c;
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if (Abs(c) < fpmin) c = fpmin;
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d = 1/d;
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del = d*c;
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h = h*del;
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if (Abs(del-1) < eps) break;
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//if (i==itmax) cout << "*GamCf(a,x)* a too large or itmax too small" << endl;
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}
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double v = Exp(-x+a*Log(x)-gln)*h;
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return (1-v);
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}
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//____________________________________________________________________________
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double MyGamma::GamSer(double a,double x)
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{
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// Computation of the incomplete gamma function P(a,x)
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// via its series representation.
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//
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// The algorithm is based on the formulas and code as denoted in
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// Numerical Recipes 2nd ed. on p. 210-212 (W.H.Press et al.).
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//
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//--- Nve 14-nov-1998 UU-SAP Utrecht
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int itmax = 100; // Maximum number of iterations
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double eps = 3.e-7; // Relative accuracy
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if (a <= 0 || x <= 0) return 0;
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double gln = LnGamma(a);
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double ap = a;
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double sum = 1/a;
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double del = sum;
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for (int n=1; n<=itmax; n++) {
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ap += 1;
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del = del*x/ap;
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sum += del;
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if (MyGamma::Abs(del) < Abs(sum*eps)) break;
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//if (n==itmax) cout << "*GamSer(a,x)* a too large or itmax too small" << endl;
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}
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double v = sum*Exp(-x+a*Log(x)-gln);
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return v;
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}
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double MyGamma::LnGamma(double z)
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{
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if (z <= 0)
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return 0;
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return std::lgamma(z);
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}
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