include/Geant4/G4RandomDirection.hh

0001 //
0002 // ********************************************************************
0003 // * License and Disclaimer                                           *
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0025 //
0026 //
0027 //
0028 //
0029 // ------------------------------------------------------------
0030 //      GEANT 4 class header file
0031 // ------------------------------------------------------------
0032 // Class description:
0033 //
0034 // Function returning a unit 3-vector homogeneously randomised over 4pi
0035 // solid angle. It can be used in any particle scattering methods
0036 // instead of:
0037 //   z=R1, x=SQRT(1-R1*R1)*SIN(2*pi*R2), y=SQRT(1-R1*R1)*COS(2*pi*R2)
0038 // providing more performant results.
0039
0040 // History:
0041 //    19.10.17 E. Tcherniaev, use of G.Marsaglia (1972) method
0042 //    06.04.17 E. Tcherniaev, added G4RandomDirection(cosTheta)
0043 //    15.03.16 E. Tcherniaev, removed unnecessary if and unit()
0044 //    18.03.08 V. Grichine, unit radius sphere surface based algorithm
0045 //      ~ 2007 M. Kossov, algorithm based on 8 Quadrants technique
0046 //
0047 // ------------------------------------------------------------
0048 #ifndef G4RANDOMDIR_HH
0049 #define G4RANDOMDIR_HH
0050
0051 #include <CLHEP/Units/PhysicalConstants.h>
0052
0053 #include "G4ThreeVector.hh"
0054 #include "Randomize.hh"
0055 #include "globals.hh"
0056
0057 // G.Marsaglia (1972) method
0058 inline G4ThreeVector G4RandomDirection()
0059 {
0060   G4double u, v, b;
0061   do
0062   {
0063     u = 2. * G4UniformRand() - 1.;
0064     v = 2. * G4UniformRand() - 1.;
0065     b = u * u + v * v;
0066   } while(b > 1.);
0067   G4double a = 2. * std::sqrt(1. - b);
0068   return G4ThreeVector(a * u, a * v, 2. * b - 1.);
0069 }
0070
0071 inline G4ThreeVector G4RandomDirection(G4double cosTheta)
0072 {
0073   G4double z   = (1. - cosTheta) * G4UniformRand() + cosTheta;
0074   G4double rho = std::sqrt((1. + z) * (1. - z));
0075   G4double phi = CLHEP::twopi * G4UniformRand();
0076   return G4ThreeVector(rho * std::cos(phi), rho * std::sin(phi), z);
0077 }
0078
0079 #endif /* G4RANDOMDIR_HH */