include/Geant4/G4ConstRK4.hh

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0025 //
0026 // G4ConstRK4
0027 //
0028 // Class description:
0029 //
0030 // G4ConstRK4 performs the integration of one step with error calculation
0031 // in constant magnetic field. The integration method is the same as in
0032 // ClassicalRK4. The field value is assumed constant for the step.
0033 // This field evaluation is called only once per step.
0034 // G4ConstRK4 can be used only for magnetic fields.
0035
0036 // Authors: J.Apostolakis, T.Nikitina (CERN), 18.09.2008
0037 // -------------------------------------------------------------------
0038 #ifndef G4CONSTRK4_HH
0039 #define G4CONSTRK4_HH
0040
0041 #include "G4MagErrorStepper.hh"
0042 #include "G4EquationOfMotion.hh"
0043 #include "G4Mag_EqRhs.hh"
0044
0045 /**
0046  * @brief G4ConstRK4 performs the integration of one step with error
0047  * calculation in constant magnetic field. The integration method is the
0048  * same as in ClassicalRK4. The field value is assumed constant for the step.
0049  * This field evaluation is called only once per step.
0050  * G4ConstRK4 can be used only for magnetic fields.
0051  */
0052
0053 class G4ConstRK4 : public G4MagErrorStepper
0054 {
0055    public:
0056
0057     /**
0058      * Constructor for G4ConstRK4.
0059      *  @param[in] EqRhs Pointer to the provided equation of motion.
0060      *  @param[in] numberOfVariables The number of integration variables.
0061      */
0062      G4ConstRK4(G4Mag_EqRhs* EquationMotion,
0063                 G4int numberOfStateVariables=8);
0064
0065     /**
0066      * Destructor.
0067      */
0068      ~G4ConstRK4() override;
0069
0070     /**
0071      * Copy constructor and assignment operator not allowed.
0072      */
0073      G4ConstRK4(const G4ConstRK4&) = delete;
0074      G4ConstRK4& operator=(const G4ConstRK4&) = delete;
0075
0076     /**
0077      * The stepper for the Runge Kutta integration.
0078      * The stepsize is fixed, with the step size given by 'h'.
0079      * Integrates ODE starting values y[0 to 6].
0080      * Outputs yout[] and its estimated error yerr[].
0081      *  @param[in] y Starting values array of integration variables.
0082      *  @param[in] dydx Derivatives array.
0083      *  @param[in] h The given step size.
0084      *  @param[out] yout Integration output.
0085      *  @param[out] yerr The estimated error.
0086      */
0087      void Stepper( const G4double y[],
0088                    const G4double dydx[],
0089                          G4double h,
0090                          G4double yout[],
0091                          G4double yerr[]  ) override;
0092
0093     /**
0094      * Given values for the variables y[0,..,n-1] and their derivatives
0095      * dydx[0,...,n-1] known at x, uses the classical 4th Runge-Kutta
0096      * method to advance the solution over an interval h and returns the
0097      * incremented variables as yout[0,...,n-1]. The user supplies the
0098      * function RightHandSide(x,y,dydx), which returns derivatives dydx at x.
0099      * The source is routine rk4 from NRC p.712-713.
0100      *  @param[in] y Starting values array of integration variables.
0101      *  @param[in] dydx Derivatives array.
0102      *  @param[in] h The given step size.
0103      *  @param[out] yout Integration output.
0104      */
0105      void DumbStepper( const G4double yIn[],
0106                        const G4double dydx[],
0107                              G4double h,
0108                              G4double yOut[] ) override ;
0109
0110     /**
0111      * Returns the distance from chord line.
0112      */
0113      G4double DistChord() const override;
0114
0115     /**
0116      * Returns the derivatives value, at position and time 'y'.
0117      *  @param[in] y The position vector plus time (x,y,z,t).
0118      *  @param[out] dydx The derivatives array.
0119      */
0120      inline void RightHandSideConst(const G4double y[], G4double dydx[] ) const;
0121
0122     /**
0123      * Returns the field values, at position and time 'y'.
0124      *  @param[in] y The position vector plus time (x,y,z,t).
0125      *  @param[out] Field The field value in output.
0126      */
0127      inline void GetConstField(const G4double y[], G4double Field[]);
0128
0129     /**
0130      * Returns the order, 4, of integration.
0131      */
0132      inline G4int IntegratorOrder() const override { return 4; }
0133
0134     /**
0135      * Returns the stepper type-ID, "kConstRK4".
0136      */
0137      inline G4StepperType StepperType() const override { return kConstRK4; }
0138
0139    private:
0140
0141      G4ThreeVector fInitialPoint, fMidPoint, fFinalPoint;
0142      // Data stored in order to find the chord
0143      G4double *dydxm, *dydxt, *yt; // scratch space - not state
0144      G4double *yInitial, *yMiddle, *dydxMid, *yOneStep;
0145      G4Mag_EqRhs* fEq = nullptr;
0146      G4double Field[3];
0147 };
0148
0149 // Inline methods
0150
0151 inline void G4ConstRK4::RightHandSideConst(const G4double y[],
0152                                                  G4double dydx[] ) const
0153 {
0154
0155   G4double momentum_mag_square = y[3]*y[3] + y[4]*y[4] + y[5]*y[5];
0156   G4double inv_momentum_magnitude = 1.0 / std::sqrt( momentum_mag_square );
0157
0158   G4double cof = fEq->FCof()*inv_momentum_magnitude;
0159
0160   dydx[0] = y[3]*inv_momentum_magnitude;       //  (d/ds)x = Vx/V
0161   dydx[1] = y[4]*inv_momentum_magnitude;       //  (d/ds)y = Vy/V
0162   dydx[2] = y[5]*inv_momentum_magnitude;       //  (d/ds)z = Vz/V
0163
0164   dydx[3] = cof*(y[4]*Field[2] - y[5]*Field[1]) ;   // Ax = a*(Vy*Bz - Vz*By)
0165   dydx[4] = cof*(y[5]*Field[0] - y[3]*Field[2]) ;   // Ay = a*(Vz*Bx - Vx*Bz)
0166   dydx[5] = cof*(y[3]*Field[1] - y[4]*Field[0]) ;   // Az = a*(Vx*By - Vy*Bx)
0167 }
0168
0169 inline void G4ConstRK4::GetConstField(const G4double y[], G4double B[])
0170 {
0171   G4double PositionAndTime[4];
0172
0173   PositionAndTime[0] = y[0];
0174   PositionAndTime[1] = y[1];
0175   PositionAndTime[2] = y[2];
0176   // Global Time
0177   PositionAndTime[3] = y[7];
0178   fEq -> GetFieldValue(PositionAndTime, B);
0179 }
0180
0181 #endif