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25 //
26 // G4MagErrorStepper implementation
27 //
28 // Author: W.Wander <wwc@mit.edu>, 09.12.1997
29 // --------------------------------------------------------------------
30
31 #include "G4MagErrorStepper.hh"
32 #include "G4LineSection.hh"
33
34 G4MagErrorStepper::~G4MagErrorStepper()
35 {
36 delete [] yMiddle;
37 delete [] dydxMid;
38 delete [] yInitial;
39 delete [] yOneStep;
40 }
41
42 void G4MagErrorStepper::Stepper( const G4double yInput[],
43 const G4double dydx[],
44 G4double hstep,
45 G4double yOutput[],
46 G4double yError [] )
47 {
48 const G4int nvar = GetNumberOfVariables();
49 const G4int maxvar = GetNumberOfStateVariables();
50
51 // correction for Richardson Extrapolation.
52 //
53 G4double correction = 1. / ( (1 << IntegratorOrder()) -1 );
54
55 // Saving yInput because yInput and yOutput can be aliases for same array
56 //
57 for(G4int i=0; i<nvar; ++i)
58 {
59 yInitial[i]=yInput[i];
60 }
61 yInitial[7] = yInput[7]; // Copy the time in case...even if not really needed
62 yMiddle[7] = yInput[7]; // Copy the time from initial value
63 yOneStep[7] = yInput[7]; // As it contributes to final value of yOutput ?
64 // yOutput[7] = yInput[7]; // -> dumb stepper does it too for RK4
65
66 for(G4int i=nvar; i<maxvar; ++i)
67 {
68 yOutput[i]=yInput[i];
69 }
70 // yError[7] = 0.0;
71
72 G4double halfStep = hstep * 0.5;
73
74 // Do two half steps
75 //
76 DumbStepper (yInitial, dydx, halfStep, yMiddle);
77 RightHandSide(yMiddle, dydxMid);
78 DumbStepper (yMiddle, dydxMid, halfStep, yOutput);
79
80 // Store midpoint, chord calculation
81 //
82 fMidPoint = G4ThreeVector( yMiddle[0], yMiddle[1], yMiddle[2]);
83
84 // Do a full Step
85 //
86 DumbStepper(yInitial, dydx, hstep, yOneStep);
87 for(G4int i=0; i<nvar; ++i)
88 {
89 yError [i] = yOutput[i] - yOneStep[i] ;
90 yOutput[i] += yError[i]*correction ;
91 // Provides accuracy increased by 1 order via Richardson Extrapolation
92 }
93
94 fInitialPoint = G4ThreeVector( yInitial[0], yInitial[1], yInitial[2]);
95 fFinalPoint = G4ThreeVector( yOutput[0], yOutput[1], yOutput[2]);
96
97 return;
98 }
99
100 G4double G4MagErrorStepper::DistChord() const
101 {
102 // Estimate the maximum distance from the curve to the chord
103 //
104 // We estimate this using the distance of the midpoint to
105 // chord (the line between
106 //
107 // Method below is good only for angle deviations < 2 pi,
108 // This restriction should not a problem for the Runge cutta methods,
109 // which generally cannot integrate accurately for large angle deviations.
110
111 G4double distLine, distChord;
112
113 if (fInitialPoint != fFinalPoint)
114 {
115 distLine = G4LineSection::Distline(fMidPoint, fInitialPoint, fFinalPoint);
116 // This is a class method that gives distance of Mid
117 // from the Chord between the Initial and Final points.
118
119 distChord = distLine;
120 }
121 else
122 {
123 distChord = (fMidPoint-fInitialPoint).mag();
124 }
125
126 return distChord;
127 }
128