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24 //
25 // G4ModifiedMidpoint implementation
26 //
27 // Author: Dmitry Sorokin, Google Summer of Code 2016
28 // Supervision: John Apostolakis, CERN
29 // --------------------------------------------------------------------
30
31 #include "G4ModifiedMidpoint.hh"
32 #include "G4FieldUtils.hh"
33
34 using namespace field_utils;
35
36 G4ModifiedMidpoint::G4ModifiedMidpoint( G4EquationOfMotion* equation,
37 G4int nvar, G4int steps )
38 : fEquation(equation), fnvar(nvar), fsteps(steps)
39 {
40 if (nvar <= 0)
41 {
42 G4Exception("G4ModifiedMidpoint::G4ModifiedMidpoint()",
43 "GeomField0002", FatalException,
44 "Invalid number of variables; must be greater than zero!");
45 }
46 }
47
48 void G4ModifiedMidpoint::DoStep( const G4double yIn[], const G4double dydyIn[],
49 G4double yOut[], G4double hstep) const
50 {
51 G4double y0[G4FieldTrack::ncompSVEC];
52 G4double y1[G4FieldTrack::ncompSVEC];
53 G4double yTemp[G4FieldTrack::ncompSVEC];
54 setValue(yIn, Value1D::LabTime, y0, y1, yTemp, yOut);
55
56 G4double dydx[G4FieldTrack::ncompSVEC];
57
58 const G4double h = hstep / fsteps;
59 const G4double h2 = 2 * h;
60
61 // y1 = yIn + h * dydx
62 //
63 for (G4int i = 0; i < fnvar; ++i)
64 {
65 y1[i] = yIn[i] + h * dydyIn[i];
66 }
67
68 fEquation->RightHandSide(y1, dydx);
69
70 copy(y0, yIn);
71
72 // general step
73 // yTemp = y1; y1 = y0 + h2 * dydx; y0 = yTemp
74 //
75 for (G4int i = 1; i < fsteps; ++i)
76 {
77 copy(yTemp, y1);
78 for (G4int j = 0; j < fnvar; ++j)
79 {
80 y1[j] = y0[j] + h2 * dydx[j];
81 }
82 copy(y0, yTemp);
83
84 fEquation->RightHandSide(y1, dydx);
85 }
86
87 // last step
88 // yOut = 0.5 * (y0 + y1 + h * dydx)
89 //
90 for (G4int i = 0; i < fnvar; ++i)
91 {
92 yOut[i] = 0.5 * (y0[i] + y1[i] + h * dydx[i]);
93 }
94 }
95
96 void G4ModifiedMidpoint::DoStep( const G4double yIn[], const G4double dydxIn[],
97 G4double yOut[], G4double hstep, G4double yMid[],
98 G4double derivs[][G4FieldTrack::ncompSVEC]) const
99 {
100 G4double y0[G4FieldTrack::ncompSVEC];
101 G4double y1[G4FieldTrack::ncompSVEC];
102 G4double yTemp[G4FieldTrack::ncompSVEC];
103 setValue(yIn, Value1D::LabTime, y0, y1, yTemp, yMid, yOut);
104
105 const G4double h = hstep / fsteps;
106 const G4double h2 = 2 * h;
107
108 // y0 = yIn
109 copy(y0, yIn);
110
111 // y1 = y0 + h * dydx
112 for (G4int i = 0; i < fnvar; ++i)
113 {
114 y1[i] = y0[i] + h * dydxIn[i];
115 }
116
117 // result of first step already gives approximation
118 // at the center of the interval
119 //
120 if(fsteps == 2)
121 {
122 copy(yMid, y1);
123 }
124
125 fEquation->RightHandSide(y1, derivs[0]);
126
127 // general step
128 // yTemp = y1; y1 = y0 + h2 * dydx; y0 = yTemp
129 //
130 for (G4int i = 1; i < fsteps; ++i)
131 {
132 copy(yTemp, y1);
133 for (G4int j = 0; j < fnvar; ++j)
134 {
135 y1[j] = y0[j] + h2 * derivs[i-1][j];
136 }
137 copy(y0, yTemp);
138
139 // save approximation at the center of the interval
140 if(i == fsteps / 2 - 1 )
141 {
142 copy(yMid, y1);
143 }
144
145 fEquation->RightHandSide(y1, derivs[i]);
146 }
147
148 // last step
149 // yOut = 0.5 * (y0 + y1 + h * dydx)
150 //
151 for (G4int i = 0; i < fnvar; ++i)
152 {
153 yOut[i] = 0.5 * (y0[i] + y1[i] + h * derivs[fsteps-1][i]);
154 }
155 }
156
157 void G4ModifiedMidpoint::copy(G4double dst[], const G4double src[]) const
158 {
159 std::memcpy(dst, src, sizeof(G4double) * fnvar);
160 }
161