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G4ModifiedMidpoint.cc
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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 
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 {
54  setValue(yIn, Value1D::LabTime, y0, y1, yTemp, yOut);
55 
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 {
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 }