geant4/source/geometry/magneticfield/include/G4QSSDriver.icc

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template <class T>
G4QSSDriver<T>::G4QSSDriver(T* pStepper) : G4InterpolationDriver<T, true>(0, pStepper)
{
  // TODO: Remove additional stepper instances - should be a separate Driver class
  this->fSteppers.resize(1);
}

template <class T>
void G4QSSDriver<T>::OnStartTracking()
{
  Base::OnStartTracking();
  if (! initializedOnFirstRun) {
    // this->SetPrecision( G4QSSMessenger::instance()->dQRel, G4QSSMessenger::instance()->dQMin);

    G4double dqRel = G4QSSMessenger::instance()->dQRel;
    G4double dQMin = G4QSSMessenger::instance()->dQMin;
    if (dqRel == 0) {
      dqRel = 0.001;
    }
    if (dQMin == 0) {
      dQMin = 0.0001;
    }
    this->SetPrecision(dqRel, dQMin);

    initializedOnFirstRun = true;
  }
}

template <class T>
void G4QSSDriver<T>::SetPrecision(G4double dq_rel, G4double dq_min)
{
  G4cout << "Setting QSS precision parameters: "
         << "dQRel = " << dq_rel << " - "
         << "dQMin = " << dq_min << G4endl;

  for (const auto& item : this->fSteppers) {
    item.stepper->SetPrecision(dq_rel, dq_min);
  }
}

template <class T>
G4double G4QSSDriver<T>::AdvanceChordLimited(
  G4FieldTrack& track, G4double hstep, G4double epsStep, G4double chordDistance)
{
  // For now, just extract functionality that we don't use from G4InterpolationDriver
  // We should probably end up making  a custom G4QSSDriver separated from it
  ++this->fTotalStepsForTrack;

  // SetPrecision(10*epsStep, epsStep); // Propagate the required accuracy to QSS

  const G4double curveLengthBegin = track.GetCurveLength();
  const G4double hend = std::min(hstep, this->fChordStepEstimate);
  G4double hdid = 0.0;
  auto it = this->fSteppers.begin();
  G4double dChordStep = 0.0;

  field_utils::State yBegin, y;
  track.DumpToArray(yBegin);
  track.DumpToArray(y);

  if (this->fFirstStep) {
    Base::GetEquationOfMotion()->RightHandSide(y, this->fdydx);
    this->fFirstStep = false;
  }

  if (this->fKeepLastStepper) {
    std::swap(*this->fSteppers.begin(), *this->fLastStepper);
    it = this->fSteppers.begin();  // new begin, update iterator
    this->fLastStepper = it;
    hdid = it->end - curveLengthBegin;
    if (hdid > hend) {
      hdid = hend;
      this->InterpolateImpl(curveLengthBegin + hdid, it, y);
    }
    else {
      field_utils::copy(y, it->stepper->GetYOut());
    }

    dChordStep = this->DistChord(yBegin, curveLengthBegin, y, curveLengthBegin + hdid);

    ++it;
  }

  // accurate advance & check chord distance
  G4double h = this->fhnext;
  for (; hdid < hend && dChordStep < chordDistance && it != this->fSteppers.end(); ++it) {
    h = hstep;  // h = std::min(h, hstep - hdid); <--- Omit

    // make one step
    hdid += OneGoodStep(it, y, this->fdydx, h, epsStep, curveLengthBegin + hdid, &track);

    // update last stepper
    this->fLastStepper = it;

    G4double dcTmp = this->DistChord(yBegin, curveLengthBegin, y, curveLengthBegin + hdid);
    // estimate chord distance
    dChordStep = std::max(dChordStep, dcTmp);
    //  this->DistChord(yBegin, curveLengthBegin, y, curveLengthBegin + hdid) );

    // std::cout << "QSSdrv:  h= " << h << " hdid= " << hdid << " dcTmp= " << dcTmp << " dChord= "
    // << dChordStep << std::endl;
  }

  // Now, either
  //   - full integration ( hdid >= hend )
  //   - estimated chord has exceeded limit 'chordDistance'
  //   - reached maximum number of steps (from number of steppers.)

  // update step estimation
  if (h > this->fMinimumStep) {
    this->fhnext = h;
  }

  // CheckState();

  // update chord step estimate
  //
  hdid = this->FindNextChord(
    yBegin, curveLengthBegin, y, curveLengthBegin + hdid, dChordStep, chordDistance);

  const G4double curveLengthEnd = curveLengthBegin + hdid;
  this->fKeepLastStepper = this->fLastStepper->end - curveLengthEnd > CLHEP::perMillion;
  track.LoadFromArray(y, this->fLastStepper->stepper->GetNumberOfVariables());
  track.SetCurveLength(curveLengthBegin + hdid);

  return hdid;
}

template <class T>
G4double G4QSSDriver<T>::OneGoodStep(typename G4InterpolationDriver<T, true>::StepperIterator it,
  field_utils::State& y, field_utils::State& dydx, G4double& hstep, G4double /*epsStep*/,
  G4double curveLength, G4FieldTrack* /*track*/)
{
  G4double yerr[G4FieldTrack::ncompSVEC], ytemp[G4FieldTrack::ncompSVEC];
  G4double h = hstep;
  it->stepper->Stepper(y, dydx, h, ytemp, yerr);

  // set interpolation inverval
  it->begin = curveLength;
  it->end = curveLength + h;
  it->inverseLength = 1. / h;

  field_utils::copy(y, ytemp);

  return h;
}