/*=========================================================================

  Program:   Visualization Toolkit
  Module:    vtkRungeKutta4.cxx

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
#include "vtkRungeKutta4.h"

#include "vtkFunctionSet.h"
#include "vtkObjectFactory.h"

vtkStandardNewMacro(vtkRungeKutta4);

vtkRungeKutta4::vtkRungeKutta4()
{
  for(int i=0; i<3; i++)
  {
    this->NextDerivs[i] = nullptr;
  }
}

vtkRungeKutta4::~vtkRungeKutta4()
{
  for(int i=0; i<3; i++)
  {
    delete[] this->NextDerivs[i];
    this->NextDerivs[i] = nullptr;
  }
}


void vtkRungeKutta4::Initialize()
{
  this->vtkInitialValueProblemSolver::Initialize();
  if (!this->FunctionSet || !this->Initialized)
  {
    return;
  }
  // Allocate memory for temporary derivatives array
  for(int i=0; i<3; i++)
  {
    delete[] this->NextDerivs[i];
    this->NextDerivs[i] =
      new double[this->FunctionSet->GetNumberOfFunctions()];
  }
}

// For a detailed description of Runge-Kutta methods,
// see, for example, Numerical Recipes in (C/Fortran/Pascal) by
// Press et al. (Cambridge University Press) or
// Applied Numerical Analysis by C. F. Gerald and P. O. Wheatley
// (Addison Wesley)
int vtkRungeKutta4::ComputeNextStep(double* xprev, double* dxprev, double* xnext,
                                    double t, double& delT, double& delTActual,
                                    double, double, double, double& error, void* userData)
{

  int i, numDerivs, numVals;

  delTActual = 0;
  error = 0;

  if (!this->FunctionSet)
  {
    vtkErrorMacro("No derivative functions are provided!");
    return NOT_INITIALIZED;
  }

  if (!this->Initialized)
  {
    vtkErrorMacro("Integrator not initialized!");
    return NOT_INITIALIZED;
  }

  numDerivs = this->FunctionSet->GetNumberOfFunctions();
  numVals = numDerivs + 1;
  for(i=0; i<numVals-1; i++)
  {
    this->Vals[i] = xprev[i];
  }
  this->Vals[numVals-1] = t;

  //  4th order
  //  1
  if (dxprev)
  {
    for(i=0; i<numDerivs; i++)
    {
      this->Derivs[i] = dxprev[i];
    }
  }
  else if ( !this->FunctionSet->FunctionValues(this->Vals, this->Derivs, userData) )
  {
    memcpy(xnext, this->Vals, (numVals-1)*sizeof(double));
    return OUT_OF_DOMAIN;
  }

  for(i=0; i<numVals-1; i++)
  {
    this->Vals[i] = xprev[i] + delT/2.0*this->Derivs[i];
  }
  this->Vals[numVals-1] = t + delT/2.0;

  // 2
  if (!this->FunctionSet->FunctionValues(this->Vals, this->NextDerivs[0], userData))
  {
    memcpy(xnext, this->Vals, (numVals-1)*sizeof(double));
    delTActual = delT/2.0; // we've been able to take half a step
    return OUT_OF_DOMAIN;
  }

  for(i=0; i<numVals-1; i++)
  {
    this->Vals[i] = xprev[i] + delT/2.0*this->NextDerivs[0][i];
  }
  this->Vals[numVals-1] = t + delT/2.0;

  // 3
  if (!this->FunctionSet->FunctionValues(this->Vals, this->NextDerivs[1], userData))
  {
    memcpy(xnext, this->Vals, (numVals-1)*sizeof(double));
    delTActual = delT/2.0; // we've been able to take half a step
    return OUT_OF_DOMAIN;
  }

  for(i=0; i<numVals-1; i++)
  {
    this->Vals[i] = xprev[i] + delT*this->NextDerivs[1][i];
  }
  this->Vals[numVals-1] = t + delT;

  // 4
  if (!this->FunctionSet->FunctionValues(this->Vals, this->NextDerivs[2], userData))
  {
    memcpy(xnext, this->Vals, (numVals-1)*sizeof(double));
    delTActual = delT; // we've been able to take a full step but couldn't finish the algorithm
    return OUT_OF_DOMAIN;
  }

  for(i=0; i<numDerivs; i++)
  {
    xnext[i] = xprev[i] + delT*(this->Derivs[i]/6.0 +
                                this->NextDerivs[0][i]/3.0 +
                                this->NextDerivs[1][i]/3.0 +
                                this->NextDerivs[2][i]/6.0);
  }
  delTActual = delT;
  return 0;
}

void vtkRungeKutta4::PrintSelf(ostream& os, vtkIndent indent)
{
  this->Superclass::PrintSelf(os,indent);
  os << indent << "Runge-Kutta 4 function derivatives: "
     << this->NextDerivs[0] << " " << this->NextDerivs[1]  << " "
     << this->NextDerivs[2] << endl;
}






