// SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
// SPDX-License-Identifier: BSD-3-Clause
#include "vtkRungeKutta4.h"

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

VTK_ABI_NAMESPACE_BEGIN
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;
}
VTK_ABI_NAMESPACE_END
