/*=========================================================================
 *
 *  Copyright NumFOCUS
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *         https://www.apache.org/licenses/LICENSE-2.0.txt
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *=========================================================================*/
#ifndef itkVectorCurvatureNDAnisotropicDiffusionFunction_hxx
#define itkVectorCurvatureNDAnisotropicDiffusionFunction_hxx


namespace itk
{
template <typename TImage>
double VectorCurvatureNDAnisotropicDiffusionFunction<TImage>::m_MIN_NORM = 1.0e-10;

template <typename TImage>
VectorCurvatureNDAnisotropicDiffusionFunction<TImage>::VectorCurvatureNDAnisotropicDiffusionFunction()

{
  unsigned int i, j;
  RadiusType   r;

  for (i = 0; i < ImageDimension; ++i)
  {
    r[i] = 1;
  }
  this->SetRadius(r);

  // Dummy neighborhood used to set up the slices.
  Neighborhood<PixelType, ImageDimension> it;
  it.SetRadius(r);

  // Slice the neighborhood
  m_Center = it.Size() / 2;

  for (i = 0; i < ImageDimension; ++i)
  {
    m_Stride[i] = it.GetStride(i);
  }

  for (i = 0; i < ImageDimension; ++i)
  {
    x_slice[i] = std::slice(m_Center - m_Stride[i], 3, m_Stride[i]);
  }

  for (i = 0; i < ImageDimension; ++i)
  {
    for (j = 0; j < ImageDimension; ++j)
    {
      // For taking derivatives in the i direction that are offset one
      // pixel in the j direction.
      xa_slice[i][j] = std::slice((m_Center + m_Stride[j]) - m_Stride[i], 3, m_Stride[i]);
      xd_slice[i][j] = std::slice((m_Center - m_Stride[j]) - m_Stride[i], 3, m_Stride[i]);
    }
  }

  // Allocate the derivative operator.
  m_DerivativeOperator.SetDirection(0); // Not relevant, we'll apply in a slice-based
                                        // fashion
  m_DerivativeOperator.SetOrder(1);
  m_DerivativeOperator.CreateDirectional();
}

template <typename TImage>
auto
VectorCurvatureNDAnisotropicDiffusionFunction<TImage>::ComputeUpdate(const NeighborhoodType & it,
                                                                     void *,
                                                                     const FloatOffsetType &) -> PixelType
{
  unsigned int i, j, k;
  double       speed;
  double       dx_forward_Cn[ImageDimension][VectorDimension];
  double       dx_backward_Cn[ImageDimension][VectorDimension];
  double       propagation_gradient;
  double       grad_mag_sq[VectorDimension];
  double       grad_mag_sq_d[VectorDimension];
  double       grad_mag[VectorDimension];
  double       grad_mag_d[VectorDimension];
  double       Cx[ImageDimension];
  double       Cxd[ImageDimension];

  const ScalarValueType ScalarValueTypeZero{};

  PixelType dx_forward[ImageDimension];
  PixelType dx_backward[ImageDimension];
  PixelType dx[ImageDimension];
  PixelType dx_aug;
  PixelType dx_dim;
  PixelType ans;

  // Calculate the partial derivatives for each dimension
  for (i = 0; i < ImageDimension; ++i)
  {
    // "Half" derivatives
    dx_forward[i] = it.GetPixel(m_Center + m_Stride[i]) - it.GetPixel(m_Center);
    dx_forward[i] = dx_forward[i] * this->m_ScaleCoefficients[i];
    dx_backward[i] = it.GetPixel(m_Center) - it.GetPixel(m_Center - m_Stride[i]);
    dx_backward[i] = dx_backward[i] * this->m_ScaleCoefficients[i];

    // Centralized differences
    dx[i] = m_InnerProduct(x_slice[i], it, m_DerivativeOperator);
    dx[i] = dx[i] * this->m_ScaleCoefficients[i];
  }

  for (k = 0; k < VectorDimension; ++k)
  {
    grad_mag_sq[k] = 0.0;
    grad_mag_sq_d[k] = 0.0;
    for (i = 0; i < ImageDimension; ++i)
    {
      // Gradient magnitude approximations
      grad_mag_sq[k] += dx_forward[i][k] * dx_forward[i][k];
      grad_mag_sq_d[k] += dx_backward[i][k] * dx_backward[i][k];
      for (j = 0; j < ImageDimension; ++j)
      {
        if (j != i)
        {
          dx_aug = m_InnerProduct(xa_slice[j][i], it, m_DerivativeOperator);
          dx_aug = dx_aug * this->m_ScaleCoefficients[j];
          dx_dim = m_InnerProduct(xd_slice[j][i], it, m_DerivativeOperator);
          dx_dim = dx_dim * this->m_ScaleCoefficients[j];
          grad_mag_sq[k] += 0.25f * (dx[j][k] + dx_aug[k]) * (dx[j][k] + dx_aug[k]);
          grad_mag_sq_d[k] += 0.25f * (dx[j][k] + dx_dim[k]) * (dx[j][k] + dx_dim[k]);
        }
      }
    }

    grad_mag[k] = std::sqrt(m_MIN_NORM + grad_mag_sq[k]);
    grad_mag_d[k] = std::sqrt(m_MIN_NORM + grad_mag_sq_d[k]);
    // this grad mag should depend only on the current k
    for (i = 0; i < ImageDimension; ++i)
    {
      dx_forward_Cn[i][k] = dx_forward[i][k] / grad_mag[k];
      dx_backward_Cn[i][k] = dx_backward[i][k] / grad_mag_d[k];
    }
  }

  double grad_mag_sq_tmp = 0.0;
  double grad_mag_sq_d_tmp = 0.0;

  for (k = 0; k < VectorDimension; ++k)
  {
    grad_mag_sq_tmp += grad_mag_sq[k];
    grad_mag_sq_d_tmp += grad_mag_sq_d[k];
  }

  // this grad mag should depend on the sum over k's
  // Conductance Terms

  for (i = 0; i < ImageDimension; ++i)
  {
    if (m_K == 0.0)
    {
      Cx[i] = 0.0;
      Cxd[i] = 0.0;
    }
    else
    {
      Cx[i] = std::exp(grad_mag_sq_tmp / m_K);
      Cxd[i] = std::exp(grad_mag_sq_d_tmp / m_K);
    }
  }

  for (k = 0; k < VectorDimension; ++k)
  {
    // First order normalized finite-difference conductance products
    speed = 0.0;
    for (i = 0; i < ImageDimension; ++i)
    {
      dx_forward_Cn[i][k] *= Cx[i];
      dx_backward_Cn[i][k] *= Cxd[i];

      // Second order conductance-modified curvature
      speed += (dx_forward_Cn[i][k] - dx_backward_Cn[i][k]);
    }

    // "Upwind" gradient magnitude term
    propagation_gradient = 0.0;
    if (speed > 0.0)
    {
      for (i = 0; i < ImageDimension; ++i)
      {
        propagation_gradient += itk::Math::sqr(std::min(dx_backward[i][k], ScalarValueTypeZero)) +
                                itk::Math::sqr(std::max(dx_forward[i][k], ScalarValueTypeZero));
      }
    }
    else
    {
      for (i = 0; i < ImageDimension; ++i)
      {
        propagation_gradient += itk::Math::sqr(std::max(dx_backward[i][k], ScalarValueTypeZero)) +
                                itk::Math::sqr(std::min(dx_forward[i][k], ScalarValueTypeZero));
      }
    }

    ans[k] = std::sqrt(propagation_gradient) * speed;
  }

  return ans;
}
} // end namespace itk

#endif
