/*=========================================================================
 *
 *  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 itkVectorCentralDifferenceImageFunction_hxx
#define itkVectorCentralDifferenceImageFunction_hxx


namespace itk
{

template <typename TInputImage, typename TCoordRep>
VectorCentralDifferenceImageFunction<TInputImage, TCoordRep>::VectorCentralDifferenceImageFunction()
{}

template <typename TInputImage, typename TCoordRep>
void
VectorCentralDifferenceImageFunction<TInputImage, TCoordRep>::PrintSelf(std::ostream & os, Indent indent) const
{
  this->Superclass::PrintSelf(os, indent);

  os << indent << "UseImageDirection: " << m_UseImageDirection << std::endl;
}

template <typename TInputImage, typename TCoordRep>
auto
VectorCentralDifferenceImageFunction<TInputImage, TCoordRep>::EvaluateAtIndex(const IndexType & index) const
  -> OutputType
{
  OutputType derivative;

  derivative.Fill(0.0);

  IndexType neighIndex = index;

  const typename InputImageType::SizeType &  size = this->GetInputImage()->GetBufferedRegion().GetSize();
  const typename InputImageType::IndexType & start = this->GetInputImage()->GetBufferedRegion().GetIndex();

  for (unsigned int dim = 0; dim < TInputImage::ImageDimension; ++dim)
  {
    // bounds checking
    if (index[dim] < start[dim] + 1 || index[dim] > (start[dim] + static_cast<OffsetValueType>(size[dim]) - 2))
    {
      for (unsigned int vdim = 0; vdim < Dimension; ++vdim)
      {
        derivative(vdim, dim) = 0.0;
      }
      continue;
    }

    // compute derivative
    const double deriv_weight = 0.5 / this->GetInputImage()->GetSpacing()[dim];

    neighIndex[dim] += 1;
    const InputPixelType & pixf = this->GetInputImage()->GetPixel(neighIndex);
    for (unsigned int vdim = 0; vdim < Dimension; ++vdim)
    {
      derivative(vdim, dim) = pixf[vdim];
    }

    neighIndex[dim] -= 2;
    const InputPixelType & pixb = this->GetInputImage()->GetPixel(neighIndex);
    for (unsigned int vdim = 0; vdim < Dimension; ++vdim)
    {
      derivative(vdim, dim) -= pixb[vdim];
      derivative(vdim, dim) *= deriv_weight;
    }

    neighIndex[dim] += 1;
  }

  if (this->m_UseImageDirection)
  {
    OutputType                                     orientedderivative;
    const typename InputImageType::DirectionType & direction = this->GetInputImage()->GetDirection();
    for (unsigned int i = 0; i < TInputImage::ImageDimension; ++i)
    {
      std::vector<double> sums(Dimension, 0.0);
      for (unsigned int j = 0; j < TInputImage::ImageDimension; ++j)
      {
        for (unsigned int vdim = 0; vdim < Dimension; ++vdim)
        {
          sums[vdim] += direction[i][j] * derivative(vdim, j);
        }
      }
      for (unsigned int vdim = 0; vdim < Dimension; ++vdim)
      {
        orientedderivative(vdim, i) = static_cast<TCoordRep>(sums[vdim]);
      }
    }
    return orientedderivative;
  }

  return (derivative);
}
} // end namespace itk

#endif