/*=========================================================================
 *
 *  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 itkFastMarchingUpwindGradientImageFilterBase_hxx
#define itkFastMarchingUpwindGradientImageFilterBase_hxx

#include "itkImageRegionIterator.h"
#include "itkNumericTraits.h"
#include "itkMath.h"
#include <algorithm>

namespace itk
{
/**
 *
 */
template <typename TInput, typename TOutput>
FastMarchingUpwindGradientImageFilterBase<TInput, TOutput>::FastMarchingUpwindGradientImageFilterBase()
{
  GradientImagePointer GradientImage = GradientImageType::New();
  this->SetNthOutput(1, GradientImage.GetPointer());
}

template <typename TInput, typename TOutput>
auto
FastMarchingUpwindGradientImageFilterBase<TInput, TOutput>::GetGradientImage() -> GradientImageType *
{
  return dynamic_cast<GradientImageType *>(this->ProcessObject::GetOutput(1));
}

/**
 *
 */
template <typename TInput, typename TOutput>
void
FastMarchingUpwindGradientImageFilterBase<TInput, TOutput>::PrintSelf(std::ostream & os, Indent indent) const
{
  Superclass::PrintSelf(os, indent);
}

/**
 *
 */
template <typename TInput, typename TOutput>
void
FastMarchingUpwindGradientImageFilterBase<TInput, TOutput>::InitializeOutput(OutputImageType * output)
{
  Superclass::InitializeOutput(output);

  // allocate memory for the GradientImage if requested
  GradientImagePointer GradientImage = this->GetGradientImage();

  GradientImage->CopyInformation(this->GetInput());
  GradientImage->SetBufferedRegion(output->GetBufferedRegion());
  GradientImage->Allocate();

  using GradientIterator = ImageRegionIterator<GradientImageType>;

  GradientIterator gradientIt(GradientImage, GradientImage->GetBufferedRegion());

  GradientPixelType zeroGradient;
  using GradientPixelValueType = typename GradientPixelType::ValueType;
  zeroGradient.Fill(GradientPixelValueType{});

  while (!gradientIt.IsAtEnd())
  {
    gradientIt.Set(zeroGradient);
    ++gradientIt;
  }
}


template <typename TInput, typename TOutput>
void
FastMarchingUpwindGradientImageFilterBase<TInput, TOutput>::UpdateNeighbors(OutputImageType * oImage,
                                                                            const NodeType &  iNode)
{
  Superclass::UpdateNeighbors(oImage, iNode);

  this->ComputeGradient(oImage, iNode);
}

/**
 *
 */
template <typename TInput, typename TOutput>
void
FastMarchingUpwindGradientImageFilterBase<TInput, TOutput>::ComputeGradient(OutputImageType * oImage,
                                                                            const NodeType &  iNode)
{
  NodeType neighIndex = iNode;

  OutputPixelType   centerPixel;
  OutputPixelType   dx_forward;
  OutputPixelType   dx_backward;
  GradientPixelType gradientPixel;

  const OutputPixelType ZERO{};

  OutputSpacingType spacing = oImage->GetSpacing();

  unsigned int xStride[Self::ImageDimension];

  centerPixel = oImage->GetPixel(iNode);

  for (unsigned int j = 0; j < ImageDimension; ++j)
  {
    neighIndex = iNode;

    // Set stride of one in each direction
    xStride[j] = 1;

    // Compute one-sided finite differences with alive neighbors
    // (the front can only come from there)
    dx_backward = ZERO;
    neighIndex[j] = iNode[j] - xStride[j];

    if (!((neighIndex[j] > this->m_LastIndex[j]) || (neighIndex[j] < this->m_StartIndex[j])))
    {
      if (this->GetLabelValueForGivenNode(neighIndex) == Traits::Alive)
      {
        dx_backward = centerPixel - oImage->GetPixel(neighIndex);
      }
    }

    dx_forward = ZERO;
    neighIndex[j] = iNode[j] + xStride[j];

    if (!((neighIndex[j] > this->m_LastIndex[j]) || (neighIndex[j] < this->m_StartIndex[j])))
    {
      if (this->GetLabelValueForGivenNode(neighIndex) == Traits::Alive)
      {
        dx_forward = oImage->GetPixel(neighIndex) - centerPixel;
      }
    }

    // Compute upwind finite differences
    if (std::max(dx_backward, -dx_forward) < ZERO)
    {
      gradientPixel[j] = ZERO;
    }
    else
    {
      if (dx_backward > -dx_forward)
      {
        gradientPixel[j] = dx_backward;
      }
      else
      {
        gradientPixel[j] = dx_forward;
      }
    }

    gradientPixel[j] /= spacing[j];
  }

  GradientImagePointer GradientImage = this->GetGradientImage();
  GradientImage->SetPixel(iNode, gradientPixel);
}
} // namespace itk

#endif
