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 *
 *  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 itkKullbackLeiblerCompareHistogramImageToImageMetric_hxx
#define itkKullbackLeiblerCompareHistogramImageToImageMetric_hxx

#include "itkHistogram.h"

// Todo: need to access Use_Padding in parent. Make in protected
// need to figure out what to do when "stuff" is not in histogram
// kernel function?

namespace itk
{
template <typename TFixedImage, typename TMovingImage>
KullbackLeiblerCompareHistogramImageToImageMetric<TFixedImage,
                                                  TMovingImage>::KullbackLeiblerCompareHistogramImageToImageMetric()
  : m_Epsilon(1e-12) // should be smaller than 1/numBins^2
{}

template <typename TFixedImage, typename TMovingImage>
void
KullbackLeiblerCompareHistogramImageToImageMetric<TFixedImage, TMovingImage>::Initialize()
{
  Superclass::Initialize();
}

template <typename TFixedImage, typename TMovingImage>
auto
KullbackLeiblerCompareHistogramImageToImageMetric<TFixedImage, TMovingImage>::EvaluateMeasure(
  HistogramType & histogram) const -> MeasureType
{
  // Two terms.
  // First the term that measures the entropy of the term
  // p(x,y) log p(x,y) - p(x,y) log q(x,y)

  MeasureType KullbackLeibler{};

  HistogramIteratorType       measured_it = histogram.Begin();
  const HistogramIteratorType measured_end = histogram.End();

  HistogramIteratorType       training_it = this->GetTrainingHistogram()->Begin();
  const HistogramIteratorType training_end = this->GetTrainingHistogram()->End();

  while (measured_it != measured_end)
  {
    // Every bin gets epsilon added to it
    const double TrainingFreq = training_it.GetFrequency() + m_Epsilon;
    const double MeasuredFreq = measured_it.GetFrequency() + m_Epsilon;

    KullbackLeibler += MeasuredFreq * std::log(MeasuredFreq / TrainingFreq);

    ++measured_it;
    ++training_it;
  }

  if (training_it != training_end)
  {
    itkWarningMacro("The Measured and Training Histograms have different number of bins.");
  }

  // Get the total frequency for each histogram.
  const HistogramFrequencyType totalTrainingFreq = this->GetTrainingHistogram()->GetTotalFrequency();
  const HistogramFrequencyType totalMeasuredFreq = histogram.GetTotalFrequency();

  // The actual number of total frequency is a bit larger
  // than the number of counts because we add m_Epsilon to every bin
  const double AdjustedTotalTrainingFreq =
    totalTrainingFreq + this->GetHistogramSize()[0] * this->GetHistogramSize()[1] * m_Epsilon;
  const double AdjustedTotalMeasuredFreq =
    totalMeasuredFreq + this->GetHistogramSize()[0] * this->GetHistogramSize()[1] * m_Epsilon;

  KullbackLeibler = KullbackLeibler / static_cast<MeasureType>(AdjustedTotalMeasuredFreq) -
                    std::log(AdjustedTotalMeasuredFreq / AdjustedTotalTrainingFreq);

  return KullbackLeibler;
}

template <typename TFixedImage, typename TMovingImage>
void
KullbackLeiblerCompareHistogramImageToImageMetric<TFixedImage, TMovingImage>::PrintSelf(std::ostream & os,
                                                                                        Indent         indent) const
{
  Superclass::PrintSelf(os, indent);

  os << indent << "Epsilon: " << m_Epsilon << std::endl;
}
} // End namespace itk

#endif // itkKullbackLeiblerCompareHistogramImageToImageMetric_hxx
