/*=========================================================================
 *
 *  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 itkMeanSquaresHistogramImageToImageMetric_hxx
#define itkMeanSquaresHistogramImageToImageMetric_hxx


namespace itk
{
template <typename TFixedImage, typename TMovingImage>
auto
MeanSquaresHistogramImageToImageMetric<TFixedImage, TMovingImage>::EvaluateMeasure(HistogramType & histogram) const
  -> MeasureType
{
  MeasureType            measure{};
  HistogramIteratorType  it = histogram.Begin();
  HistogramIteratorType  end = histogram.End();
  HistogramFrequencyType totalNumberOfSamples{};

  while (it != end)
  {
    HistogramFrequencyType freq = it.GetFrequency();
    if (freq > 0)
    {
      HistogramMeasurementVectorType value = it.GetMeasurementVector();
      measure += (value[0] - value[1]) * (value[0] - value[1]) * freq;
      totalNumberOfSamples += freq;
    }
    ++it;
  }

  measure /= totalNumberOfSamples;

  return measure;
}
} // End namespace itk

#endif // itkMeanSquaresHistogramImageToImageMetric_hxx
