/*=========================================================================
 *
 *  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 itkIsoDataThresholdCalculator_hxx
#define itkIsoDataThresholdCalculator_hxx

#include "itkMath.h"
#include "itkProgressReporter.h"

namespace itk
{

template <typename THistogram, typename TOutput>
void
IsoDataThresholdCalculator<THistogram, TOutput>::GenerateData()
{
  const HistogramType * histogram = this->GetInput();

  if (histogram->GetTotalFrequency() == 0)
  {
    itkExceptionMacro("Histogram is empty");
  }
  SizeValueType    size = histogram->GetSize(0);
  ProgressReporter progress(this, 0, size);
  if (size == 1)
  {
    this->GetOutput()->Set(static_cast<OutputType>(histogram->GetMeasurement(0, 0)));
    return;
  }

  InstanceIdentifier currentPos = 0;
  while (true)
  {
    // Skip the empty bins to speed up things
    for (InstanceIdentifier i = currentPos; i < size; ++i)
    {
      if (histogram->GetFrequency(i, 0) > 0)
      {
        currentPos = i;
        break;
      }
      progress.CompletedPixel();
    }
    if (currentPos >= size)
    {
      // Can't compute the isodata value - use the mean instead
      this->GetOutput()->Set(static_cast<OutputType>(histogram->Mean(0)));
      return;
    }
    // Compute the mean of the lower values
    double l = 0;
    double totl = 0;

    for (InstanceIdentifier i = 0; i <= currentPos; ++i)
    {
      totl += static_cast<double>(histogram->GetFrequency(i, 0));
      l += static_cast<double>(histogram->GetMeasurement(i, 0)) * static_cast<double>(histogram->GetFrequency(i, 0));
    }
    // Compute the mean of the higher values
    double h = 0;
    double toth = 0;

    for (InstanceIdentifier i = currentPos + 1; i < size; ++i)
    {
      toth += static_cast<double>(histogram->GetFrequency(i, 0));
      h += static_cast<double>(histogram->GetMeasurement(i, 0)) * static_cast<double>(histogram->GetFrequency(i, 0));
    }
    // Avoid a potential division by 0
    if ((totl > itk::Math::eps) && (toth > itk::Math::eps))
    {
      l /= totl;
      h /= toth;

      if (histogram->GetMeasurement(currentPos, 0) >= (l + h) * 0.5)
      {
        this->GetOutput()->Set(static_cast<OutputType>(histogram->GetMeasurement(currentPos, 0)));
        return;
      }
    }

    ++currentPos;
    progress.CompletedPixel();
  }
}

} // end namespace itk

#endif
