/*=========================================================================
 *
 *  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 itkFastSymmetricForcesDemonsRegistrationFunction_h
#define itkFastSymmetricForcesDemonsRegistrationFunction_h

#include "itkPDEDeformableRegistrationFunction.h"
#include "itkPoint.h"
#include "itkInterpolateImageFunction.h"
#include "itkCentralDifferenceImageFunction.h"
#include "itkWarpImageFilter.h"
#include <mutex>

namespace itk
{
/**
 * \class FastSymmetricForcesDemonsRegistrationFunction
 *
 * This class provides a substantially faster implementation of the
 * symmetric demons registration force. Speed is improved by keeping
 * a deformed copy of the moving image for gradient evaluation.
 *
 * \sa SymmetricForcesDemonsRegistrationFunction
 * \sa SymmetricForcesDemonsRegistrationFilter
 * \sa DemonsRegistrationFilter
 * \sa DemonsRegistrationFunction
 * \ingroup FiniteDifferenceFunctions
 *
 * \author Torsten Rohlfing, Neuroscience Program, SRI International
 * \ingroup ITKPDEDeformableRegistration
 */
template <typename TFixedImage, typename TMovingImage, typename TDisplacementField>
class ITK_TEMPLATE_EXPORT FastSymmetricForcesDemonsRegistrationFunction
  : public PDEDeformableRegistrationFunction<TFixedImage, TMovingImage, TDisplacementField>
{
public:
  ITK_DISALLOW_COPY_AND_MOVE(FastSymmetricForcesDemonsRegistrationFunction);

  /** Standard class type aliases. */
  using Self = FastSymmetricForcesDemonsRegistrationFunction;
  using Superclass = PDEDeformableRegistrationFunction<TFixedImage, TMovingImage, TDisplacementField>;
  using Pointer = SmartPointer<Self>;
  using ConstPointer = SmartPointer<const Self>;

  /** Method for creation through the object factory. */
  itkNewMacro(Self);

  /** \see LightObject::GetNameOfClass() */
  itkOverrideGetNameOfClassMacro(FastSymmetricForcesDemonsRegistrationFunction);

  /** MovingImage image type. */
  using typename Superclass::MovingImageType;
  using typename Superclass::MovingImagePointer;

  /** FixedImage image type. */
  using typename Superclass::FixedImageType;
  using typename Superclass::FixedImagePointer;
  using IndexType = typename FixedImageType::IndexType;
  using SizeType = typename FixedImageType::SizeType;
  using SpacingType = typename FixedImageType::SpacingType;

  /** Deformation field type. */
  using typename Superclass::DisplacementFieldType;
  using typename Superclass::DisplacementFieldTypePointer;

  /** Inherit some enums from the superclass. */
  static constexpr unsigned int ImageDimension = Superclass::ImageDimension;

  /** Inherit some enums from the superclass. */
  using typename Superclass::PixelType;
  using typename Superclass::RadiusType;
  using typename Superclass::NeighborhoodType;
  using typename Superclass::FloatOffsetType;
  using typename Superclass::TimeStepType;

  /** Interpolator type. */
  using CoordinateType = double;
  using CoordRepType = CoordinateType;
  using InterpolatorType = InterpolateImageFunction<MovingImageType, CoordRepType>;
  using InterpolatorPointer = typename InterpolatorType::Pointer;
  using PointType = typename InterpolatorType::PointType;
  using DefaultInterpolatorType = LinearInterpolateImageFunction<MovingImageType, CoordRepType>;

  /** Warper type */
  using WarperType = WarpImageFilter<MovingImageType, MovingImageType, DisplacementFieldType>;
  using WarperPointer = typename WarperType::Pointer;

  /** Covariant vector type. */
  using CovariantVectorType = CovariantVector<double, Self::ImageDimension>;

  /** Gradient calculator types. */
  using GradientCalculatorType = CentralDifferenceImageFunction<FixedImageType>;
  using GradientCalculatorPointer = typename GradientCalculatorType::Pointer;

  using MovingGradientCalculatorType = CentralDifferenceImageFunction<MovingImageType>;
  using MovingGradientCalculatorPointer = typename MovingGradientCalculatorType::Pointer;

  /** Set the moving image interpolator. */
  void
  SetMovingImageInterpolator(InterpolatorType * ptr)
  {
    m_MovingImageInterpolator = ptr;
    m_MovingImageWarper->SetInterpolator(ptr);
  }

  /** Get the moving image interpolator. */
  InterpolatorType *
  GetMovingImageInterpolator()
  {
    return m_MovingImageInterpolator;
  }

  /** This class uses a constant timestep of 1. */
  TimeStepType
  ComputeGlobalTimeStep(void * itkNotUsed(GlobalData)) const override
  {
    return m_TimeStep;
  }

  /** Return a pointer to a global data structure that is passed to
   * this object from the solver at each calculation.  */
  void *
  GetGlobalDataPointer() const override
  {
    auto * global = new GlobalDataStruct();

    global->m_SumOfSquaredDifference = 0.0;
    global->m_NumberOfPixelsProcessed = 0L;
    global->m_SumOfSquaredChange = 0;
    return global;
  }

  /** Release memory for global data structure.
   *
   * Updates the metric and releases the per-thread-global data.
   */
  void
  ReleaseGlobalDataPointer(void * GlobalData) const override;

  /** Set the object's state before each iteration. */
  void
  InitializeIteration() override;

  /** Compute update at a non boundary neighbourhood.
   *
   * Called by a finite difference solver image filter at each pixel that does not lie on a data set boundary.
   */
  PixelType
  ComputeUpdate(const NeighborhoodType & neighborhood,
                void *                   globalData,
                const FloatOffsetType &  offset = FloatOffsetType(0.0)) override;

  /** Get the metric value. The metric value is the mean square difference
   * in intensity between the fixed image and transforming moving image
   * computed over the overlapping region between the two images. */
  virtual double
  GetMetric() const
  {
    return m_Metric;
  }

  /** Get the rms change in deformation field. */
  virtual const double &
  GetRMSChange() const
  {
    return m_RMSChange;
  }

  /** Set/Get the threshold below which the absolute difference of
   * intensity yields a match. When the intensities match between a
   * moving and fixed image pixel, the update vector (for that
   * iteration) will be the zero vector. Default is 0.001. */
  virtual void
  SetIntensityDifferenceThreshold(double);

  virtual double
  GetIntensityDifferenceThreshold() const;

protected:
  FastSymmetricForcesDemonsRegistrationFunction();
  ~FastSymmetricForcesDemonsRegistrationFunction() override = default;
  void
  PrintSelf(std::ostream & os, Indent indent) const override;

  /** FixedImage image neighborhood iterator type. */
  using FixedImageNeighborhoodIteratorType = ConstNeighborhoodIterator<FixedImageType>;

  /** A global data type for this class of equation. Used to store
   * iterators for the fixed image. */
  struct GlobalDataStruct
  {
    double        m_SumOfSquaredDifference;
    SizeValueType m_NumberOfPixelsProcessed;
    double        m_SumOfSquaredChange;
  };

private:
  /** Cache fixed image information. */
  double m_Normalizer{};

  /** Function to compute derivatives of the fixed image. */
  GradientCalculatorPointer m_FixedImageGradientCalculator{};

  /** Function to compute derivatives of the fixed image. */
  MovingGradientCalculatorPointer m_WarpedMovingImageGradientCalculator{};

  /** Function to interpolate the moving image. */
  InterpolatorPointer m_MovingImageInterpolator{};

  /** Filter to warp moving image for fast gradient computation. */
  WarperPointer m_MovingImageWarper{};

  /** The global timestep. */
  TimeStepType m_TimeStep{};

  /** Threshold below which the denominator term is considered zero. */
  double m_DenominatorThreshold{};

  /** Threshold below which two intensity value are assumed to match. */
  double m_IntensityDifferenceThreshold{};

  /** The metric value is the mean square difference in intensity between
   * the fixed image and transforming moving image computed over the
   * the overlapping region between the two images. */
  mutable double        m_Metric{};
  mutable double        m_SumOfSquaredDifference{};
  mutable SizeValueType m_NumberOfPixelsProcessed{};
  mutable double        m_RMSChange{};
  mutable double        m_SumOfSquaredChange{};

  /** Mutex lock to protect modification to metric. */
  mutable std::mutex m_MetricCalculationMutex{};
};
} // end namespace itk

#ifndef ITK_MANUAL_INSTANTIATION
#  include "itkFastSymmetricForcesDemonsRegistrationFunction.hxx"
#endif

#endif
