/*=========================================================================
 *
 *  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 itkObjectToObjectMultiMetricv4_h
#define itkObjectToObjectMultiMetricv4_h

#include "itkObjectToObjectMetric.h"
#include "itkArray.h"
#include <deque>

namespace itk
{
/** \class ObjectToObjectMultiMetricv4
 * \brief This class takes one ore more ObjectToObject metrics and assigns weights to their derivatives
 * to compute a single result.
 *
 * This class takes N ObjectToObject-derived component metrics and assigns a weight to each of the metrics'
 * derivatives. It then computes a weighted measure of the metric. The GetValue() and
 * GetValueAndDerivative() methods compute the measure and derivative using the following calculation:
 *
 * metric value = Sum_j ( w_j * M_j ) (see important note below)
 *
 * and the GetDerivative() method computes the derivative by computing:
 *
 * derivative = Sum_j ( w_j * dM_j / ||dM_j|| ) * ( Sum_j( ||dM_j|| ) / J )
 *
 * \note The metric value is unit-less, and thus it is difficult to compute a combined metric.
 * This metric returns the metric value in three ways:
 *  1) GetValue() returns the computed value of only the *first* component metric. This result
 *     is stored in m_Value and also returned by GetCurrentValue().
 *  2) GetValueArray() returns an itkArray of metric values, one for each component metric. It
 *     only has meaning after a call to GetValue(), GetDerivative() or GetValueAndDerivative().
 *  3) GetWeightedValue() returns a combined metric value of all component metrics, using the
 *     assigned weights. It only has meaning after a call to GetValue(), GetDerivative()
 *     or GetValueAndDerivative().
 *
 * The assigned weights are normalized internally to sum to one before use, and the weights
 * default to 1/N, where N is the number of component metrics.
 *
 * \note Each component metric must use the same transform parameters object. That is, each metric
 * must be evaluating the same parameters by evaluating the same transform. Except, if a component
 * transform is a CompositeTransform, in which case it must be set to optimize a single transform,
 * and that transform must be the same as the transform in other component metrics.
 *
 * \note Each component metric must be setup independently, except for the metric transforms
 * which can optionally be set from this class. That is, each component's images or point sets,
 * fixed transforms and options must be set independently. The only methods in this metric for setting
 * up the component metrics is SetMovingTransform(). The corresponding
 * Set accesor is also available. When Set{Fixed/Moving}Transform() is not used
 * this metric's m_{Fixed/Moving}Transform member is assigned to the
 * fixed/moving transform assigned to the first component metric.
 *
 * Each component will be initialized by this metric in the call to Initialize().
 *
 * \note When used with an itkRegistrationParameterScalesEstimator estimator, and the multi-metric
 * holds one or more point-set metrics, the user must assign a virtual domain point set for sampling
 * to ensure proper sampling within the point set metrics. In order to generate valid shift estimations,
 * such a virtual domain point set must include mapped points from the fixed point set.
 * See RegistrationParameterScalesEstimator::SetVirtualDomainPointSet() and
 * PointSetToPointSetMetricv4::GetVirtualTransformedPointSet(). If there are two different point sets,
 * then the virtual domain point set should be a union of the two for completeness.
 *
 * \note If the user does not explicitly assign a virtual domain, then the first valid virtual
 * domain found in the component metrics will be used a virtual domain for this multi-metric,
 * which will be queried by classes such as registration parameter estimators.
 * Each component metric will still use its own virtual domain for internal calculations when
 * evaluated, so it is possible to use different virtual domains for each metric if desired.
 * If no component metric has a virtual domain defined, then by default the virtual domain is
 * unbounded.
 * When the transform is high dimensional (e.g. DisplacementFieldTransform) then there must
 * be a virtual domain that matches the space of the transform field. Note that when used
 * with a DisplacementFieldTransform, both Image and PointSet metrics will automatically
 * create a matching virtual domain during initialization if one has not been assigned by the user.
 *
 * \ingroup ITKMetricsv4
 */
template <unsigned int TFixedDimension,
          unsigned int TMovingDimension,
          typename TVirtualImage = Image<double, TFixedDimension>,
          class TInternalComputationValueType = double>
class ITK_TEMPLATE_EXPORT ObjectToObjectMultiMetricv4
  : public ObjectToObjectMetric<TFixedDimension, TMovingDimension, TVirtualImage, TInternalComputationValueType>
{
public:
  ITK_DISALLOW_COPY_AND_MOVE(ObjectToObjectMultiMetricv4);

  /** Standard class type aliases */
  using Self = ObjectToObjectMultiMetricv4;
  using Superclass =
    ObjectToObjectMetric<TFixedDimension, TMovingDimension, TVirtualImage, TInternalComputationValueType>;
  using Pointer = SmartPointer<Self>;
  using ConstPointer = SmartPointer<const Self>;

  /** \see LightObject::GetNameOfClass() */
  itkOverrideGetNameOfClassMacro(ObjectToObjectMultiMetricv4);

  /** New macro for creation of through a Smart Pointer */
  itkNewMacro(Self);

  /** Types inherited from Superclass. */
  using typename Superclass::MeasureType;
  using typename Superclass::DerivativeType;
  using typename Superclass::DerivativeValueType;
  using typename Superclass::ParametersType;
  using typename Superclass::ParametersValueType;
  using typename Superclass::NumberOfParametersType;
  using typename Superclass::CoordinateRepresentationType;
  using typename Superclass::MovingTransformType;
  using typename Superclass::FixedTransformType;

  /** type alias related to the metric queue */
  using MetricType = Superclass;
  using MetricBasePointer = typename MetricType::Pointer;
  using MetricBaseConstPointer = typename MetricType::ConstPointer;
  using MetricQueueType = std::deque<MetricBasePointer>;

  using ObjectType = typename Superclass::ObjectType;

  using WeightValueType = typename DerivativeType::ValueType;
  using WeightsArrayType = Array<WeightValueType>;
  using MetricValueArrayType = Array<MeasureType>;

  itkSetMacro(MetricWeights, WeightsArrayType);
  itkGetMacro(MetricWeights, WeightsArrayType);

  /** Add a metric to the queue. */
  void
  AddMetric(MetricType * metric);

  /** Clear the metric queue. */
  void
  ClearMetricQueue();

  /** Get the number of metrics. */
  SizeValueType
  GetNumberOfMetrics() const;

  void
  Initialize() override;

  /** Set fixed object (image, point set, etc.)*/
  void
  SetFixedObject(const ObjectType * itkNotUsed(object)) override
  {
    itkExceptionMacro("A single object should not be specified for the multi metric.");
  }

  /** Set moving object (image, point set, etc.)*/
  void
  SetMovingObject(const ObjectType * itkNotUsed(object)) override
  {
    itkExceptionMacro("A single object should not be specified for the multi metric.");
  }

  /** Set each of the component metrics to use this moving transform. */
  void
  SetMovingTransform(MovingTransformType *) override;

  /** Set each of the component metrics to use this fixed transform. */
  void
  SetFixedTransform(FixedTransformType *) override;

  /** Evaluate the metrics and return the value of only the *first* metric.
   * \sa GetValueArray
   * \sa GetWeightedValue
   */
  MeasureType
  GetValue() const override;

  void
  GetDerivative(DerivativeType &) const override;

  /** Evaluate the metric value and derivative.
   * \param firstValue will contain the value of only the *first* metric on return.
   * \param derivativeResult holds the combined derivative on return.
   *
   * \sa GetValueArray
   * \sa GetWeightedValue */
  void
  GetValueAndDerivative(MeasureType & firstValue, DerivativeType & derivativeResult) const override;

  /** Returns an itkArray of metric values, one for each component metric. It
   *  only has meaning after a call to GetValue(), GetDerivative() or GetValueAndDerivative(). */
  MetricValueArrayType
  GetValueArray() const;

  /**  Returns a combined metric value of all component metrics, using the
   *   assigned weights. It only has meaning after a call to GetValue(), GetDerivative() or GetValueAndDerivative(). */
  MeasureType
  GetWeightedValue() const;

  /** Get the metrics queue */
  const MetricQueueType &
  GetMetricQueue() const;

  bool
  SupportsArbitraryVirtualDomainSamples() const override;

  using typename Superclass::MetricCategoryType;

  /** Get metric category */
  MetricCategoryType
  GetMetricCategory() const override
  {
    return MetricCategoryType::MULTI_METRIC;
  }

protected:
  ObjectToObjectMultiMetricv4();
  ~ObjectToObjectMultiMetricv4() override = default;
  void
  PrintSelf(std::ostream & os, Indent indent) const override;

private:
  MetricQueueType              m_MetricQueue{};
  WeightsArrayType             m_MetricWeights{};
  mutable MetricValueArrayType m_MetricValueArray{};
};

} // end namespace itk

#ifndef ITK_MANUAL_INSTANTIATION
#  include "itkObjectToObjectMultiMetricv4.hxx"
#endif

#endif