/*=========================================================================
 *
 *  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 itkAtanRegularizedHeavisideStepFunction_hxx
#define itkAtanRegularizedHeavisideStepFunction_hxx

#include "itkMath.h"

namespace itk
{

template <typename TInput, typename TOutput>
auto
AtanRegularizedHeavisideStepFunction<TInput, TOutput>::Evaluate(const InputType & input) const -> OutputType
{
  const RealType t = static_cast<RealType>(input) * this->GetOneOverEpsilon();
  return 0.5 + static_cast<OutputType>(itk::Math::one_over_pi * std::atan(t));
}

/** Evaluate the derivative at the specified input position */
template <typename TInput, typename TOutput>
auto
AtanRegularizedHeavisideStepFunction<TInput, TOutput>::EvaluateDerivative(const InputType & input) const -> OutputType
{
  const RealType oneOverEpsilon = this->GetOneOverEpsilon();
  const RealType t = static_cast<RealType>(input) * oneOverEpsilon;

  return static_cast<OutputType>(Math::one_over_pi * oneOverEpsilon / (1.0 + t * t));
}

} // namespace itk

#endif
