/**
 * @file softshrink.hpp
 * @author Lakshya Ojha
 *
 * The soft shrink function has threshold proportional to the noise level given
 * by the user.
 * The use of a Soft Shrink activation function provides adaptive denoising at
 * various noise levels using a single CNN(Convolution Neural) without a
 * requirement to train a unique CNN for each noise level.
 *
 * mlpack is free software; you may redistribute it and/or modify it under the
 * terms of the 3-clause BSD license.  You should have received a copy of the
 * 3-clause BSD license along with mlpack.  If not, see
 * http://www.opensource.org/licenses/BSD-3-Clause for more information.
 */
#ifndef MLPACK_METHODS_ANN_LAYER_SOFTSHRINK_HPP
#define MLPACK_METHODS_ANN_LAYER_SOFTSHRINK_HPP

#include <mlpack/prereqs.hpp>

namespace mlpack {
namespace ann /** Artifical Neural Network. */ {

/**
 * Soft Shrink operator is defined as,
 * @f{eqnarray*}{
 * f(x) &=& \left\{
 *   \begin{array}{lr}
 *     x - lambda & : x >  lambda \\
 *     x + lambda & : x < -lambda \\
 *     0 & : otherwise
 *   \end{array} \\
 * \right.
 * f'(x) &=& \left\{
 *   \begin{array}{lr}
 *     1 & : x >  lambda \\
 *     1 & : x < -lambda \\
 *     0 & : otherwise
 *   \end{array}
 * \right.
 * @f}
 *
 * @tparam InputDataType Type of the input data (arma::colvec, arma::mat,
 *         arma::sp_mat or arma::cube).
 * @tparam OutputDataType Type of the output data (arma::colvec, arma::mat,
 *         arma::sp_mat or arma::cube).
 */
template <
    typename InputDataType = arma::mat,
    typename OutputDataType = arma::mat
>
class SoftShrink
{
 public:
  /**
   * Create Soft Shrink object using specified hyperparameter lambda.
   *
   * @param lambda The noise level of an image depends on settings of an
   *        imaging device. The settings can be used to select appropriate
   *        parameters for denoising methods. It is proportional to the noise
   *        level entered by the user.
   *        And it is calculated by multiplying the
   *        noise level sigma of the input(noisy image) and a
   *        coefficient 'a' which is one of the training parameters.
   *        Default value of lambda is 0.5.
   */
  SoftShrink(const double lambda = 0.5);

  /**
   * Ordinary feed forward pass of a neural network, evaluating the function
   * f(x) by propagating the activity forward through f.
   * 
   * @param input Input data used for evaluating the Soft Shrink function.
   * @param output Resulting output activation
   */
  template<typename InputType, typename OutputType>
  void Forward(const InputType& input, OutputType& output);

  /**
   * Ordinary feed backward pass of a neural network, calculating the function
   * f(x) by propagating x backwards through f. Using the results from the feed
   * forward pass.
   * 
   * @param input The propagated input activation f(x).
   * @param gy The backpropagated error.
   * @param g The calculated gradient
   */
  template<typename DataType>
  void Backward(const DataType& input,
                DataType& gy,
                DataType& g);

  //! Get the output parameter.
  OutputDataType const& OutputParameter() const { return outputParameter; }
  //! Modify the output parameter.
  OutputDataType& OutputParameter() { return outputParameter; }

  //! Get the delta.
  OutputDataType const& Delta() const { return delta; }
  //! Modify the delta.
  OutputDataType& Delta() { return delta; }

  //! Get the hyperparameter lambda.
  double const& Lambda() const { return lambda; }
  //! Modify the hyperparameter lambda.
  double& Lambda() { return lambda; }

  /**
   * Serialize the layer.
   */
  template<typename Archive>
  void serialize(Archive& ar, const unsigned int /* version */);

 private:
  //! Locally-stored delta object.
  OutputDataType delta;

  //! Locally-stored output parameter object.
  OutputDataType outputParameter;

  //! Locally-stored hyperparamater lambda.
  double lambda;
}; // class SoftShrink

} // namespace ann
} // namespace mlpack

// Include implementation.
#include "softshrink_impl.hpp"

#endif