// This is core/vnl/vnl_cost_function.h
#ifndef vnl_cost_function_h_
#define vnl_cost_function_h_
//:
//  \file
//  \brief Vector->Real function
//  \author Andrew W. Fitzgibbon, Oxford RRG
//  \date   23 Oct 97
//
// \verbatim
// Modifications
//  971023 AWF Initial version.
//  LSB (Manchester) 26/3/01 Tidied documentation
//   Feb.2002 - Peter Vanroose - brief doxygen comment placed on single line
// \endverbatim
//
//-----------------------------------------------------------------------------

#include <vnl/vnl_unary_function.h>
#include <vnl/vnl_vector.h>
#include "vnl/vnl_export.h"

//:   An object that represents a function from R^n -> R.
//    It is commonly used to express the
//    interface of a minimizer.
class VNL_EXPORT vnl_cost_function : public vnl_unary_function<double, vnl_vector<double> >
{
 public:

  //! Default constructor
  vnl_cost_function():dim(0) {}

  //! Construct with a specified number of unknowns
  vnl_cost_function(int number_of_unknowns):dim(number_of_unknowns) {}

  ~vnl_cost_function() override = default;

  //:  The main function.  Given the parameter vector x, compute the value of f(x).
  double f(vnl_vector<double> const& x) override;

  //:  Calculate the gradient of f at parameter vector x.
  virtual void gradf(vnl_vector<double> const& x, vnl_vector<double>& gradient);

  //:  Compute one or both of f and g.
  //   Normally implemented in terms of the above two, but may be faster if specialized. f != 0 => compute f
  virtual void compute(vnl_vector<double> const& x, double *f, vnl_vector<double>* g);

  //:  Return the number of unknowns
  int get_number_of_unknowns() const { return dim; }

  //:  Compute finite-difference gradient
  void fdgradf(vnl_vector<double> const& x, vnl_vector<double>& gradient, double stepsize = 1e-5);

  //:  Called when error is printed for user.
  virtual double reported_error(double f_value) { return f_value; }

  //:  Conveniences for printing grad, fdgrad
  vnl_vector<double> gradf(vnl_vector<double> const& x);
  vnl_vector<double> fdgradf(vnl_vector<double> const& x);

protected:

    //! Set number of unknowns.
    void set_number_of_unknowns(int number_of_unknowns) { dim=number_of_unknowns; }

public:
    int dim;
};

#endif // vnl_cost_function_h_
