/*
 * ChargeSmearer.hpp
 *
 *  Created on: Sep 2, 2015
 *      Author: heber
 */

#ifndef CHARGESMEARER_HPP_
#define CHARGESMEARER_HPP_

// include config.h
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include "CodePatterns/Singleton.hpp"

namespace VMG {
  class Grid;
  class GridIterator;
  class Index;

  namespace Particle {
    class BSpline;
  }
}

/** This class smears out a given (peak-like) charge value by using a radial
 * b-spline of compile-defined degree and runtime-defined width ("near field
 * cells").
 *
 * This is used to smear out the electronic charge distribution in the same
 * way as is done with the (point-like) nuclei charge distribution to increase
 * accuracy for the long-range calculations.
 *
 * The trick relies on two distant and non-overlapping charge distributions
 * behave actually as two point charges and vice versa. Core electron charge
 * distributions are strongly peaked and hence almost appear like the point-like
 * nuclei charge distributions. Hence, we perform the same trick as is done
 * with the nuclei. The charges are smeared out to such a degree that charge
 * distributions do not overlap except within a single fragment. Within
 * fragments the short-range Coulomb energy and forces is calculated accurately.
 * In between fragments the long-range calculations via vmg is used and hence
 * charges there may not overlap. The maximum distance allowed and hence the
 * maximum near field cells can be obtained from the Interfragmenter class.
 *
 * This class is a singleton as we only need to precalculate the spline values
 * for a given nfc once and then may use of all FragmentJobs.
 *
 * The code here is taken from vmg's units/particles/bspline.cpp::SetSpline().
 */
class ChargeSmearer : public Singleton<ChargeSmearer>
{
  friend class Singleton<ChargeSmearer>;

private:
  /** Private cstor for ChargeSmearer.
   *
   * Must be private as is singleton.
   *
   */
  ChargeSmearer();

  /** Dstor for ChargeSmearer.
   *
   */
  ~ChargeSmearer();

public:
  /** Setter for the values defining the grid and the spline extent.
   *
   * This function recalculates the spline values but only if the
   * values have changed with respect to the old ones.
   *
   * \param _spl spline to evaluate
   * \param _nfc near field cells
   * \param _meshwdith mesh spacing
   */
  void initializeSplineArray(
      const VMG::Particle::BSpline &_spl,
      const unsigned int _nfc,
      const double _meshwidth);

  /** Getter for the width of the smearing area, i.e. number of near field
   * cells.
   *
   * \return number of near field cells, i.e. 2nfc+1 equals width of array
   */
  unsigned int getNearFielCells() const
  { return nfc; }

  /** Getter for the meshwidth.
   *
   * \return meshwidth
   */
  unsigned int getMeshwidth() const
  { return meshwidth; }

  /** Smears out the given charge \a _value with the current \a nfc and bspline
   * on the \a _grid.
   *
   * \param _grid grid to smear out charge on
   * \param _iter central node on grid where to add smearing
   * \param _charge charge to smear out
   */
  void operator()(
      VMG::Grid& _grid,
      const VMG::GridIterator &_iter,
      const double _charge) const;

private:
  //!> typedef for the visitor functor
  typedef boost::function<void (const VMG::Index &, const double)> visitor_t;

  /** Visitor pattern for going over bspline support and executing the given
   * functor at each valid node.
   *
   * \param _iter iterator giving central node for support on grid
   * \param _visitor visitor functor to execute at each node (that's still inside grid)
   */
  void visitBSplineDomain(
      const VMG::GridIterator &_iter,
      const visitor_t &_visitor
      ) const;

private:
  //!> near field radius for bspline
  unsigned int nfc;

  //!> mesh width of the grid
  double meshwidth;

  //!> contains the array of pre-compiled spline values
  double *vals;

  //!> integrated value of spline array for normalization
  double int_val;
};


#endif /* CHARGESMEARER_HPP_ */