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//---------------------------------------------------------------------------
//
// ClassName:   G4AtomicFormFactor
//
// Description: Contains the function for the evaluation of the atomic form
//              factor. The tabulated data are available on IUCr website
//
// Class description:
//
// XXX
//
// 21-04-16, created by E.Bagli

#ifndef G4ATOMICFORMFACTOR_HH
#define G4ATOMICFORMFACTOR_HH 1

#include "G4Exp.hh"
#include <CLHEP/Units/PhysicalConstants.h>
#include "globals.hh"

#include <map>
#include <vector>

class G4AtomicFormFactor
{
 public:
  static G4AtomicFormFactor* GetManager()
  {
    if (s_G4AtomicFormFactorManager == nullptr) {
      s_G4AtomicFormFactorManager = new G4AtomicFormFactor();
    }
    return s_G4AtomicFormFactorManager;
  }

  G4double Get(G4double kScatteringVector, G4int Z, G4int charge = 0)
  {
    if (loadedIndex != GetIndex(Z, charge)) {
      LoadCoefficiencts(GetIndex(Z, charge));
    }
    G4double result = 0.;
    // Convert k from mm^-1 to Å^-1 and divide by 4π , then square.
    const G4double kVecOn4Pi = (kScatteringVector * 1.0e-7) * (1.0 / (4.0 * CLHEP::pi));
    G4double kVecOn4PiSquared = kVecOn4Pi * kVecOn4Pi;  // (k/(4π))^2 in Å^-2

    for (unsigned int i0 = 0; i0 < 4; i0++) {
      result += theCoefficients[i0 * 2] * G4Exp(-theCoefficients[i0 * 2 + 1] * kVecOn4PiSquared);
    }
    result += theCoefficients[8];
    return result;
  }

 protected:
  G4AtomicFormFactor(); 
  ~G4AtomicFormFactor() = default;

 private:
  void InsertCoefficients(G4int index, const std::vector<G4double>& aDoubleVec)
  {
    theCoefficientsMap.insert(std::pair<G4int, std::vector<G4double>>(index, aDoubleVec));
  }

  // LoadCoefficiencts() method allows the evaluation of the atomic form
  // factor coefficients and the storage in theCoefficients.
  // If theCoefficients are already correct, no need to get new ones
  // Reference: International Tables for Crystallography (2006).
  // Vol. C, ch. 6.1, pp. 554-595
  // doi: 10.1107/97809553602060000600
  // Chapter 6.1. Intensity of diffracted intensities
  // IUCr Eq. 6.1.1.15, Coefficients Table 6.1.1.4
  void LoadCoefficiencts(G4int index)
  {
    loadedIndex = index;
    for (unsigned int i0 = 0; i0 < 9; i0++) {
      theCoefficients[i0] = theCoefficientsMap[index][i0];
    }
  }

  // Get() function gives back the Atomic Form Factor of the Z material
  inline G4int GetIndex(G4int Z, G4int charge = 0) { return Z * 100 + charge; }

 private:
  inline static G4AtomicFormFactor* s_G4AtomicFormFactorManager = nullptr;

  // theCoefficientsMap stores the coefficients for the form factor
  // calculations. It can be loaded only by LoadCoefficiencts()
  // and accessed by theCoefficients[].
  std::map<G4int, std::vector<G4double>> theCoefficientsMap;
  G4double theCoefficients[9];
  G4int loadedIndex;
};
#endif
