/**********************************************************

This software is part of J.-S. Caux's ABACUS library.

Copyright (c) J.-S. Caux.

-----------------------------------------------------------

File:  src/ODSLF/ODSLF_Matrix_Element_Contrib.cc

Purpose:  handles the generic call for a matrix element.

***********************************************************/

#include "ABACUS.h"

using namespace std;
using namespace ABACUS;

namespace ABACUS {

  DP Compute_Matrix_Element_Contrib (char whichDSF, int iKmin, int iKmax, ODSLF_XXZ_Bethe_State& LeftState,
				     ODSLF_XXZ_Bethe_State& RefState, DP Chem_Pot, fstream& DAT_outfile)
  {
    // This function prints the matrix element information to the fstream,
    // and returns a weighed `data_value' to be multiplied by sumrule_factor,
    // to determine if scanning along this thread should be continued.

    // Identify which matrix is needed from the number of particles in Left and Right states:

    bool fixed_iK = (iKmin == iKmax);

    DP ME = 0.0;
    if (whichDSF == 'Z')
      ME = LeftState.E - RefState.E;
    else if (whichDSF == 'm')
      ME = exp(real(ln_Smin_ME (RefState, LeftState)));
    else if (whichDSF == 'z') {
      if (LeftState.base_id == RefState.base_id && LeftState.type_id == RefState.type_id && LeftState.id == RefState.id)
	ME = sqrt(RefState.chain.Nsites * 0.25) * (1.0 - 2.0*RefState.base.Mdown/RefState.chain.Nsites);
      else ME = exp(real(ln_Sz_ME (RefState, LeftState)));
    }
    else if (whichDSF == 'p')
      ME = exp(real(ln_Smin_ME (LeftState, RefState)));
    else ABACUSerror("Wrong whichDSF in Compute_Matrix_Element_Contrib.");

    if (is_nan(ME)) ME = 0.0;

    // Do the momentum business:
    int iKout = LeftState.iK - RefState.iK;
    while(iKout < 0) iKout += RefState.chain.Nsites;
    while(iKout >= RefState.chain.Nsites) iKout -= RefState.chain.Nsites;

    DAT_outfile << setprecision(16);
    // Print information to fstream:
    if (iKout >= iKmin && iKout <= iKmax) {
      if (whichDSF == 'Z') {
	DAT_outfile << endl << LeftState.E - RefState.E - (LeftState.base.Mdown - RefState.base.Mdown) * Chem_Pot << "\t"
		    << iKout << "\t"
		    << LeftState.base_id << "\t" << LeftState.type_id << "\t" << LeftState.id;
      }
      else {
	DAT_outfile << endl << LeftState.E - RefState.E - (LeftState.base.Mdown - RefState.base.Mdown) * Chem_Pot << "\t"
		    << iKout << "\t"
		    << ME << "\t"
		    << LeftState.base_id << "\t" << LeftState.type_id << "\t" << LeftState.id;
      }
    } // if iKmin <= iKout <= iKmax

    // Calculate and return the data_value:
    DP data_value = ME * ME;
    if (whichDSF == 'Z') // use 1/(1 + omega)
      data_value = 1.0/(1.0 + LeftState.E - RefState.E - (LeftState.base.Mdown - RefState.base.Mdown) * Chem_Pot);
    else if (fixed_iK) // data value is MEsq * omega:
      data_value = ME * ME * (LeftState.E - RefState.E - (LeftState.base.Mdown - RefState.base.Mdown) * Chem_Pot);

    return(data_value);
  }

} // namespace ABACUS