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// $Id$
//
#ifndef G4NeutronHPFissionSpectrum_h
#define G4NeutronHPFissionSpectrum_h 1

#include <fstream>
#include <CLHEP/Units/SystemOfUnits.h>

#include "globals.hh"
#include "G4ios.hh"
#include "Randomize.hh"
#include "G4NeutronHPVector.hh"
#include "G4VNeutronHPEDis.hh"

// we will need a List of these .... one per term.

class G4NeutronHPFissionSpectrum : public G4VNeutronHPEDis
{
  public:
  G4NeutronHPFissionSpectrum()
  {
    expm1 = std::exp(-1.);
  }
  ~G4NeutronHPFissionSpectrum()
  {
  }
  
  inline void Init(std::ifstream & aDataFile)
  {
    theFractionalProb.Init(aDataFile, CLHEP::eV);
    theThetaDist.Init(aDataFile, CLHEP::eV);
  }
  
  inline G4double GetFractionalProbability(G4double anEnergy)
  {
    return theFractionalProb.GetY(anEnergy);
  }
  
  inline G4double Sample(G4double anEnergy) 
  {
    G4double theta = theThetaDist.GetY(anEnergy);
    // here we need to sample Maxwells distribution, if 
    // need be.
    G4double result, cut;
    G4double range =50*CLHEP::MeV;
    G4double max = Maxwell((theta*CLHEP::eV)/2., theta);
    G4double value;
    do
    {
      result = range*G4UniformRand();
      value = Maxwell(result, theta);
      cut = G4UniformRand();
    }
    while(cut > value/max);
    return result;
  }
  
  private:
 
  // this is the function to sample from. 
  inline G4double Maxwell(G4double anEnergy, G4double theta)
  {
    G4double result = std::sqrt(anEnergy/CLHEP::eV)*std::exp(-anEnergy/CLHEP::eV/theta);
    return result;
  }
  
  private:
  
  G4double expm1;
  
  G4NeutronHPVector theFractionalProb;
  
  G4NeutronHPVector theThetaDist;
  
};

#endif