// GammaKinematics.h is a part of the PYTHIA event generator.
// Copyright (C) 2026 Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.

// Header file for kinematics selection of photons from lepton beams.

#ifndef Pythia8_GammaKinematics_H
#define Pythia8_GammaKinematics_H

#include "Pythia8/Basics.h"
#include "Pythia8/BeamParticle.h"
#include "Pythia8/Info.h"
#include "Pythia8/PhysicsBase.h"
#include "Pythia8/Settings.h"

namespace Pythia8 {

//==========================================================================

// Class to sample the virtuality and transverse momentum of emitted photons.

class GammaKinematics : public PhysicsBase {

public:

  // Constructor.
  GammaKinematics() : Q2maxGamma(), Wmin(), Wmax(), eCM(), sCM(), m2BeamA(),
    m2BeamB(), Q2min1(), Q2min2(), xGamma1(), xGamma2(), Q2gamma1(),
    Q2gamma2(), phi1(), phi2(), kT1(), kT2(), kz1(), kz2(), mGmGm(), m2GmGm(),
    theta1(), theta2(), theta1Max(), theta2Max(), eCM2A(), eCM2B(), sHatNew(),
    kT(), kz(), phi(), theta(), xGammaMax1(), xGammaMax2(), m2eA(), m2eB(),
    alphaEM(), log2xMinA(), log2xMinB(), log2xMaxA(), log2xMaxB(),
    sigmaEstimate(), wt(), gammaMode(), subInA(), subInB(), hasGammaA(),
    hasGammaB(), sampleQ2(), gammaA(), gammaB(),
    hasApproxFluxA(), hasApproxFluxB(), sHatScaled() {}

  // Sample the trial or final event kinematics.
  bool init();

  // Sampling of the kinematics of the intermediate photon.
  bool   sampleKTgamma(bool nonDiff = false);
  bool   deriveKin(double xGamma, double Q2gamma, double m2beam, double eCM2);
  bool   finalize();
  bool   trialKinSoftPhaseSpaceSampling();
  double fluxWeight();
  double setupSoftPhaseSpaceSampling(double sigmaMax);

  // Calculate and return rescaled sHat according to the process.
  double calcNewSHat( double sHatOld);

  // Methods to pass along the sampled values.
  double getQ2gamma1()   const {return Q2gamma1;}
  double getQ2gamma2()   const {return Q2gamma2;}
  double getPhi1()       const {return phi1;}
  double getPhi2()       const {return phi2;}
  double getKT1()        const {return kT1;}
  double getKT2()        const {return kT2;}
  double eCMsub()        const {return mGmGm;}
  double weight()        const {return wt;}
  int    idInA()         const {return subInA;}
  int    idInB()         const {return subInB;}
  bool   hasNewSHat()    const {return sHatScaled;}

private:

  // Kinematics variables.
  double Q2maxGamma, Wmin, Wmax, eCM, sCM, m2BeamA, m2BeamB, Q2min1, Q2min2,
         xGamma1, xGamma2, Q2gamma1, Q2gamma2, phi1, phi2, kT1, kT2, kz1, kz2,
         mGmGm, m2GmGm, theta1, theta2, theta1Max, theta2Max, eCM2A, eCM2B,
         sHatNew, kT, kz, phi, theta, xGammaMax1, xGammaMax2, m2eA, m2eB,
         alphaEM, log2xMinA, log2xMinB, log2xMaxA, log2xMaxB, sigmaEstimate,
         wt;

  // Direct or resolved processes.
  int    gammaMode, subInA, subInB;

  // Sample one or two photon kinematics.
  bool hasGammaA, hasGammaB, sampleQ2, gammaA, gammaB,
    hasApproxFluxA, hasApproxFluxB, sHatScaled;

};

//==========================================================================

} // end namespace Pythia8

#endif // Pythia8_GammaKinematics_H
