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/// \file PrimaryGeneratorAction2.cc
/// \brief Implementation of the PrimaryGeneratorAction2 class

#include "PrimaryGeneratorAction2.hh"

#include "PrimaryGeneratorAction.hh"

#include "G4Event.hh"
#include "G4ParticleDefinition.hh"
#include "G4ParticleGun.hh"
#include "G4ParticleTable.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "Randomize.hh"

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PrimaryGeneratorAction2::PrimaryGeneratorAction2(G4ParticleGun* gun) : fParticleGun(gun)
{
  // energy distribution
  //
  InitFunction();
}

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void PrimaryGeneratorAction2::GeneratePrimaries(G4Event* anEvent)
{
  // uniform solid angle
  G4double cosAlpha = 1. - 2 * G4UniformRand();  // cosAlpha uniform in [-1,+1]
  G4double sinAlpha = std::sqrt(1. - cosAlpha * cosAlpha);
  G4double psi = twopi * G4UniformRand();  // psi uniform in [0, 2*pi]
  G4ThreeVector dir(sinAlpha * std::cos(psi), sinAlpha * std::sin(psi), cosAlpha);

  fParticleGun->SetParticleMomentumDirection(dir);

  // set energy from a tabulated distribution
  //
  // G4double energy = RejectAccept();
  G4double energy = InverseCumul();
  fParticleGun->SetParticleEnergy(energy);

  // create vertex
  //
  fParticleGun->GeneratePrimaryVertex(anEvent);
}
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void PrimaryGeneratorAction2::InitFunction()
{
  // tabulated function
  // Y is assumed positive, linear per segment, continuous
  //
  fNPoints = 16;
  const G4double xx[] = {37 * keV,  39 * keV,  45 * keV,  51 * keV, 57 * keV, 69 * keV,
                         71 * keV,  75 * keV,  83 * keV,  91 * keV, 97 * keV, 107 * keV,
                         125 * keV, 145 * keV, 159 * keV, 160 * keV};

  const G4double yy[] = {0.000,  0.077,  0.380,  2.044,  5.535, 15.077, 12.443, 14.766,
                         17.644, 18.518, 17.772, 14.776, 8.372, 3.217,  0.194,  0.000};

  // copy arrays in std::vector and compute fMax
  //
  fX.resize(fNPoints);
  fY.resize(fNPoints);
  fYmax = 0.;
  for (G4int j = 0; j < fNPoints; j++) {
    fX[j] = xx[j];
    fY[j] = yy[j];
    if (fYmax < fY[j]) fYmax = fY[j];
  };

  // compute slopes
  //
  fSlp.resize(fNPoints);
  for (G4int j = 0; j < fNPoints - 1; j++) {
    fSlp[j] = (fY[j + 1] - fY[j]) / (fX[j + 1] - fX[j]);
  };

  // compute cumulative function
  //
  fYC.resize(fNPoints);
  fYC[0] = 0.;
  for (G4int j = 1; j < fNPoints; j++) {
    fYC[j] = fYC[j - 1] + 0.5 * (fY[j] + fY[j - 1]) * (fX[j] - fX[j - 1]);
  };
}

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G4double PrimaryGeneratorAction2::RejectAccept()
{
  // tabulated function
  // Y is assumed positive, linear per segment, continuous
  // (see Particle Data Group: pdg.lbl.gov --> Monte Carlo techniques)
  //
  G4double Xrndm = 0., Yrndm = 0., Yinter = -1.;

  while (Yrndm > Yinter) {
    // choose a point randomly
    Xrndm = fX[0] + G4UniformRand() * (fX[fNPoints - 1] - fX[0]);
    Yrndm = G4UniformRand() * fYmax;
    // find bin
    G4int j = fNPoints - 2;
    while ((fX[j] > Xrndm) && (j > 0))
      j--;
    // compute Y(x_rndm) by linear interpolation
    Yinter = fY[j] + fSlp[j] * (Xrndm - fX[j]);
  };
  return Xrndm;
}

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G4double PrimaryGeneratorAction2::InverseCumul()
{
  // tabulated function
  // Y is assumed positive, linear per segment, continuous
  // --> cumulative function is second order polynomial
  // (see Particle Data Group: pdg.lbl.gov --> Monte Carlo techniques)

  // choose y randomly
  G4double Yrndm = G4UniformRand() * fYC[fNPoints - 1];
  // find bin
  G4int j = fNPoints - 2;
  while ((fYC[j] > Yrndm) && (j > 0))
    j--;
  // y_rndm --> x_rndm :  fYC(x) is second order polynomial
  G4double Xrndm = fX[j];
  G4double a = fSlp[j];
  if (a != 0.) {
    G4double b = fY[j] / a, c = 2 * (Yrndm - fYC[j]) / a;
    G4double delta = b * b + c;
    G4int sign = 1;
    if (a < 0.) sign = -1;
    Xrndm += sign * std::sqrt(delta) - b;
  }
  else if (fY[j] > 0.) {
    Xrndm += (Yrndm - fYC[j]) / fY[j];
  };
  return Xrndm;
}

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