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//
// Abstract base class for multibody "phase space" generators.  Subclasses
// implement a specific algorithm, such as Kopylov, GENBOD, or Makoto's
// NBody.  Subclasses are used by G4HadPhaseSpaceGenerator.
//
// Author:	Michael Kelsey (SLAC) <kelsey@slac.stanford.edu>

#include "G4VHadDecayAlgorithm.hh"
#include "G4HadronicException.hh"
#include "G4PhysicalConstants.hh"
#include "G4SystemOfUnits.hh"
#include "G4ThreeVector.hh"
#include "Randomize.hh"
#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>


// Initial state (rest mass) and list of final masses

void G4VHadDecayAlgorithm::Generate(G4double initialMass,
				    const std::vector<G4double>& masses,
				    std::vector<G4LorentzVector>& finalState) {
  if (verboseLevel) G4cout << GetName() << "::Generate" << G4endl;

  // Initialization and sanity check
  finalState.clear();
  if (!IsDecayAllowed(initialMass, masses)) return;

  // Allow different procedures for two-body or N-body distributions
  if (masses.size() == 2U) 
    GenerateTwoBody(initialMass, masses, finalState);
  else 
    GenerateMultiBody(initialMass, masses, finalState);
}


// Base class does very simple validation of configuration

G4bool G4VHadDecayAlgorithm::
IsDecayAllowed(G4double initialMass,
	       const std::vector<G4double>& masses) const {
  G4bool okay =
    (initialMass > 0. && masses.size() >= 2 &&
     initialMass >= std::accumulate(masses.begin(),masses.end(),0.));

  if (verboseLevel) {
    G4cout << GetName() << "::IsDecayAllowed? initialMass " << initialMass
	   << " " << masses.size() << " masses sum "
	   << std::accumulate(masses.begin(),masses.end(),0.) << G4endl;

    if (verboseLevel>1) PrintVector(masses," ",G4cout);

    G4cout << " Returning " << okay << G4endl;
  }

  return okay;
}


// Momentum function (c.f. PDK() function from CERNLIB W515)

G4double G4VHadDecayAlgorithm::TwoBodyMomentum(G4double M0, G4double M1,
					       G4double M2) const {
  G4double PSQ = (M0+M1+M2)*(M0+M1-M2)*(M0-M1+M2)*(M0-M1-M2);
  if (PSQ < 0.) {
    G4cout << GetName() << ":  problem of decay of M(GeV) " << M0/GeV 
	   << " to M1(GeV) " << M1/GeV << " and M2(GeV) " << M2/GeV
	   << " PSQ(MeV) " << PSQ/MeV << " < 0" << G4endl;
    // exception only if the problem is numerically significant
    if (PSQ < -CLHEP::eV) {
      throw G4HadronicException(__FILE__, __LINE__,"Error in decay kinematics");
    }

    PSQ = 0.;
  }

  return std::sqrt(PSQ)/(2.*M0);
}

// Convenience functions for uniform angular distributions

G4double G4VHadDecayAlgorithm::UniformTheta() const {
  return std::acos(2.0*G4UniformRand() - 1.0);
}

G4double G4VHadDecayAlgorithm::UniformPhi() const {
  return twopi*G4UniformRand();
}


// Dump contents of vector to output

void G4VHadDecayAlgorithm::
PrintVector(const std::vector<G4double>& v,
	    const G4String& vname, std::ostream& os) const {
  os << " " << vname << "(" << v.size() << ") ";
  std::copy(v.begin(), v.end(), std::ostream_iterator<G4double>(os, " "));
  os << std::endl;
}