Qjets.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
 
#include "Qjets.h"
 
using namespace std;
using namespace JetSubStructureUtils;
 
Qjets::Qjets(double zcut, double dcut_fctr, double exp_min, double exp_max, double rigidity, double truncation_fctr)
: m_rand(),
  m_zcut(zcut), 
  m_dcut(-1.), 
  m_dcut_fctr(dcut_fctr),
  m_exp_min(exp_min),
  m_exp_max(exp_max),
  m_rigidity(rigidity),
  m_truncation_fctr(truncation_fctr)
{
}
 
void Qjets::SetRandSeed(unsigned int seed){
  m_rand.SetSeed(seed);
}
 
bool Qjets::JetUnmerged(int num) const{
  return m_merged_jets.find(num) == m_merged_jets.end();
}
 
bool Qjets::JetsUnmerged(const jet_distance& jd) const{
  return JetUnmerged(jd.j1) && JetUnmerged(jd.j2);
}
 
jet_distance Qjets::GetNextDistance(){
  vector< pair<jet_distance, double> > popped_distances;
  double norm(0.);
  jet_distance ret{};
  ret.j1 = -1;
  ret.j2 = -1;
  ret.dij = -1.;
  bool dmin_set(false);
  double dmin(0.);
 
  while(!m_distances.empty()){
    jet_distance dist = m_distances.top();
    m_distances.pop();
    if(JetsUnmerged(dist)){
      if(!dmin_set){
        dmin = dist.dij;
        dmin_set = true;
      }
      double weight = exp(-m_rigidity * (dist.dij-dmin) /dmin);
      popped_distances.emplace_back(dist,weight);        
      norm += weight; 
      if(weight/norm < m_truncation_fctr)
        break;            
    }
  }
 
  double rand(Rand()), tot_weight(0.);
  if (!popped_distances.empty()) {
    const double inv_norm = norm == 0 ? 1 : 1. / norm;
    for(vector<pair<jet_distance, double> >::iterator it = popped_distances.begin(); it != popped_distances.end(); ++it){
      tot_weight += (*it).second * inv_norm;
      if(tot_weight >= rand){
        ret = (*it).first;
        break;
      }
    }
  }
 
  // repopulate in reverse (maybe quicker?)
  for(vector<pair<jet_distance, double> >::reverse_iterator it = popped_distances.rbegin(); it != popped_distances.rend(); ++it)
    if(JetsUnmerged((*it).first))
      m_distances.push((*it).first);
 
  return ret;
}
 
void Qjets::Cluster(fastjet::ClusterSequence & cs){
  ComputeDCut(cs);
 
  // Populate all the distances
  ComputeAllDistances(cs.jets());
  jet_distance jd = GetNextDistance();
 
  while(!m_distances.empty() && jd.dij != -1.){
    if(!Prune(jd,cs)){
      //      m_merged_jets.push_back(jd.j1);
      //      m_merged_jets.push_back(jd.j2);
      m_merged_jets[jd.j1] = true;
      m_merged_jets[jd.j2] = true;
 
      int new_jet;
      cs.plugin_record_ij_recombination(jd.j1, jd.j2, 1., new_jet);
      assert(JetUnmerged(new_jet));
      ComputeNewDistanceMeasures(cs,new_jet);
    } else {
      double j1pt = cs.jets()[jd.j1].perp();
      double j2pt = cs.jets()[jd.j2].perp();
      if(j1pt>j2pt){
        //  m_merged_jets.push_back(jd.j2);
        m_merged_jets[jd.j2] = true;
        cs.plugin_record_iB_recombination(jd.j2, 1.);
      } else {
        //  m_merged_jets.push_back(jd.j1);
        m_merged_jets[jd.j1] = true;
        cs.plugin_record_iB_recombination(jd.j1, 1.);
      }
    }
    jd = GetNextDistance();
  } 
 
  // merge remaining jets with beam
  [[maybe_unused]] int num_merged_final(0);
 
  for(unsigned int i = 0 ; i < cs.jets().size(); i++)
    if(JetUnmerged(i)){
      num_merged_final++;
      cs.plugin_record_iB_recombination(i,1.);
    }
 
  assert(num_merged_final < 2);
}
 
void Qjets::ComputeDCut(fastjet::ClusterSequence & cs){
  // assume all jets in cs form a single jet.  compute mass and pt
  fastjet::PseudoJet sum(0.,0.,0.,0.);
  for(vector<fastjet::PseudoJet>::const_iterator it = cs.jets().begin(); it != cs.jets().end(); ++it)
    sum += (*it);
  m_dcut = 2. * m_dcut_fctr * sum.m()/sum.perp(); 
}
 
bool Qjets::Prune(jet_distance& jd,fastjet::ClusterSequence & cs){
  double pt1 = cs.jets()[jd.j1].perp();
  double pt2 = cs.jets()[jd.j2].perp();
  fastjet::PseudoJet sum_jet = cs.jets()[jd.j1]+cs.jets()[jd.j2];
  double sum_pt = sum_jet.perp();
  double z = min(pt1,pt2)/sum_pt;
  double d = sqrt(cs.jets()[jd.j1].plain_distance(cs.jets()[jd.j2]));
  return (d > m_dcut) && (z < m_zcut);
}
 
void Qjets::ComputeAllDistances(const vector<fastjet::PseudoJet>& inp){
  for(unsigned int i = 0 ; i < inp.size()-1; i++){
    // jet-jet distances
    for(unsigned int j = i+1 ; j < inp.size(); j++){
      jet_distance jd{};
      jd.j1 = i;
      jd.j2 = j;
      if(jd.j1 != jd.j2){
        jd.dij = d_ij(inp[i],inp[j]);
        m_distances.push(jd);
      }
    }    
  }
}
 
void Qjets::ComputeNewDistanceMeasures(fastjet::ClusterSequence & cs, int new_jet){
  // jet-jet distances
  for(unsigned int i = 0; i < cs.jets().size(); i++)
    if(JetUnmerged(i) && ((int)i) != new_jet){
      jet_distance jd{};
      jd.j1 = new_jet;
      jd.j2 = i;
      jd.dij = d_ij(cs.jets()[jd.j1], cs.jets()[jd.j2]);
      m_distances.push(jd);
    }
}
 
double Qjets::d_ij(const fastjet::PseudoJet& v1,const  fastjet::PseudoJet& v2) const{
  double p1 = v1.perp();
  double p2 = v2.perp();
  double ret = pow(min(p1,p2),m_exp_min) * pow(max(p1,p2),m_exp_max) * v1.squared_distance(v2);
  assert(!std::isnan(ret));
  return ret; 
}
 
double Qjets::Rand(){
  return m_rand.Rndm();
}