Loops.h

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/*
  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
*/
 
#ifndef XAOD_ANALYSIS
#ifndef TRUTHUTILS_LOOPS_H
#define TRUTHUTILS_LOOPS_H
#include <algorithm>
#include <vector>
#include <memory>
 
namespace MC {
template <class Evt, class Prt, class Vtx>
class Loops  {
public:

    Loops() {}

    Loops(const Evt* evt) {
        findLoops(evt,true);
    }

    bool isLoop(const Prt& p) const {
        return std::find(m_loop_particles.begin(), m_loop_particles.end(), p) != m_loop_particles.end();
    }

    bool isLoop(const Vtx& v) const {
        return std::find(m_loop_vertices.begin(), m_loop_vertices.end(), v) != m_loop_vertices.end();
    }

    const std::vector<Prt>& loop_particles() const {
        return m_loop_particles;
    }

    const std::vector<Vtx>& loop_vertices() const {
        return m_loop_vertices;
    }

    int findLoops(const Evt* evt, bool force) {
        // If the event is empty, return -1 (an error code, basically)
        if (!evt) return -1;
        // If we have already indexed the event and are not forced to do it again, return
        if (evt == m_evt &&  !force) return 0;
 
        // Update the local event pointer. This is now the most recent event that has been indexed.
        m_evt = evt;
        // Clear the lists of particles and vertices. These will now refer to this event.
        m_loop_particles.clear();
        m_loop_vertices.clear();
 
        // Create a map, keeping track of whether particles are in loops.
        // 0 for "unknown", 1 for "in a loop", -1 for "not in a loop"
        std::map<Prt, int> incycle;
 
        // Easiest case to deal with: if a particle has no production or end vertex, it is not in a loop
        for (const auto & p: *m_evt){
            if (!p->end_vertex()||!p->production_vertex()){
              incycle[p] = -1;
            } else {
              incycle[p] = 0;
            }
        }
 
        // Now begin interating through particles to try to find loops
        size_t minincycle = m_evt->particles_size();
        for (;;) {
            // Number of particles still unknown in the current iteration
            size_t unknown = 0;
            for (const auto & p: *m_evt) {
 
                // If the particle was already identified as in/not in a loop, skip it
                if (incycle[p] != 0) continue;
                // Otherwise it is unknown on this iteration
                unknown++;
 
                // Start from the end vertex of the particle
                auto ev = p->end_vertex();
                if (ev) {
                    // If the end vertex of a particle exclusively has particles not in loops
                    // then this particle cannot be in a loop either
                    bool goodo = true;
                    for (auto& po: *ev) goodo = goodo && (incycle[po] == -1);
                    if (goodo) incycle[p] = -1;
                }
 
                // Now start from the production vertex of the particle
                auto pv = p->production_vertex();
                if (pv) {
                    // If the production vertex of a particle exclusively has particles not
                    // in loops, then this particle cannot be in a loop either
                    bool goodi = true;
                    for (auto& pi: ev->particles_in()) goodi = goodi && (incycle[pi] == -1);
                    if (goodi) incycle[p] = -1;
                }
            }
            // If the number of unknown particles has not changed, we are done iterating
            if (minincycle == unknown) break;
            // If the number of unknown particles has changed, update at iterate again
            minincycle = std::min(minincycle, unknown);
        } // Outer loop - iterate until all particles have been dealt with
 
        // Any remaining particles that have not been identified are a part of a loop
        // Add them to our list of looping particles
        for (const auto & p: *m_evt){
          if (incycle[p] == 0) incycle[p] = 1;
          if (incycle[p] == 1) m_loop_particles.push_back(p);
        }
 
        // Now loop over all vertices.
        for (auto & v: m_evt->vertices()) {
            bool push = false;
            // First check incoming particles.
            // If any incoming are in loops, consider this vertex to be in a loop.
            for ( auto& pin: v->particles_in()){
                if (incycle[pin] == 1) {
                      push = true;
                      break;
                }
            } // End of loop over incoming particles
            // In case we have not yet identified the vertex as being in a loop
            // then check the outgoing particles from the vertex.
            // If any outgoing are in loops, consider this vertex to be in a loop.
            if(!push) {
                for ( const auto& pou: *v) {
                    if (incycle[pou] == 1) {
                        push = true;
                        break;
                    }
                }
            }
            // Update the records if this vertex is in a loop.
            if (push) m_loop_vertices.push_back(v);
        } // End of loop over vertices
 
        // All finished. Return success.
        return 0;
    } // End of the findLoops function
 
private:

    const Evt* m_evt = nullptr;

    std::vector<Prt> m_loop_particles;

    std::vector<Vtx> m_loop_vertices;
};
}
#endif
#endif