old.PhotonBackTracker_service.cc

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//
//
// \file: PhotonBackTracker_service.cc
//
//jason.stock@mines.sdsmt.edu
//Based on the BackTracker_service by Brian Rebel
//
#include <map>

// Framework includes
#include "art/Framework/Principal/Event.h"
#include "art/Framework/Principal/View.h"
#include "messagefacility/MessageLogger/MessageLogger.h"

// LArSoft includes
#include "larsim/MCCheater/PhotonBackTracker.h"
#include "larcore/Geometry/Geometry.h"
#include "larcorealg/Geometry/GeometryCore.h"
#include "lardata/Utilities/AssociationUtil.h"
#include "lardataobj/RecoBase/OpHit.h"
#include "lardataobj/RecoBase/SpacePoint.h"
#include "nutools/ParticleNavigation/EmEveIdCalculator.h"
#include "nusimdata/SimulationBase/MCParticle.h"
#include "lardataobj/Simulation/sim.h"
#include "larsim/Simulation/SimListUtils.h"
#include "lardata/DetectorInfoServices/DetectorClocksService.h"

namespace cheat{

  //----------------------------------------------------------------------
  PhotonBackTracker::PhotonBackTracker(const fhicl::ParameterSet& pset,
         art::ActivityRegistry& reg)
  {
    reconfigure(pset);

    reg.sPreProcessEvent.watch(this, &PhotonBackTracker::Rebuild);
  }

  //----------------------------------------------------------------------
  PhotonBackTracker::~PhotonBackTracker()
  {
  }

  //----------------------------------------------------------------------
  void PhotonBackTracker::reconfigure(const fhicl::ParameterSet& pset)
  {
    fG4ModuleLabel          = pset.get<std::string>("G4ModuleLabel",            "largeant");
    fMinOpHitEnergyFraction = pset.get<double     >("MinimumOpHitEnergyFraction", 0.1);
    fDelay                  = pset.get< double > ("Delay");
    have_complained         = false;
  }

  //----------------------------------------------------------------------
  void PhotonBackTracker::Rebuild(const art::Event& evt)
  {
    // do nothing if this is data
    if(evt.isRealData()) return;

    // get the particles from the event
    art::Handle<std::vector<simb::MCParticle>> pHandle;
    evt.getByLabel(fG4ModuleLabel, pHandle);

    // first check to see if we got called to early, that is the particles are
    // and sim channels are not made yet in a MC production job
    // if that is the case, we'll take care of it later
    if(pHandle.failedToGet()){
      mf::LogWarning("PhotonBackTracker") << "failed to get handle to simb::MCParticle from "
            << fG4ModuleLabel
            << ", return";
      return;
    }

    // Clear out anything remaining from previous calls to Rebuild
    fParticleList.clear();
    fMCTruthList .clear();
    cOpDetBacktrackerRecords .clear();
    //fVoxelList   .clear();

    art::FindOneP<simb::MCTruth> fo(pHandle, evt, fG4ModuleLabel);

    if( fo.isValid() ){
      for(size_t p = 0; p < pHandle->size(); ++p){

        simb::MCParticle *part = new simb::MCParticle(pHandle->at(p));
        fParticleList.Add(part);

        // get the simb::MCTruth associated to this sim::ParticleList
        try{
          art::Ptr<simb::MCTruth> mct = fo.at(p);
          if(fMCTruthList.size() < 1) fMCTruthList.push_back(mct);
          else{
            // check that we are not adding a simb::MCTruth twice to the collection
            // we know that all the particles for a given simb::MCTruth are put into the
            // collection of particles at the same time, so we can just check that the
            // current art::Ptr has a different id than the last one put
            if(!(mct == fMCTruthList.back())) fMCTruthList.push_back(mct);
          }
          // fill the track id to mctruth index map
          fTrackIDToMCTruthIndex[pHandle->at(p).TrackId()] = fMCTruthList.size() - 1;
        }
        catch(cet::exception &ex){
          mf::LogWarning("PhotonBackTracker") << "unable to find MCTruth from ParticleList "
          << "created in " << fG4ModuleLabel << " "
          << "any attempt to get the MCTruth objects from "
          << "the photon backtracker will fail\n"
          << "message from caught exception:\n" << ex;
        }
      }// end loop over particles to get MCTruthList
    }// end if fo.isValid()

    // grab the sim::OpDetBacktrackerRecords for this event

    /*
    try{evt.getView(fG4ModuleLabel, cOpDetBacktrackerRecords);}
    catch(art::Exception const& e){
      if(e.categoryCode() != art::errors::ProductNotFound) throw;
      if(have_complained==false){
      }
    }
    */

    art::Handle< std::vector< sim::OpDetBacktrackerRecord> > cPBTRHandle;
    //    std::vector< art::Ptr< sim::OpDetBacktrackerRecords > > cOpDetBacktrackerRecords;
    evt.getByLabel(fG4ModuleLabel, cPBTRHandle);
    if(cPBTRHandle.failedToGet()){//Failed to get products. Prepare for controlled freak out. Assuming this is because there is no OpDetBacktrackerRecords, we will not cause things to fail, but will prepare to fail if a user tries to call backtracker functionality.
      auto failMode = cPBTRHandle.whyFailed();
      if(failMode->categoryCode() != art::errors::ProductNotFound) throw;
      else if(have_complained==false){
        std::cout<<"FAILED BECAUSE "<<(*failMode)<<"\n";
        have_complained=true;
        mf::LogWarning("PhotonBackTracker")<<"Failed to get BackTrackerRecords from this event. All calls to the PhotonBackTracker will fail.\n"
          <<"This message will be generated only once per lar invokation. If this is event one, be aware the PhotonBackTracker may not work on any events from this file.\n"
          <<"Please change the log level to debug if you need more information for each event.\n"
          <<"Failed with :"<<(*failMode)<<"\n";
        mf::LogDebug("PhotonBackTracker")<<"Failed to get BackTrackerRecords from this event.\n";
      }else{
        mf::LogDebug("PhotonBackTracker")<<"Failed to get BackTrackerRecords from this event.\n";
      }
    }else{//Did not fail to get products. All is well. Run as expected.
      art::fill_ptr_vector(cOpDetBacktrackerRecords, cPBTRHandle);
    }


    // grab the voxel list for this event
    //fVoxelList = sim::SimListUtils::GetLArVoxelList(evt, fG4ModuleLabel);

    fParticleList.AdoptEveIdCalculator(new sim::EmEveIdCalculator);

    LOG_DEBUG("PhotonBackTracker") << "PhotonBackTracker has " << cOpDetBacktrackerRecords.size()
           << " sim::OpDetBacktrackerRecords and " << GetSetOfTrackIDs().size()
           << " tracks.  The particles are:\n"
           << fParticleList
           << "\n the MCTruth information is\n";
    for(size_t mc = 0; mc < fMCTruthList.size(); ++mc)
      LOG_DEBUG("PhotonBackTracker") << *(fMCTruthList.at(mc).get());

    return;
  }

  //----------------------------------------------------------------------
  const void PhotonBackTracker::shouldThisFail() const{
    //I need to revisit this and see if this check is too aggressive, as it only takes one failed event to set have_complained to true for the rest of the file.
    //I currently do believe this is okay, as have_complained only flips on ProductNotFound errors, and if that happens in one event of a file,
    // it should happen in all events of the file.
    if( have_complained==true ){
      throw cet::exception("PhotonBackTracker1") << "PhotonBackTracker methods called on a file without OpDetPhotonBacktrackerRecords. Backtracked information is not available.";
    }
  }

  //----------------------------------------------------------------------
  const simb::MCParticle* PhotonBackTracker::TrackIDToParticle(int const& id) const
  {
    shouldThisFail();
    sim::ParticleList::const_iterator part_it = fParticleList.find(id);

    if(part_it == fParticleList.end()){
      mf::LogWarning("PhotonBackTracker") << "can't find particle with track id "
            << id << " in sim::ParticleList"
            << " returning null pointer";
      return 0;
    }

    return part_it->second;
  }

  //----------------------------------------------------------------------
  const simb::MCParticle* PhotonBackTracker::TrackIDToMotherParticle(int const& id) const
  {
    shouldThisFail();
    // get the mother id from the particle navigator
    // the EveId was adopted in the Rebuild method

    return this->TrackIDToParticle(fParticleList.EveId(abs(id)));
  }

  //----------------------------------------------------------------------
  const art::Ptr<simb::MCTruth>& PhotonBackTracker::TrackIDToMCTruth(int const& id) const
  {
    shouldThisFail();
    // find the entry in the MCTruth collection for this track id
    size_t mct = fTrackIDToMCTruthIndex.find(abs(id))->second;

    if(/* mct < 0 || */ mct > fMCTruthList.size() )
      throw cet::exception("PhotonBackTracker") << "attempting to find MCTruth index for "
            << "out of range value: " << mct
            << "/" << fMCTruthList.size() << "\n";

    return fMCTruthList[mct];
  }

  //----------------------------------------------------------------------
  std::vector<sim::SDP> PhotonBackTracker::TrackIDToSimSDP(int const& id) const
  {
    shouldThisFail();
    std::vector<sim::SDP> sdps;

    // loop over all sim::OpDetBacktrackerRecords and fill a vector
    // of sim::SDP objects for the given track id
    for(size_t sc = 0; sc < cOpDetBacktrackerRecords.size(); ++sc){
      const auto & pdTimeSDPmap = cOpDetBacktrackerRecords[sc]->timePDclockSDPsMap();

      // loop over the SDPMAP
      for(auto mapitr = pdTimeSDPmap.begin(); mapitr != pdTimeSDPmap.end(); mapitr++){

  // loop over the vector of SDP objects.
  const std::vector<sim::SDP>& sdpvec = (*mapitr).second;
  for(size_t iv = 0; iv < sdpvec.size(); ++iv){
    if( abs(sdpvec[iv].trackID) == id) sdps.push_back(sdpvec[iv]);
  }

      } // end loop over map from sim::OpDetBacktrackerRecord
    } // end loop over sim::OpDetBacktrackerRecords

    return sdps;
  }

  //----------------------------------------------------------------------
  const art::Ptr<simb::MCTruth>& PhotonBackTracker::ParticleToMCTruth(const simb::MCParticle* p) const
  {
    shouldThisFail();
    return this->TrackIDToMCTruth(p->TrackId());
  }

  //----------------------------------------------------------------------
  std::vector<const simb::MCParticle*> PhotonBackTracker::MCTruthToParticles(art::Ptr<simb::MCTruth> const& mct) const
  {
    shouldThisFail();
    std::vector<const simb::MCParticle*> ret;

    // sim::ParticleList::value_type is a pair (track ID, particle pointer)
    for (const sim::ParticleList::value_type& TrackIDpair: fParticleList) {
      if( TrackIDToMCTruth(TrackIDpair.first) == mct )
        ret.push_back(TrackIDpair.second);
    }

    return ret;
  }

  //----------------------------------------------------------------------
  std::vector<sim::TrackSDP> PhotonBackTracker::OpHitToTrackSDPs(art::Ptr<recob::OpHit> const& opHit)
  {
    shouldThisFail();
    std::vector<sim::TrackSDP> trackSDPs;
    const double pTime = opHit->PeakTime();
    const double pWidth= opHit->Width();
    const double start = (pTime-pWidth)*1000-fDelay;
    const double end   = (pTime+pWidth)*1000-fDelay;

    this->ChannelToTrackSDPs(trackSDPs, opHit->OpChannel(), start, end);

    return trackSDPs;
  }

  //----------------------------------------------------------------------
  const std::vector<std::vector<art::Ptr<recob::OpHit>>> PhotonBackTracker::TrackIDsToOpHits(std::vector<art::Ptr<recob::OpHit>> const& allOpHits,
                       std::vector<int> const& tkIDs)
  {
    shouldThisFail();
    // returns a subset of the opHits in the allOpHits collection that are matched
    // to MC particles listed in tkIDs

    // temporary vector of TrackIDs and Ptrs to opHits so only one
    // loop through the (possibly large) allOpHits collection is needed
    std::vector<std::pair<int, art::Ptr<recob::OpHit>>> opHitList;
    std::vector<sim::TrackSDP> tids;
    for(auto itr = allOpHits.begin(); itr != allOpHits.end(); ++itr) {
      tids.clear();
      art::Ptr<recob::OpHit> const& opHit = *itr;
      const double pTime = opHit->PeakTime(), pWidth= opHit->Width();
      const double start = (pTime-pWidth)*1000.0-fDelay, end = (pTime+pWidth)*1000.0-fDelay;
      this->ChannelToTrackSDPs(tids, opHit->OpChannel(), start, end);
      for(auto itid = tids.begin(); itid != tids.end(); ++itid) {
        for(auto itkid = tkIDs.begin(); itkid != tkIDs.end(); ++itkid) {
          if(itid->trackID == *itkid) {
            if(itid->energyFrac > fMinOpHitEnergyFraction)
              opHitList.push_back(std::make_pair(*itkid, opHit));
          }
        } // itkid
      } // itid
    } // itr

    // now build the truOpHits vector that will be returned to the caller
    std::vector<std::vector<art::Ptr<recob::OpHit>>> truOpHits;
    // temporary vector containing opHits assigned to one MC particle
    std::vector<art::Ptr<recob::OpHit>> tmpOpHits;
    for(auto itkid = tkIDs.begin(); itkid != tkIDs.end(); ++itkid) {
      tmpOpHits.clear();
      for(auto itr = opHitList.begin(); itr != opHitList.end(); ++itr) {
        if(*itkid == (*itr).first) tmpOpHits.push_back((*itr).second);
      }
      truOpHits.push_back(tmpOpHits);
    }

    return truOpHits;
  }

  //----------------------------------------------------------------------

  std::vector<sim::TrackSDP> PhotonBackTracker::OpHitToEveSDPs(art::Ptr<recob::OpHit> const& opHit)
  {
    shouldThisFail();
    std::vector<sim::TrackSDP> trackSDPs = this->OpHitToTrackSDPs(opHit);

    // make a map of evd ID values and fraction of energy represented by
    // that eve id in this opHit
    std::map<int, float> eveIDtoEfrac;

    double totalE = 0.;
    for(size_t t = 0; t < trackSDPs.size(); ++t){
      eveIDtoEfrac[fParticleList.EveId( trackSDPs[t].trackID )] += trackSDPs[t].energy;
      totalE += trackSDPs[t].energy;
    }

    // now fill the eveSDPs vector from the map
    std::vector<sim::TrackSDP> eveSDPs;
    eveSDPs.reserve(eveIDtoEfrac.size());
    for(auto itr = eveIDtoEfrac.begin(); itr != eveIDtoEfrac.end(); itr++){
      sim::TrackSDP temp;
      temp.trackID    = (*itr).first;
      temp.energyFrac = (*itr).second/totalE;
      temp.energy     = (*itr).second;
      eveSDPs.push_back(std::move(temp));
    }

    return eveSDPs;
  }
  std::vector<sim::TrackSDP> PhotonBackTracker::OpHitToEveID(art::Ptr<recob::OpHit> const& opHit)
  {
    mf::LogWarning("PhotonBackTracker") << "PhotonBackTracker::OpHitToEveID is being replaced with PhotonBackTracker::OpHitToEveSDPs. Please \n update your code accordingly.\n ";
    std::vector<sim::TrackSDP> eveSDPs = OpHitToEveSDPs(opHit);
    return eveSDPs;
  }

  //----------------------------------------------------------------------
  std::set<int> PhotonBackTracker::GetSetOfEveIDs()
  {
    shouldThisFail();
    std::set<int> eveIDs;

    sim::ParticleList::const_iterator plitr = fParticleList.begin();
    while(plitr != fParticleList.end() ){
      int eveID = fParticleList.EveId((*plitr).first);
      // look to see if this eveID is already in the set
      if( eveIDs.find(eveID) == eveIDs.end() ) eveIDs.insert(eveID);
      plitr++;
    }

    return eveIDs;
  }

  //----------------------------------------------------------------------
  std::set<int> PhotonBackTracker::GetSetOfTrackIDs()
  {
    shouldThisFail();
    // fParticleList::value_type is a pair (track, particle pointer)
    std::set<int> trackIDs;
    for (const sim::ParticleList::value_type& pl: fParticleList)
      trackIDs.insert(pl.first);

    return trackIDs;
  }

  //----------------------------------------------------------------------
  std::set<int> PhotonBackTracker::GetSetOfEveIDs(std::vector< art::Ptr<recob::OpHit> > const& opHits)
  {
    shouldThisFail();
    std::set<int> eveIDs;

    std::vector< art::Ptr<recob::OpHit> >::const_iterator itr = opHits.begin();
    while(itr != opHits.end() ){

      // get the eve ids corresponding to this opHit
      const std::vector<sim::TrackSDP> sdps = OpHitToEveID(*itr);

      // loop over the sdps and extract the track ids
      for(size_t i = 0; i < sdps.size(); ++i) eveIDs.insert(sdps[i].trackID);

      itr++;
    }

    return eveIDs;
  }

  //----------------------------------------------------------------------
  std::set<int> PhotonBackTracker::GetSetOfTrackIDs(std::vector< art::Ptr<recob::OpHit> > const& opHits)
  {
    shouldThisFail();
    std::set<int> trackIDs;

    std::vector< art::Ptr<recob::OpHit> >::const_iterator itr = opHits.begin();
    while(itr != opHits.end() ){

      std::vector<sim::TrackSDP> trackSDPs;

      // get the track ids corresponding to this opHit
      const double pTime = (*itr)->PeakTime();
      const double pWidth= (*itr)->Width();
      const double start = (pTime-pWidth)*1000.0-fDelay;
      const double end   = (pTime+pWidth)*1000.0-fDelay;
     // const double start = (*itr)->PeakTimeMinusRMS();
     // const double end   = (*itr)->PeakTimePlusRMS();

      this->ChannelToTrackSDPs(trackSDPs, (*itr)->OpChannel(), start, end);

      // loop over the sdps and extract the track ids
      for(size_t i = 0; i < trackSDPs.size(); ++i) {
  trackIDs.insert(trackSDPs[i].trackID);
      }

      itr++;
    }

    return trackIDs;
  }

  //----------------------------------------------------------------------
  double PhotonBackTracker::OpHitCollectionPurity(std::set<int>                              trackIds,
            std::vector< art::Ptr<recob::OpHit> > const& opHits)
  {
    shouldThisFail();
    // get the list of EveIDs that correspond to the opHits in this collection
    // if the EveID shows up in the input list of trackIDs, then it counts
    float total   = 1.*opHits.size();;
    float desired = 0.;
    for(size_t h = 0; h < opHits.size(); ++h){
      art::Ptr<recob::OpHit> opHit = opHits[h];
      std::vector<sim::TrackSDP> opHitTrackSDPs = this->OpHitToTrackSDPs(opHit);
      // don't double count if this opHit has more than one of the
      // desired track IDs associated with it
      for(size_t e = 0; e < opHitTrackSDPs.size(); ++e){
  if(trackIds.find(opHitTrackSDPs[e].trackID) != trackIds.end()){
    desired += 1.;
    break;
  }
      }
    }// end loop over opHits
    double purity = 0.;
    if(total > 0) purity = desired/total;
    return purity;
  }

  //----------------------------------------------------------------------
  double PhotonBackTracker::OpHitChargeCollectionPurity(std::set<int>                              trackIDs,
      std::vector< art::Ptr<recob::OpHit> > const& opHits)
  {
    shouldThisFail();
    // get the list of EveIDs that correspond to the opHits in this collection
    // if the EveID shows up in the input list of trackIDs, then it counts
    float total   = 0;
    float desired = 0.;
    // don't have to check the view in the opHits collection because
    // those are assumed to be from the object we are testing and will
    // the correct view by definition then.
    for(size_t h = 0; h < opHits.size(); ++h){
      art::Ptr<recob::OpHit> opHit = opHits[h];
      std::vector<sim::TrackSDP> opHitTrackIDs = this->OpHitToTrackSDPs(opHit);
      total+=opHit->Area(); // sum up the charge in the cluster
      // don't double count if this opHit has more than one of the
      // desired track IDs associated with it
      for(size_t e = 0; e < opHitTrackIDs.size(); ++e){
        if(trackIDs.find(opHitTrackIDs[e].trackID) != trackIDs.end()){
          desired += opHit->Area();
          break;
        }
      }
    }// end loop over opHits
    double purity = 0.;
    if(total > 0) purity = desired/total;
    return purity;
  }


  //----------------------------------------------------------------------
  double PhotonBackTracker::OpHitCollectionEfficiency(std::set<int>                              trackIDs,
      std::vector< art::Ptr<recob::OpHit> > const& opHits,
      std::vector< art::Ptr<recob::OpHit> > const& allOpHits,
      geo::View_t                         const& view)
  {
    throw cet::exception("PhotonBackTracker")<<"This function is not supported. OpHits do not have type View.\n";
  }

  double PhotonBackTracker::OpHitCollectionEfficiency(std::set<int> trackIds, std::vector< art::Ptr<recob::OpHit> > const& opHits, std::vector< art::Ptr<recob::OpHit> > const& allOpHits)
  {
    shouldThisFail();
    float desired = 0.;
    float total   = 0.;
    for(size_t h = 0; h < opHits.size(); ++h){
      art::Ptr<recob::OpHit> opHit = opHits[h];
      std::vector<sim::TrackSDP> opHitTrackSDPs = this->OpHitToTrackSDPs(opHit);
      // also don't double count if this opHit has more than one of the
      // desired track IDs associated with it
      for(size_t e = 0; e < opHitTrackSDP.size(); ++e){
        if(trackIDs.find(opHitTrackSDPs[e].trackID) != trackIDs.end() &&
 opHitTrackSDPs[e].energyFrac >= fMinOpHitEnergyFraction){
          desired += 1.;
          break;
        }
      }
    }// end loop over opHits
    // now figure out how many opHits in the whole collection are associated with this id
    for(size_t h = 0; h < allOpHits.size(); ++h){
      art::Ptr<recob::OpHit> opHit = allOpHits[h];
      std::vector<sim::TrackSDP> opHitTrackSDPs = this->OpHitToTrackSDPs(opHit);
      for(size_t e = 0; e < opHitTrackSDPs.size(); ++e){
        // don't worry about opHits where the energy fraction for the chosen
        // trackID is < 0.1
        // also don't double count if this opHit has more than one of the
        // desired track IDs associated with it
        if(trackIDs.find(opHitTrackSDPs[e].trackID) != trackIDs.end() &&
            opHitTrackIDs[e].energyFrac             >= fMinOpHitEnergyFraction){
          total += 1.;
          break;
        }
      }
    }// end loop over all opHits
    double efficiency = 0.;
    if(total > 0.) efficiency = desired/total;
    return efficiency;
  }

  //----------------------------------------------------------------------
  double PhotonBackTracker::OpHitChargeCollectionEfficiency(std::set<int>                              trackIDs,
      std::vector< art::Ptr<recob::OpHit> > const& opHits,
      std::vector< art::Ptr<recob::OpHit> > const& allOpHits,
      geo::View_t                         const& view)
  {
    throw cet::exception("PhotonBackTracker")<<"This function is not supported. OpHits do not have type View.\n";
  }
  double PhotonBackTracker::OpHitChargeCollectionEfficiency(std::set<int>                              trackIDs,
      std::vector< art::Ptr<recob::OpHit> > const& opHits,
      std::vector< art::Ptr<recob::OpHit> > const& allOpHits)
  {
    shouldThisFail();
    // get the list of EveIDs that correspond to the opHits in this collection
    // and the energy associated with the desired trackID
    float desired = 0.;
    float total   = 0.;

    // don't have to check the view in the opHits collection because
    // those are assumed to be from the object we are testing and will
    // the correct view by definition then.
    for(size_t h = 0; h < opHits.size(); ++h){

      art::Ptr<recob::OpHit> opHit = opHits[h];
      std::vector<sim::TrackSDP> opHitTrackIDs = this->OpHitToTrackSDPs(opHit);

      // don't worry about opHits where the energy fraction for the chosen
      // trackID is < 0.1
      // also don't double count if this opHit has more than one of the
      // desired track IDs associated with it
      for(size_t e = 0; e < opHitTrackIDs.size(); ++e){
        if(trackIDs.find(opHitTrackIDs[e].trackID) != trackIDs.end() &&
            opHitTrackIDs[e].energyFrac             >= fMinOpHitEnergyFraction){
          desired += opHit->Area();
          break;
        }
      }
    }// end loop over opHits

    // now figure out how many opHits in the whole collection are associated with this id
    for(size_t h = 0; h < allOpHits.size(); ++h){

      art::Ptr<recob::OpHit> opHit = allOpHits[h];

      // check that we are looking at the appropriate view here
      // in the case of 3D objects we take all opHits
      //if(opHit->View() != view && view != geo::k3D ) continue;

      std::vector<sim::TrackSDP> opHitTrackIDs = this->OpHitToTrackSDPs(opHit);

      for(size_t e = 0; e < opHitTrackIDs.size(); ++e){
        // don't worry about opHits where the energy fraction for the chosen
        // trackID is < 0.1
        // also don't double count if this opHit has more than one of the
        // desired track IDs associated with it
        if(trackIDs.find(opHitTrackIDs[e].trackID) != trackIDs.end() &&
            opHitTrackIDs[e].energyFrac             >= fMinOpHitEnergyFraction){
          total += opHit->Area();
          break;
        }
      }

    }// end loop over all opHits

    double efficiency = 0.;
    if(total > 0.) efficiency = desired/total;

    return efficiency;
  }



  //----------------------------------------------------------------------
  const art::Ptr< sim::OpDetBacktrackerRecord > PhotonBackTracker::FindOpDetBacktrackerRecord(int opDetNum) const
  {
    shouldThisFail();
    art::Ptr< sim::OpDetBacktrackerRecord > opDet;

    for(size_t sc = 0; sc < cOpDetBacktrackerRecords.size(); ++sc){
      //This could become a bug. What if it occurs twice (shouldn't happen in correct recorts, but still, no error handeling included for the situation
      if(cOpDetBacktrackerRecords[sc]->OpDetNum() == opDetNum) opDet = cOpDetBacktrackerRecords[sc];
    }

    if(!opDet)
    {
      throw cet::exception("PhotonBackTracker2") << "No sim::OpDetBacktrackerRecord corresponding "
        << "to opDetNum: " << opDetNum << "\n";
    }

    return opDet;
  }

  //----------------------------------------------------------------------
  void PhotonBackTracker::ChannelToTrackSDPs(std::vector<sim::TrackSDP>&   trackSDPs,
      int channel,
      const double opHit_start_time,
      const double opHit_end_time)
  {
    shouldThisFail();
    trackSDPs.clear();

    double totalE = 0.;

    try{
      const art::Ptr< sim::OpDetBacktrackerRecord > schannel = this->FindOpDetBacktrackerRecord( geom->OpDetFromOpChannel(channel) );

      // loop over the photons in the channel and grab those that are in time
      // with the identified opHit start and stop times
      //const detinfo::DetectorClocks* ts = lar::providerFrom<detinfo::DetectorClocksService>();
      //int start_tdc = ts->OpticalG4Time2TDC( opHit_start_time );
      //int end_tdc   = ts->OpticalG4Time2TDC( opHit_end_time   );
      //      if(start_tdc<0) start_tdc = 0;
      //      if(end_tdc<0) end_tdc = 0;
      std::vector<sim::SDP> simSDPs = schannel->TrackIDsAndEnergies(opHit_start_time, opHit_end_time);

      // first get the total energy represented by all track ids for
      // this channel and range of tdc values
      for(size_t e = 0; e < simSDPs.size(); ++e)
        totalE += simSDPs[e].energy;


      // protect against a divide by zero below
      if(totalE < 1.e-5) totalE = 1.;

      // loop over the entries in the map and fill the input vectors

      for(size_t e = 0; e < simSDPs.size(); ++e){

        if(simSDPs[e].trackID == sim::NoParticleId) continue;

        sim::TrackSDP info;
        info.trackID    = simSDPs[e].trackID;
        info.energyFrac = simSDPs[e].energy/totalE;
        info.energy     = simSDPs[e].energy;

        trackSDPs.push_back(info);

      }
    }// end try
    catch(cet::exception e){
      mf::LogWarning("PhotonBackTracker") << "caught exception \n"
        << e;
    }

    return;
  }

  //----------------------------------------------------------------------
  void PhotonBackTracker::OpHitToSDPs(recob::OpHit const& opHit,
      std::vector<sim::SDP>&      sdps) const
  {
    shouldThisFail();
    // Get services.
    //const detinfo::DetectorClocks* ts = lar::providerFrom<detinfo::DetectorClocksService>();

    double fPeakTime       = opHit.PeakTime();
    double fWidth          = opHit.Width();
    sim::OpDetBacktrackerRecord::timePDclock_t start_time = ((fPeakTime-fWidth)*1000.0)-fDelay;
    sim::OpDetBacktrackerRecord::timePDclock_t end_time = ((fPeakTime+fWidth)*1000.0)-fDelay;

    sdps = FindOpDetBacktrackerRecord( geom->OpDetFromOpChannel(opHit.OpChannel()) )->TrackIDsAndEnergies(start_time, end_time);

  }

  //----------------------------------------------------------------------
  std::vector<double> PhotonBackTracker::SimSDPsToXYZ(std::vector<sim::SDP> const& sdps)
  {
    shouldThisFail();
    std::vector<double> xyz(3, -999.);

    double x = 0.;
    double y = 0.;
    double z = 0.;
    double w = 0.;

    // loop over photons.

    for(auto const& sdp : sdps) {

      double weight = sdp.numPhotons;

      w += weight;
      x += weight * sdp.x;
      y += weight * sdp.y;
      z += weight * sdp.z;

    }// end loop over sim::SDPs

    //If the sum of the weights is still zero, then fail to return a value.
    //A hit with no contributing photons does't make sense.
    if(w < 1.e-5)
      throw cet::exception("PhotonBackTracker") << "No sim::SDPs providing non-zero number of photons"
        << " can't determine originating location from truth\n";

    xyz[0] = x/w;
    xyz[1] = y/w;
    xyz[2] = z/w;

    return xyz;
  }

  //----------------------------------------------------------------------
  std::vector<double> PhotonBackTracker::OpHitToXYZ(art::Ptr<recob::OpHit> const& opHit)
  {
    shouldThisFail();
    std::vector<sim::SDP> sdps;
    OpHitToSDPs(opHit, sdps);
    return SimSDPsToXYZ(sdps);
  }

} // namespace

namespace cheat{
  DEFINE_ART_SERVICE(PhotonBackTracker)
}