BackTracker.cc

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//
//
// \file BackTracker.cc
// \brief The functions needed for the BackTracker class needed by the BackTracker service
//        in order to connect truth information with reconstruction.
// \author jason.stock@mines.sdsmt.edu
//
// Based on the original BackTracker by brebel@fnal.gov
//
//

#include "messagefacility/MessageLogger/MessageLogger.h"

#include "BackTracker.h"
#include "lardataobj/Simulation/sim.h"

#include "larcorealg/Geometry/GeometryCore.h"
#include "larcorealg/Geometry/WireReadoutGeom.h"
#include "larcoreobj/SimpleTypesAndConstants/geo_types.h"
#include "lardataalg/DetectorInfo/DetectorClocks.h"

namespace cheat {

  //-----------------------------------------------------------------------
  BackTracker::BackTracker(const fhiclConfig& config,
                           const cheat::ParticleInventory* partInv,
                           geo::GeometryCore const* geom,
                           geo::WireReadoutGeom const* wireReadoutGeom)
    : fPartInv(partInv)
    , fWireReadoutGeom(wireReadoutGeom)
    , fG4ModuleLabel(config.G4ModuleLabel())
    , fSimChannelModuleLabel(config.SimChannelModuleLabel())
    , fHitLabel(config.DefaultHitModuleLabel())
    , fMinHitEnergyFraction(config.MinHitEnergyFraction())
    , fOverrideRealData(config.OverrideRealData())
    , fHitTimeRMS(config.HitTimeRMS())
  {}

  //-----------------------------------------------------------------------
  BackTracker::BackTracker(const fhicl::ParameterSet& pSet,
                           const cheat::ParticleInventory* partInv,
                           geo::GeometryCore const* geom,
                           geo::WireReadoutGeom const* wireReadoutGeom)
    : fPartInv(partInv)
    , fWireReadoutGeom(wireReadoutGeom)
    , fG4ModuleLabel(pSet.get<art::InputTag>("G4ModuleLabel", "largeant"))
    , fSimChannelModuleLabel(pSet.get<art::InputTag>("SimChannelModuleLabel", fG4ModuleLabel))
    , // -- D.R. if not provided, default behavior is to use the G4ModuleLabel
    fHitLabel(pSet.get<art::InputTag>("DefaultHitModuleLabel", "hitfd"))
    , fMinHitEnergyFraction(pSet.get<double>("MinHitEnergyFraction", 0.010))
    , fOverrideRealData(pSet.get<bool>("OverrideRealData", false))
    , fHitTimeRMS(pSet.get<double>("HitTimeRMS", 1.0))
  {}

  //-----------------------------------------------------------------------
  void BackTracker::ClearEvent()
  {
    fSimChannels.clear();
    //    fAllHitList.clear();
  }

  //-----------------------------------------------------------------------
  std::vector<const sim::IDE*> BackTracker::TrackIdToSimIDEs_Ps(int const& id) const
  {
    std::vector<const sim::IDE*> ideps;
    for (size_t sc = 0; sc < fSimChannels.size(); ++sc) {
      const auto& tdcidemap = fSimChannels[sc]->TDCIDEMap(); // This returns a reference.
      for (auto mapitr = tdcidemap.begin(); mapitr != tdcidemap.end(); mapitr++) {
        // loop over the vector of IDE objects.
        const std::vector<sim::IDE>& idevec =
          (*mapitr).second; // note, mapitr.second returns the actual data from
                            // the map, not a copy
        for (size_t iv = 0; iv < idevec.size(); ++iv) {
          if (abs(idevec[iv].trackID) == id) ideps.push_back(&(idevec[iv]));
        } // end for index in idevec
      }   // end loop over map from sim::SimChannel
    }     // end loop over sim::SimChannels
    return ideps;
  }

  //-----------------------------------------------------------------------
  std::vector<const sim::IDE*> BackTracker::TrackIdToSimIDEs_Ps(int const& id,
                                                                const geo::View_t view) const
  {
    std::vector<const sim::IDE*> ide_Ps;
    for (const art::Ptr<sim::SimChannel> sc : fSimChannels) {
      if (fWireReadoutGeom->View(sc->Channel()) != view) continue;

      for (const auto& item : sc->TDCIDEMap()) {

        // loop over the vector of IDE objects.
        for (const sim::IDE& ide : item.second) {
          if (abs(ide.trackID) == id) ide_Ps.push_back(&ide);
        }
      } // end loop over map from sim::SimChannel
    }   // end loop over sim::SimChannels

    return ide_Ps;
  }

  //-----------------------------------------------------------------------
  art::Ptr<sim::SimChannel> BackTracker::FindSimChannelPtr(raw::ChannelID_t channel) const
  {
    auto ilb = std::lower_bound(fSimChannels.begin(),
                                fSimChannels.end(),
                                channel,
                                [](art::Ptr<sim::SimChannel> const& a, raw::ChannelID_t channel) {
                                  return (a->Channel() < channel);
                                });
    return ((ilb != fSimChannels.end()) && ((*ilb)->Channel() == channel)) ?
             *ilb :
             art::Ptr<sim::SimChannel>{};
  }

  //-----------------------------------------------------------------------
  art::Ptr<sim::SimChannel> BackTracker::FindSimChannel(raw::ChannelID_t channel) const
  {
    if (auto const chan = FindSimChannelPtr(channel)) return chan;
    throw cet::exception("BackTracker") << "No sim::SimChannel corresponding "
                                        << "to channel: " << channel << "\n";
  }

  //-----------------------------------------------------------------------
  std::vector<sim::TrackIDE> BackTracker::ChannelToTrackIDEs(
    detinfo::DetectorClocksData const& clockData,
    raw::ChannelID_t channel,
    const double hit_start_time,
    const double hit_end_time) const
  {
    art::Ptr<sim::SimChannel> schannel = FindSimChannelPtr(channel);
    if (!schannel) return {};

    std::vector<sim::TrackIDE> trackIDEs;
    double totalE = 0.;

    // loop over the electrons in the channel and grab those that are in time with the
    // identified hit start and stop times
    int start_tdc = clockData.TPCTick2TDC(hit_start_time);
    int end_tdc = clockData.TPCTick2TDC(hit_end_time);
    if (start_tdc < 0) start_tdc = 0;
    if (end_tdc < 0) end_tdc = 0;
    std::vector<sim::IDE> simides = schannel->TrackIDsAndEnergies(start_tdc, end_tdc);

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

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

      sim::TrackIDE info;
      info.trackID = simides[e].trackID;
      info.energyFrac = simides[e].energy / totalE;
      info.energy = simides[e].energy;
      info.numElectrons = simides[e].numElectrons;

      trackIDEs.push_back(info);
    }

    return trackIDEs;
  }

  //-----------------------------------------------------------------------
  std::vector<sim::TrackIDE> BackTracker::HitToTrackIDEs(
    detinfo::DetectorClocksData const& clockData,
    recob::Hit const& hit) const
  {
    const double start = hit.PeakTimeMinusRMS(fHitTimeRMS);
    const double end = hit.PeakTimePlusRMS(fHitTimeRMS);
    return ChannelToTrackIDEs(clockData, hit.Channel(), start, end);
  }

  //-----------------------------------------------------------------------
  std::vector<int> BackTracker::HitToTrackIds(detinfo::DetectorClocksData const& clockData,
                                              recob::Hit const& hit) const
  {
    std::vector<int> retVec;
    for (auto const trackIDE : HitToTrackIDEs(clockData, hit)) {
      retVec.push_back(trackIDE.trackID);
    }
    return retVec;
  }

  // These don't exist in the event. They are generated on the fly.
  //----------------------------------------------------------------------------
  std::vector<sim::TrackIDE> BackTracker::HitToEveTrackIDEs(
    detinfo::DetectorClocksData const& clockData,
    recob::Hit const& hit) const
  {
    std::vector<sim::TrackIDE> eveIDEs;
    std::vector<sim::TrackIDE> trackIDEs = HitToTrackIDEs(clockData, hit);
    std::map<int, std::pair<double, double>> eveToEMap;
    double totalE = 0.0;
    for (const auto& trackIDE : trackIDEs) {
      auto check = eveToEMap.emplace(fPartInv->TrackIdToEveTrackId(trackIDE.trackID),
                                     std::make_pair(trackIDE.energy, trackIDE.numElectrons));
      if (check.second == false) {
        check.first->second.first += trackIDE.energy;
        check.first->second.second += trackIDE.numElectrons;
      }
      totalE += trackIDE.energy;
    } // End for trackIDEs
    eveIDEs.reserve(eveToEMap.size());
    for (const auto& eveToE : eveToEMap) {
      sim::TrackIDE eveTrackIDE_tmp;

      eveTrackIDE_tmp.trackID = eveToE.first;
      eveTrackIDE_tmp.energy = eveToE.second.first;
      eveTrackIDE_tmp.energyFrac = (eveTrackIDE_tmp.energy) / (totalE);
      eveTrackIDE_tmp.numElectrons = eveToE.second.second;

      eveIDEs.push_back(eveTrackIDE_tmp);
    } // END eveToEMap loop
    return eveIDEs;
  }

  //-----------------------------------------------------------------------
  std::vector<art::Ptr<recob::Hit>> BackTracker::TrackIdToHits_Ps(
    detinfo::DetectorClocksData const& clockData,
    const int tkId,
    std::vector<art::Ptr<recob::Hit>> const& hitsIn) const
  {
    // returns a subset of the hits in the hitsIn collection that are matched to the given
    // track

    // temporary vector of TrackIds and Ptrs to hits so only one loop through the
    // (possibly large) hitsIn collection is needed
    std::vector<art::Ptr<recob::Hit>> hitList;
    std::vector<sim::TrackIDE> trackIDE;
    for (auto itr = hitsIn.begin(); itr != hitsIn.end(); ++itr) {
      trackIDE.clear();
      art::Ptr<recob::Hit> const& hit = *itr;
      trackIDE = ChannelToTrackIDEs(clockData,
                                    hit->Channel(),
                                    hit->PeakTimeMinusRMS(fHitTimeRMS),
                                    hit->PeakTimePlusRMS(fHitTimeRMS));
      for (auto itr_trakIDE = trackIDE.begin(); itr_trakIDE != trackIDE.end(); ++itr_trakIDE) {
        if (itr_trakIDE->trackID == tkId && itr_trakIDE->energyFrac > fMinHitEnergyFraction)
          hitList.push_back(hit);
      } // itr_trakIDE
    }   // itr
    return hitList;
  }

  //-----------------------------------------------------------------------
  // This function could clearly be made by calling TrackIdToHits for each trackId, but
  // that would be significantly slower because we would loop through all hits many times.
  std::vector<std::vector<art::Ptr<recob::Hit>>> BackTracker::TrackIdsToHits_Ps(
    detinfo::DetectorClocksData const& clockData,
    std::vector<int> const& tkIds,
    std::vector<art::Ptr<recob::Hit>> const& hitsIn) const
  {
    // returns a subset of the hits in the hitsIn collection that are matched to MC
    // particles listed in tkIds

    // temporary vector of TrackIds and Ptrs to hits so only one loop through the
    // (possibly large) hitsIn collection is needed
    std::vector<std::pair<int, art::Ptr<recob::Hit>>> hitList;
    std::vector<sim::TrackIDE> tids;
    for (auto itr = hitsIn.begin(); itr != hitsIn.end(); ++itr) {
      tids.clear();
      art::Ptr<recob::Hit> const& hit = *itr;
      tids = ChannelToTrackIDEs(clockData,
                                hit->Channel(),
                                hit->PeakTimeMinusRMS(fHitTimeRMS),
                                hit->PeakTimePlusRMS(fHitTimeRMS));
      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 > fMinHitEnergyFraction)
              hitList.push_back(std::make_pair(*itkid, hit));
          }
        } // itkid
      }   // itid
    }     // itr

    // now build the truHits vector that will be returned to the caller
    std::vector<std::vector<art::Ptr<recob::Hit>>> truHits;
    // temporary vector containing hits assigned to one MC particle
    std::vector<art::Ptr<recob::Hit>> tmpHits;
    for (auto itkid = tkIds.begin(); itkid != tkIds.end(); ++itkid) {
      tmpHits.clear();
      for (auto itr = hitList.begin(); itr != hitList.end(); ++itr) {
        if (*itkid == (*itr).first) tmpHits.push_back((*itr).second);
      }
      truHits.push_back(tmpHits);
    }
    return truHits;
  }

  //-----------------------------------------------------------------------
  //Cannot be returned as a reference, as these IDEs do not exist in the event. They are constructed on the fly.
  std::vector<sim::IDE> BackTracker::HitToAvgSimIDEs(detinfo::DetectorClocksData const& clockData,
                                                     recob::Hit const& hit) const
  {
    int start_tdc = clockData.TPCTick2TDC(hit.PeakTimeMinusRMS(fHitTimeRMS));
    int end_tdc = clockData.TPCTick2TDC(hit.PeakTimePlusRMS(fHitTimeRMS));
    if (start_tdc < 0) start_tdc = 0;
    if (end_tdc < 0) end_tdc = 0;

    return FindSimChannel(hit.Channel())->TrackIDsAndEnergies(start_tdc, end_tdc);
  }

  //-----------------------------------------------------------------------
  std::vector<const sim::IDE*> BackTracker::HitToSimIDEs_Ps(
    detinfo::DetectorClocksData const& clockData,
    recob::Hit const& hit) const
  {
    std::vector<const sim::IDE*> retVec;
    int start_tdc = clockData.TPCTick2TDC(hit.PeakTimeMinusRMS(fHitTimeRMS));
    int end_tdc = clockData.TPCTick2TDC(hit.PeakTimePlusRMS(fHitTimeRMS));
    if (start_tdc < 0) start_tdc = 0;
    if (end_tdc < 0) end_tdc = 0;

    if (start_tdc > end_tdc) { throw; }

    // The following does not return a std::map. It returns a vector... with no guarantee
    // that it is sorted...
    auto const& tdcIDEMap = FindSimChannel(hit.Channel())->TDCIDEMap();
    std::vector<const std::pair<unsigned short, std::vector<sim::IDE>>*> tdcIDEMap_SortedPointers;
    for (auto& pair : tdcIDEMap) {
      tdcIDEMap_SortedPointers.push_back(&pair);
    }

    // This is a bunch of extra steps, due to needing a vector we can sort, and needing
    // those items in the sorted vector to be the items from the sim channels (so a
    // pointer to the IDEs inside the sim channels can be made).  The work around is to
    // make a vector of pointers to IDEs inside the TDCIDEMap (which is a constant
    // reference to the fTDCIDEs in the SimChannel.)
    auto pairSort = [](auto& a, auto& b) { return a->first < b->first; };
    if (!std::is_sorted(
          tdcIDEMap_SortedPointers.begin(), tdcIDEMap_SortedPointers.end(), pairSort)) {
      std::sort(tdcIDEMap_SortedPointers.begin(), tdcIDEMap_SortedPointers.end(), pairSort);
    }

    std::vector<sim::IDE> dummyVec; // I need something to stick in a pair to compare
                                    // pair<tdcVal, IDE>. This is an otherwise useless
                                    // "hack".
    std::pair<double, std::vector<sim::IDE>> start_tdcPair =
      std::make_pair(start_tdc, dummyVec); // This pair is a "hack" to make my comparison
                                           // work for lower and upper bound.
    std::pair<double, std::vector<sim::IDE>> end_tdcPair = std::make_pair(end_tdc, dummyVec);
    auto start_tdcPair_P = &start_tdcPair;
    auto end_tdcPair_P = &end_tdcPair;
    auto mapFirst = std::lower_bound(
      tdcIDEMap_SortedPointers.begin(), tdcIDEMap_SortedPointers.end(), start_tdcPair_P, pairSort);

    // iterator to just after the last interesting IDE
    auto mapLast = std::upper_bound(
      tdcIDEMap_SortedPointers.begin(), tdcIDEMap_SortedPointers.end(), end_tdcPair_P, pairSort);
    for (auto& mapitr = mapFirst; mapitr != mapLast; ++mapitr) {
      for (auto& ide : (*mapitr)->second) {
        retVec.push_back(&ide);
      } // Add all interesting IDEs to the retVec
    }
    return retVec;
  }

  //------------------------------------------------------------------------------
  std::vector<double> BackTracker::SimIDEsToXYZ(std::vector<sim::IDE> const& ides) const
  {
    std::vector<double> xyz(3, 0.0);
    double w = 0.0;
    for (auto const& ide : ides) {
      double weight = ide.numElectrons;
      w += weight;
      xyz[0] += (weight * ide.x);
      xyz[1] += (weight * ide.y);
      xyz[2] += (weight * ide.z);
    }
    if (w < 1.e-5)
      throw cet::exception("BackTracker") << "No sim::IDEs providing non-zero number of electrons"
                                          << " can't determine originating location from truth\n";
    xyz[0] = xyz[0] / w;
    xyz[1] = xyz[1] / w;
    xyz[2] = xyz[2] / w;
    return xyz;
  }

  //-------------------------------------------------------------------------------
  std::vector<double> BackTracker::SimIDEsToXYZ(std::vector<const sim::IDE*> const& ide_Ps) const
  {
    std::vector<sim::IDE> ides;
    for (auto ide_P : ide_Ps) {
      ides.push_back(*ide_P);
    }
    return SimIDEsToXYZ(ides);
  }

  //--------------------------------------------------------------------------------
  std::vector<double> BackTracker::HitToXYZ(detinfo::DetectorClocksData const& clockData,
                                            const recob::Hit& hit) const
  {
    return SimIDEsToXYZ(HitToSimIDEs_Ps(clockData, hit));
  }

  //-----------------------------------------------------------------------------------
  double BackTracker::HitCollectionPurity(detinfo::DetectorClocksData const& clockData,
                                          std::set<int> const& trackIds,
                                          std::vector<art::Ptr<recob::Hit>> const& hits) const
  {
    int desired = 0;
    for (const auto& hit : hits) {
      std::vector<sim::TrackIDE> hitTrackIDEs = HitToTrackIDEs(clockData, hit);
      for (const auto& tIDE : hitTrackIDEs) {
        if (trackIds.find(tIDE.trackID) != trackIds.end()) {
          ++desired;
          break;
        } // End if TID Found
      }   // END for trackIDE in TrackIDEs
    }     // End for hit in hits
    if (hits.size() > 0) { return double(double(desired) / double(hits.size())); }
    return 0;
  }

  //-----------------------------------------------------------------------------------
  double BackTracker::HitChargeCollectionPurity(detinfo::DetectorClocksData const& clockData,
                                                std::set<int> const& trackIds,
                                                std::vector<art::Ptr<recob::Hit>> const& hits) const
  {
    double totalCharge = 0., desired = 0.;
    for (const auto& hit : hits) {
      totalCharge += hit->Integral();
      std::vector<sim::TrackIDE> trackIDEs = HitToTrackIDEs(clockData, hit);
      for (const auto& trackIDE : trackIDEs) {
        if (trackIds.find(trackIDE.trackID) != trackIds.end()) {
          desired += hit->Integral();
          break;
        } // End if trackId in trackIds.
      }   // End for trackIDE in trackIDEs
    }     // End for Hit in Hits
    if (totalCharge > 0.0) { return (desired / totalCharge); }
    return 0.0;
  }

  //-----------------------------------------------------------------------------------
  double BackTracker::HitCollectionEfficiency(detinfo::DetectorClocksData const& clockData,
                                              std::set<int> const& trackIds,
                                              std::vector<art::Ptr<recob::Hit>> const& hits,
                                              std::vector<art::Ptr<recob::Hit>> const& allHits,
                                              geo::View_t const& view) const
  {

    int desired = 0, total = 0;

    for (const auto& hit : hits) {
      std::vector<sim::TrackIDE> hitTrackIDEs = HitToTrackIDEs(clockData, hit);
      for (const auto& trackIDE : hitTrackIDEs) {
        if (trackIds.find(trackIDE.trackID) != trackIds.end() &&
            trackIDE.energyFrac >= fMinHitEnergyFraction) {
          ++desired;
          break;
        } // End if trackID in trackIds.
      }   // end for trackIDE in TrackIDEs
    }     // end for hit in hits

    for (const auto& hit : allHits) {
      if (hit->View() != view && view != geo::k3D) {
        continue;
      } // End if hit.view = view or view = geo::k3D
      std::vector<sim::TrackIDE> hitTrackIDEs = HitToTrackIDEs(clockData, hit);
      for (const auto& hitIDE : hitTrackIDEs) {
        if (trackIds.find(hitIDE.trackID) != trackIds.end() &&
            hitIDE.energyFrac >= fMinHitEnergyFraction) {
          ++total;
          break;
        }
      } // END for all IDEs in HitTrackIDEs.
    }   // end for hit in allHits.
    if (total >= 0) { return double(double(desired) / double(total)); }
    return 0.;
  }

  //-----------------------------------------------------------------------------------
  double BackTracker::HitChargeCollectionEfficiency(
    detinfo::DetectorClocksData const& clockData,
    std::set<int> const& trackIds,
    std::vector<art::Ptr<recob::Hit>> const& hits,
    std::vector<art::Ptr<recob::Hit>> const& allHits,
    geo::View_t const& view) const
  {
    double desired = 0., total = 0.;
    for (const auto& hit : hits) {
      std::vector<sim::TrackIDE> hitTrackIDEs = HitToTrackIDEs(clockData, hit);
      for (const auto& hitIDE : hitTrackIDEs) {
        if (trackIds.find(hitIDE.trackID) != trackIds.end() &&
            hitIDE.energyFrac >= fMinHitEnergyFraction) {
          desired += hit->Integral();
          break;
        } // end if hit id matches and energy sufficient.
      }   // End for IDE in HitTrackIDEs.
    }     // End for hit in hits.

    for (const auto& hit : allHits) {
      if (hit->View() != view && view != geo::k3D) { continue; }
      std::vector<sim::TrackIDE> hitTrackIDEs = HitToTrackIDEs(clockData, hit);
      for (const auto& hitIDE : hitTrackIDEs) {
        if (trackIds.find(hitIDE.trackID) != trackIds.end() &&
            hitIDE.energyFrac >= fMinHitEnergyFraction) {
          total += hit->Integral();
          break;
        } // end if hit matches
      }   // end for ide in ides
    }     // End for hit in allHits

    if (total > 0.) { return desired / total; }
    return 0.;
  }

  //-----------------------------------------------------------------------------------
  std::set<int> BackTracker::GetSetOfTrackIds(detinfo::DetectorClocksData const& clockData,
                                              std::vector<art::Ptr<recob::Hit>> const& hits) const
  {
    std::set<int> tids;
    for (const auto& hit : hits) {
      const double start = hit->PeakTimeMinusRMS(fHitTimeRMS);
      const double end = hit->PeakTimePlusRMS(fHitTimeRMS);
      std::vector<sim::TrackIDE> trackIDEs =
        ChannelToTrackIDEs(clockData, hit->Channel(), start, end);
      for (const auto& ide : trackIDEs) {
        tids.insert(ide.trackID);
      } // End for TrackIDEs
    }   // End for hits
    return tids;
  } // End GetSetOfTrackIds

  //-----------------------------------------------------------------------------------
  std::set<int> BackTracker::GetSetOfEveIds(detinfo::DetectorClocksData const& clockData,
                                            std::vector<art::Ptr<recob::Hit>> const& hits) const
  {
    std::set<int> eveIds;
    for (const auto& hit : hits) {
      const std::vector<sim::TrackIDE> ides = HitToEveTrackIDEs(clockData, hit);
      for (const auto& ide : ides) {
        eveIds.insert(ide.trackID);
      } // end ides
    }   // End for hits
    return eveIds;
  }

  // This function definitely needs a new implementation. There must be a better way than
  // so many loops.
  std::vector<double> BackTracker::SpacePointHitsToWeightedXYZ(
    detinfo::DetectorClocksData const& clockData,
    std::vector<art::Ptr<recob::Hit>> const& hits) const
  {
    std::vector<std::vector<std::vector<int>>> numHits(fGeom->Ncryostats());
    std::vector<std::vector<std::vector<double>>> hitWeight(fGeom->Ncryostats());
    std::vector<std::vector<std::vector<std::vector<double>>>> hitPos(fGeom->Ncryostats());
    // Do we need to resize everything...
    for (size_t c = 0; c < numHits.size(); ++c) {
      geo::CryostatID const cid(c);
      numHits[c].resize(fGeom->NTPC(cid));
      hitWeight[c].resize(fGeom->NTPC(cid));
      hitPos[c].resize(fGeom->NTPC(cid));
      for (size_t t = 0; t < numHits[c].size(); ++t) {
        geo::TPCID const tpcid(cid, t);
        numHits[c][t].resize(fWireReadoutGeom->Nplanes(tpcid));
        hitWeight[c][t].resize(fWireReadoutGeom->Nplanes(tpcid));
        hitPos[c][t].resize(fWireReadoutGeom->Nplanes(tpcid));
      }
    }

    for (auto const& hit_ptr : hits) {
      const recob::Hit& hit = *hit_ptr;

      // use the HitToXYZ and Geometry::PositionToTPC to figure out which drift volume the
      // hit originates from
      std::vector<double> hitOrigin = HitToXYZ(clockData, hit_ptr);
      geo::Point_t const worldLoc{hitOrigin[0], hitOrigin[1], hitOrigin[2]};
      auto const tpcid = fGeom->PositionToTPCID(worldLoc);

      auto const [cstat, tpc] = std::make_tuple(tpcid.Cryostat, tpcid.TPC);

      if (hit.WireID().Cryostat == cstat && hit.WireID().TPC == tpc) {
        ++numHits[cstat][tpc][hit.WireID().Plane];
        hitWeight[cstat][tpc][hit.WireID().Plane] = hit.Integral();
        hitPos[cstat][tpc][hit.WireID().Plane] = hitOrigin;
      }
    }

    // loop over the vectors we made and find the average position for the hits
    // in the future we might take a weighted average
    int nhits = 0;
    std::vector<double> xyz(3);
    for (size_t c = 0; c < numHits.size(); ++c) {
      for (size_t t = 0; t < numHits[c].size(); ++t) {
        for (size_t p = 0; p < numHits[c][t].size(); ++p) {

          if (numHits[c][t][p] == 1) {
            ++nhits;
            xyz[0] += hitPos[c][t][p][0];
            xyz[1] += hitPos[c][t][p][1];
            xyz[2] += hitPos[c][t][p][2];
          }

        } // end loop over planes
      }   // end loop over tpcs
    }     // end loop over cryostats

    // get the average position
    if (nhits < 1)
      throw cet::exception("BackTracker")
        << "No hits to determine originating location from truth\n";

    xyz[0] /= nhits;
    xyz[1] /= nhits;
    xyz[2] /= nhits;

    // Done.
    return xyz;
  }

} // End namespace cheat