QuickTrackAssociatorByHitsImpl.cc

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#include <iostream>
#include <fstream>

#include "QuickTrackAssociatorByHitsImpl.h"

#include "SimTracker/TrackerHitAssociation/interface/TrackerHitAssociator.h"

#include "DataFormats/TrackerRecHit2D/interface/SiPixelRecHit.h"
#include "DataFormats/TrackerRecHit2D/interface/SiStripMatchedRecHit2D.h"
#include "DataFormats/TrackerRecHit2D/interface/SiStripRecHit2D.h"
#include "DataFormats/TrackerRecHit2D/interface/SiStripRecHit1D.h"
#include "DataFormats/TrackerRecHit2D/interface/Phase2TrackerRecHit1D.h"
#include "DataFormats/SiStripDetId/interface/SiStripDetId.h"
#include "DataFormats/SiPixelDetId/interface/PixelSubdetector.h"
#include "FWCore/MessageLogger/interface/MessageLogger.h"
#include "DataFormats/Math/interface/deltaR.h"

//
// Use the unnamed namespace for utility functions only used in this file
//
namespace
{
    //
    // All of these functions are pretty straightforward but the implementation is type dependent.
    // The templated methods call these for the type specific parts and the compiler will resolve
    // the type and call the correct overload.
    //

    template<class T_element>
    size_t collectionSize( const edm::RefToBaseVector<T_element>& collection )
    {
        return collection.size();
    }

    template<class T_element>
    size_t collectionSize( const edm::Handle<T_element>& hCollection )
    {
        return hCollection->size();
    }

    template<class T_element>
    size_t collectionSize( const edm::RefVector<T_element>& collection )
    {
        return collection.size();
    }

    const reco::Track* getTrackAt( const edm::RefToBaseVector<reco::Track>& trackCollection, size_t index )
    {
        return &*trackCollection[index]; // pretty obscure dereference
    }

    const reco::Track* getTrackAt( const edm::Handle<edm::View<reco::Track> >& pTrackCollection, size_t index )
    {
        return &(*pTrackCollection.product())[index];
    }

    const TrackingParticle* getTrackingParticleAt( const edm::Handle<TrackingParticleCollection>& pCollection, size_t index )
    {
        return &(*pCollection.product())[index];
    }

    const TrackingParticle* getTrackingParticleAt( const edm::RefVector<TrackingParticleCollection>& collection, size_t index )
    {
        return &*collection[index];
    }

    edm::RefToBase<reco::Track> getRefToTrackAt( const edm::RefToBaseVector<reco::Track>& trackCollection, size_t index )
    {
        return trackCollection[index];
    }

    edm::RefToBase<reco::Track> getRefToTrackAt( const edm::Handle<edm::View<reco::Track> >& pTrackCollection, size_t index )
    {
        return edm::RefToBase<reco::Track>( pTrackCollection, index );
    }

    edm::Ref<TrackingParticleCollection> getRefToTrackingParticleAt( const edm::Handle<TrackingParticleCollection>& pCollection, size_t index )
    {
        return edm::Ref<TrackingParticleCollection>( pCollection, index );
    }

    edm::Ref<TrackingParticleCollection> getRefToTrackingParticleAt( const edm::RefVector<TrackingParticleCollection>& collection, size_t index )
    {
        return collection[index];
    }

  template <typename T>
  void fillKeys(std::unordered_set<T>& keys, const edm::RefVector<TrackingParticleCollection>& collection) {
    for(const auto& ref: collection) {
      keys.insert(ref.key());
    }
  }

  template <typename Coll>
  void checkClusterMapProductID(const ClusterTPAssociation& clusterToTPMap, const Coll& collection) {
    clusterToTPMap.checkMappedProductID(collection.id());
  }

  template <typename Coll>
  void checkClusterMapProductID(const TrackerHitAssociator& hitAssociator, const Coll& collection) {}

} // end of the unnamed namespace

QuickTrackAssociatorByHitsImpl::QuickTrackAssociatorByHitsImpl(edm::EDProductGetter const& productGetter,
                                                               std::unique_ptr<const TrackerHitAssociator> hitAssoc,
                                                               const ClusterTPAssociation *clusterToTPMap,

                                                               bool absoluteNumberOfHits,
                                                               double qualitySimToReco,
                                                               double puritySimToReco,
                                                               double pixelHitWeight,
                                                               double cutRecoToSim,
                                                               bool threeHitTracksAreSpecial,
                                                               SimToRecoDenomType simToRecoDenominator):
  productGetter_(&productGetter),
  hitAssociator_(std::move(hitAssoc)),
  clusterToTPMap_(clusterToTPMap),
  qualitySimToReco_(qualitySimToReco),
  puritySimToReco_(puritySimToReco),
  pixelHitWeight_(pixelHitWeight),
  cutRecoToSim_(cutRecoToSim),
  simToRecoDenominator_(simToRecoDenominator) ,
  threeHitTracksAreSpecial_(threeHitTracksAreSpecial),
  absoluteNumberOfHits_(absoluteNumberOfHits)
  {}


reco::RecoToSimCollection QuickTrackAssociatorByHitsImpl::associateRecoToSim( const edm::Handle<edm::View<reco::Track> >& trackCollectionHandle, 
                                                                              const edm::Handle<TrackingParticleCollection>& trackingParticleCollectionHandle) const
{
    // Only pass the one that was successfully created to the templated method.
    if( not clusterToTPMap_ ) return associateRecoToSimImplementation( trackCollectionHandle, trackingParticleCollectionHandle, nullptr, *hitAssociator_ );
    else return associateRecoToSimImplementation( trackCollectionHandle, trackingParticleCollectionHandle, nullptr, *clusterToTPMap_ );
}

reco::SimToRecoCollection QuickTrackAssociatorByHitsImpl::associateSimToReco( const edm::Handle<edm::View<reco::Track> >& trackCollectionHandle, 
                                                                              const edm::Handle<TrackingParticleCollection>& trackingParticleCollectionHandle) const
{
    // Only pass the one that was successfully created to the templated method.
    if( not clusterToTPMap_ ) return associateSimToRecoImplementation( trackCollectionHandle, trackingParticleCollectionHandle, nullptr, *hitAssociator_ );
    else return associateSimToRecoImplementation( trackCollectionHandle, trackingParticleCollectionHandle, nullptr, *clusterToTPMap_ );
}

reco::RecoToSimCollection QuickTrackAssociatorByHitsImpl::associateRecoToSim( const edm::RefToBaseVector<reco::Track>& trackCollection,
                                                                              const edm::RefVector<TrackingParticleCollection>& trackingParticleCollection ) const 
{
    // Only pass the one that was successfully created to the templated method.
    if( not clusterToTPMap_ ) return associateRecoToSimImplementation( trackCollection, trackingParticleCollection, nullptr, *hitAssociator_ );
    else {
          TrackingParticleRefKeySet tpKeys;
          fillKeys(tpKeys, trackingParticleCollection);
          return associateRecoToSimImplementation( trackCollection, trackingParticleCollection, &tpKeys, *clusterToTPMap_ );
        }
}

reco::SimToRecoCollection QuickTrackAssociatorByHitsImpl::associateSimToReco( const edm::RefToBaseVector<reco::Track>& trackCollection, 
                                                                              const edm::RefVector<TrackingParticleCollection>& trackingParticleCollection) const
{
    // Only pass the one that was successfully created to the templated method.
    if( not clusterToTPMap_ ) return associateSimToRecoImplementation( trackCollection, trackingParticleCollection, nullptr, *hitAssociator_ );
    else {
          TrackingParticleRefKeySet tpKeys;
          fillKeys(tpKeys, trackingParticleCollection);
          return associateSimToRecoImplementation( trackCollection, trackingParticleCollection, &tpKeys, *clusterToTPMap_ );
        }
}


template<class T_TrackCollection, class T_TrackingParticleCollection, class T_hitOrClusterAssociator>
reco::RecoToSimCollection QuickTrackAssociatorByHitsImpl::associateRecoToSimImplementation( const T_TrackCollection& trackCollection, const T_TrackingParticleCollection& trackingParticleCollection, const TrackingParticleRefKeySet *trackingParticleKeys, T_hitOrClusterAssociator hitOrClusterAssociator ) const
{
    reco::RecoToSimCollection returnValue(productGetter_);
        if(::collectionSize(trackingParticleCollection) == 0)
          return returnValue;

        checkClusterMapProductID(hitOrClusterAssociator, trackingParticleCollection);

    size_t collectionSize=::collectionSize(trackCollection); // Delegate away type specific part

    for( size_t i=0; i < collectionSize; ++i )
    {
        const reco::Track* pTrack=::getTrackAt(trackCollection,i); // Get a normal pointer for ease of use. This part is type specific so delegate.

        // The return of this function has first as the index and second as the number of associated hits
        std::vector < std::pair<edm::Ref<TrackingParticleCollection>,double> > trackingParticleQualityPairs=associateTrack( hitOrClusterAssociator, trackingParticleCollection, trackingParticleKeys, pTrack->recHitsBegin(), pTrack->recHitsEnd() );

        // int nt = 0;
        for( auto iTrackingParticleQualityPair=
                trackingParticleQualityPairs.begin(); iTrackingParticleQualityPair != trackingParticleQualityPairs.end();
                ++iTrackingParticleQualityPair )
        {
            const edm::Ref<TrackingParticleCollection>& trackingParticleRef=iTrackingParticleQualityPair->first;
            double numberOfSharedHits=iTrackingParticleQualityPair->second;
            double numberOfValidTrackHits=weightedNumberOfTrackHits(*pTrack);

            if( numberOfSharedHits == 0.0 ) continue; // No point in continuing if there was no association

            //if electron subtract double counting
            if( abs( trackingParticleRef->pdgId() ) == 11 && (trackingParticleRef->g4Track_end() - trackingParticleRef->g4Track_begin()) > 1 )
            {
                numberOfSharedHits-=getDoubleCount( hitOrClusterAssociator, pTrack->recHitsBegin(), pTrack->recHitsEnd(), trackingParticleRef );
            }

            double quality;
            if( absoluteNumberOfHits_ ) quality = numberOfSharedHits;
            else if( numberOfValidTrackHits != 0.0 ) quality = numberOfSharedHits / numberOfValidTrackHits;
            else quality=0;
            if( quality > cutRecoToSim_ && !(threeHitTracksAreSpecial_ && pTrack->numberOfValidHits() == 3 && numberOfSharedHits < 3.0) )
            {
                // Getting the RefToBase is dependent on the type of trackCollection, so delegate that to an overload.
                returnValue.insert( ::getRefToTrackAt(trackCollection,i), std::make_pair( trackingParticleRef, quality ) );
            }
        }
    }
    returnValue.post_insert();
    return returnValue;
}

template<class T_TrackCollection, class T_TrackingParticleCollection, class T_hitOrClusterAssociator>
reco::SimToRecoCollection QuickTrackAssociatorByHitsImpl::associateSimToRecoImplementation( const T_TrackCollection& trackCollection, const T_TrackingParticleCollection& trackingParticleCollection, const TrackingParticleRefKeySet *trackingParticleKeys, T_hitOrClusterAssociator hitOrClusterAssociator ) const
{
    reco::SimToRecoCollection returnValue(productGetter_);
        if(::collectionSize(trackingParticleCollection) == 0)
          return returnValue;

        checkClusterMapProductID(hitOrClusterAssociator, trackingParticleCollection);

    size_t collectionSize=::collectionSize(trackCollection); // Delegate away type specific part

    for( size_t i=0; i<collectionSize; ++i )
    {
        const reco::Track* pTrack=::getTrackAt(trackCollection,i); // Get a normal pointer for ease of use. This part is type specific so delegate.

        // The return of this function has first as an edm:Ref to the associated TrackingParticle, and second as the number of associated hits
        std::vector < std::pair<edm::Ref<TrackingParticleCollection>,double> > trackingParticleQualityPairs=associateTrack( hitOrClusterAssociator, trackingParticleCollection, trackingParticleKeys, pTrack->recHitsBegin(), pTrack->recHitsEnd() );

        // int nt = 0;
        for( auto iTrackingParticleQualityPair=trackingParticleQualityPairs.begin();
                iTrackingParticleQualityPair!=trackingParticleQualityPairs.end(); ++iTrackingParticleQualityPair )
        {
            const edm::Ref<TrackingParticleCollection>& trackingParticleRef=iTrackingParticleQualityPair->first;
            double numberOfSharedHits=iTrackingParticleQualityPair->second;
            double numberOfValidTrackHits=weightedNumberOfTrackHits(*pTrack);
            size_t numberOfSimulatedHits=0; // Set a few lines below, but only if required.

            if( numberOfSharedHits==0.0 ) continue; // No point in continuing if there was no association

            if( simToRecoDenominator_==denomsim || (numberOfSharedHits<3.0 && threeHitTracksAreSpecial_) ) // the numberOfSimulatedHits is not always required, so can skip counting in some circumstances
            {
                // Note that in the standard TrackAssociatorByHits, all of the hits in associatedTrackingParticleHits are checked for
                // various things.  I'm not sure what these checks are for but they depend on the UseGrouping and UseSplitting settings.
                // This associator works as though both UseGrouping and UseSplitting were set to true, i.e. just counts the number of
                // hits in the tracker.
                numberOfSimulatedHits=trackingParticleRef->numberOfTrackerHits();
            }

            //if electron subtract double counting
            if (abs(trackingParticleRef->pdgId())==11 && (trackingParticleRef->g4Track_end() - trackingParticleRef->g4Track_begin()) > 1 )
            {
                numberOfSharedHits -= getDoubleCount( hitOrClusterAssociator, pTrack->recHitsBegin(), pTrack->recHitsEnd(), trackingParticleRef );
            }

            double purity = numberOfSharedHits/numberOfValidTrackHits;
            double quality;
            if( absoluteNumberOfHits_ ) quality = numberOfSharedHits;
            else if( simToRecoDenominator_==denomsim && numberOfSimulatedHits != 0 ) quality = numberOfSharedHits/static_cast<double>(numberOfSimulatedHits);
            else if( simToRecoDenominator_==denomreco && numberOfValidTrackHits != 0 ) quality=purity;
            else quality=0;

            if( quality>qualitySimToReco_ && !( threeHitTracksAreSpecial_ && numberOfSimulatedHits==3 && numberOfSharedHits<3.0 ) && ( absoluteNumberOfHits_ || (purity>puritySimToReco_) ) )
            {
                // Getting the RefToBase is dependent on the type of trackCollection, so delegate that to an overload.
                returnValue.insert( trackingParticleRef, std::make_pair( ::getRefToTrackAt(trackCollection,i), quality ) );
            }
        }
    }
    returnValue.post_insert();
    return returnValue;

}

template<typename T_TPCollection,typename iter> std::vector<std::pair<edm::Ref<TrackingParticleCollection>,double> > QuickTrackAssociatorByHitsImpl::associateTrack( const TrackerHitAssociator& hitAssociator, const T_TPCollection& trackingParticles, const TrackingParticleRefKeySet *trackingParticleKeys, iter begin, iter end ) const
{
    // The pairs in this vector have a Ref to the associated TrackingParticle as "first" and the weighted number of associated hits as "second"
    std::vector< std::pair<edm::Ref<TrackingParticleCollection>,double> > returnValue;

    // The pairs in this vector have first as the sim track identifiers, and second the number of reco hits associated to that sim track.
    // Most reco hits will probably have come from the same sim track, so the number of entries in this vector should be fewer than the
    // number of reco hits.  The pair::second entries should add up to the total number of reco hits though.
    std::vector< std::pair<SimTrackIdentifiers,double> > hitIdentifiers=getAllSimTrackIdentifiers( hitAssociator, begin, end );

    // Loop over the TrackingParticles
    size_t collectionSize=::collectionSize(trackingParticles);

    for( size_t i=0; i<collectionSize; ++i )
    {
        const TrackingParticle* pTrackingParticle=getTrackingParticleAt( trackingParticles, i );

        // Historically there was a requirement that pTrackingParticle->numberOfHits() > 0
        // However, in TrackingTruthAccumulator, the numberOfHits is calculated from a subset
        // of the SimHits of the SimTracks of a TrackingParticle (essentially limiting the
        // processType and particleType to those of the "first" hit, and particleType to the pdgId of the SimTrack).
        // But, here the association between tracks and TrackingParticles is done with *all* the hits of
        // TrackingParticle, so we should not rely on the numberOfHits() calculated with a subset of SimHits.

        double numberOfAssociatedHits=0;
        // Loop over all of the sim track identifiers and see if any of them are part of this TrackingParticle. If they are, add
        // the number of reco hits associated to that sim track to the total number of associated hits.
        for( const auto& identifierCountPair: hitIdentifiers)
        {
            if( trackingParticleContainsIdentifier( pTrackingParticle, identifierCountPair.first ) ) numberOfAssociatedHits+=identifierCountPair.second;
        }

        if( numberOfAssociatedHits>0 )
        {
            returnValue.push_back( std::make_pair( getRefToTrackingParticleAt(trackingParticles,i), numberOfAssociatedHits ) );
        }
    }

    return returnValue;
}

template<typename T_TPCollection,typename iter> std::vector< std::pair<edm::Ref<TrackingParticleCollection>,double> > QuickTrackAssociatorByHitsImpl::associateTrack( const ClusterTPAssociation& clusterToTPMap, const T_TPCollection& trackingParticles, const TrackingParticleRefKeySet *trackingParticleKeys, iter begin, iter end ) const
{
    // Note that the trackingParticles parameter is not actually required since all the information is in clusterToTPMap,
    // but the method signature has to match the other overload because it is called from a templated method.

    // Note further, that we can't completely ignore the
    // trackingParticles parameter, in case it is a subset of those
    // TrackingParticles used to construct clusterToTPMap (via the
    // TrackingParticleRefVector overloads). The trackingParticles
    // parameter is still ignored since looping over it on every call
    // would be expensive, but the keys of the TrackingParticleRefs are
    // cached to an unordered_set (trackingParticleKeys) which is used
    // as a fast search structure.

    // The pairs in this vector have a Ref to the associated TrackingParticle as "first" and the weighted number of associated clusters as "second"
    // Note: typedef edm::Ref<TrackingParticleCollection> TrackingParticleRef;
    std::vector < std::pair<edm::Ref<TrackingParticleCollection>,double> > returnValue;
    if( clusterToTPMap.empty() ) return returnValue;

    // The pairs in this vector have first as the TP, and second the number of reco clusters associated to that TP.
    // Most reco clusters will probably have come from the same sim track (i.e TP), so the number of entries in this
    // vector should be fewer than the number of clusters. The pair::second entries should add up to the total
    // number of reco clusters though.
    std::vector<OmniClusterRef> oClusters=getMatchedClusters( begin, end );

    std::map < TrackingParticleRef, double > lmap;
    for( std::vector<OmniClusterRef>::const_iterator it=oClusters.begin(); it != oClusters.end(); ++it )
    {
        auto range = clusterToTPMap.equal_range(*it);
        const double weight = it->isPixel() ? pixelHitWeight_ : 1.0;
        if( range.first != range.second )
        {
            for( auto ip=range.first; ip != range.second; ++ip )
            {

                const TrackingParticleRef trackingParticle=(ip->second);

                                if(trackingParticleKeys && trackingParticleKeys->find(trackingParticle.key()) == trackingParticleKeys->end())
                                  continue;

                // Historically there was a requirement that pTrackingParticle->numberOfHits() > 0
                // However, in TrackingTruthAccumulator, the numberOfHits is calculated from a subset
                // of the SimHits of the SimTracks of a TrackingParticle (essentially limiting the
                // processType and particleType to those of the "first" hit, and particleType to the pdgId of the SimTrack).
                // But, here the association between tracks and TrackingParticles is done with *all* the hits of
                // TrackingParticle, so we should not rely on the numberOfHits() calculated with a subset of SimHits.

                /* Alternative implementation to avoid the use of lmap... memory slightly improved but slightly slower...
                 std::pair<edm::Ref<TrackingParticleCollection>,size_t> tpIntPair(trackingParticle, 1);
                 auto tp_range = std::equal_range(returnValue.begin(), returnValue.end(), tpIntPair, tpIntPairGreater);
                 if ((tp_range.second-tp_range.first)>1) {
                 edm::LogError("TrackAssociator") << ">>> Error in counting TPs!" << " file: " << __FILE__ << " line: " << __LINE__;
                 }
                 if(tp_range.first != tp_range.second) {
                 tp_range.first->second++;
                 } else {
                 returnValue.push_back(tpIntPair);
                 std::sort(returnValue.begin(), returnValue.end(), tpIntPairGreater);
                 }
                 */
                auto jpos=lmap.find( trackingParticle );
                if( jpos != lmap.end() ) jpos->second += weight;
                else lmap.insert( std::make_pair( trackingParticle, weight ) );
            }
        }
    }
    // now copy the map to returnValue
    for( auto ip=lmap.begin(); ip != lmap.end(); ++ip )
    {
        returnValue.push_back( std::make_pair( ip->first, ip->second ) );
    }
    return returnValue;
}

template<typename iter> std::vector<OmniClusterRef> QuickTrackAssociatorByHitsImpl::getMatchedClusters(iter begin, iter end) const
{
  std::vector<OmniClusterRef> returnValue;
  for (iter iRecHit = begin; iRecHit != end; ++iRecHit) {
    const TrackingRecHit* rhit = getHitFromIter(iRecHit);
    if (rhit->isValid()) {
      int subdetid = rhit->geographicalId().subdetId();
      if (subdetid==PixelSubdetector::PixelBarrel||subdetid==PixelSubdetector::PixelEndcap) {
    const SiPixelRecHit* pRHit = dynamic_cast<const SiPixelRecHit*>(rhit);
    if (!pRHit->cluster().isNonnull())
      edm::LogError("TrackAssociator") << ">>> RecHit does not have an associated cluster!" << " file: " << __FILE__ << " line: " << __LINE__;
    returnValue.push_back(pRHit->omniClusterRef());
      }
      else if (subdetid==SiStripDetId::TIB||subdetid==SiStripDetId::TOB||subdetid==SiStripDetId::TID||subdetid==SiStripDetId::TEC) {
    const std::type_info &tid = typeid(*rhit);
    if (tid == typeid(SiStripMatchedRecHit2D)) {
      const SiStripMatchedRecHit2D* sMatchedRHit = dynamic_cast<const SiStripMatchedRecHit2D*>(rhit);
      if (!sMatchedRHit->monoHit().cluster().isNonnull() || !sMatchedRHit->stereoHit().cluster().isNonnull())
        edm::LogError("TrackAssociator") << ">>> RecHit does not have an associated cluster!" << " file: " << __FILE__ << " line: " << __LINE__;
      returnValue.push_back(sMatchedRHit->monoClusterRef());
      returnValue.push_back(sMatchedRHit->stereoClusterRef());
    }
    else if (tid == typeid(SiStripRecHit2D)) {
      const SiStripRecHit2D* sRHit = dynamic_cast<const SiStripRecHit2D*>(rhit);
      if (!sRHit->cluster().isNonnull())
        edm::LogError("TrackAssociator") << ">>> RecHit does not have an associated cluster!" << " file: " << __FILE__ << " line: " << __LINE__;
      returnValue.push_back(sRHit->omniClusterRef());
    }
    else if (tid == typeid(SiStripRecHit1D)) {
      const SiStripRecHit1D* sRHit = dynamic_cast<const SiStripRecHit1D*>(rhit);
      if (!sRHit->cluster().isNonnull())
        edm::LogError("TrackAssociator") << ">>> RecHit does not have an associated cluster!" << " file: " << __FILE__ << " line: " << __LINE__;
      returnValue.push_back(sRHit->omniClusterRef());
    }
    else if (tid == typeid(Phase2TrackerRecHit1D)) {
      const Phase2TrackerRecHit1D* ph2Hit = dynamic_cast<const Phase2TrackerRecHit1D*>(rhit);
          if (!ph2Hit->cluster().isNonnull() )
        edm::LogError("TrackAssociator") << ">>> RecHit does not have an associated cluster!" << " file: " << __FILE__ << " line: " << __LINE__;
      returnValue.push_back(ph2Hit->omniClusterRef());
        }
    else {
      auto const & thit = static_cast<BaseTrackerRecHit const&>(*rhit);
      if ( thit.isProjected() ) {
      } else {
        edm::LogError("TrackAssociator") << ">>> getMatchedClusters: TrackingRecHit not associated to any SiStripCluster! subdetid = " << subdetid;
      }
    }
      }
      else {
    edm::LogError("TrackAssociator") << ">>> getMatchedClusters: TrackingRecHit not associated to any cluster! subdetid = " << subdetid;
      }
    }
  }
  return returnValue;
}

template<typename iter> std::vector< std::pair<QuickTrackAssociatorByHitsImpl::SimTrackIdentifiers,double> > QuickTrackAssociatorByHitsImpl::getAllSimTrackIdentifiers( const TrackerHitAssociator& hitAssociator, iter begin, iter end ) const
{
    // The pairs in this vector have first as the sim track identifiers, and second the number of reco hits associated to that sim track.
    std::vector < std::pair<SimTrackIdentifiers,double> > returnValue;

    std::vector<SimTrackIdentifiers> simTrackIdentifiers;
    // Loop over all of the rec hits in the track
    //iter tRHIterBeginEnd = getTRHIterBeginEnd( pTrack );
    for( iter iRecHit=begin; iRecHit != end; ++iRecHit )
    {
        if( getHitFromIter( iRecHit )->isValid() )
        {
            simTrackIdentifiers.clear();

            // Get the identifiers for the sim track that this hit came from. There should only be one entry unless clusters
            // have merged (as far as I know).
            hitAssociator.associateHitId( *(getHitFromIter( iRecHit )), simTrackIdentifiers ); // This call fills simTrackIdentifiers

                        const auto subdetId = getHitFromIter(iRecHit)->geographicalId().subdetId();
                        const double weight = (subdetId == PixelSubdetector::PixelBarrel || subdetId == PixelSubdetector::PixelEndcap) ?  pixelHitWeight_ : 1.0;

            // Loop over each identifier, and add it to the return value only if it's not already in there
            for( std::vector<SimTrackIdentifiers>::const_iterator iIdentifier=simTrackIdentifiers.begin(); iIdentifier != simTrackIdentifiers.end();
                    ++iIdentifier )
            {
                std::vector<std::pair<SimTrackIdentifiers,double> >::iterator iIdentifierCountPair;
                for(auto iIdentifierCountPair=returnValue.begin(); iIdentifierCountPair != returnValue.end(); ++iIdentifierCountPair )
                {
                    if( iIdentifierCountPair->first.first == iIdentifier->first && iIdentifierCountPair->first.second == iIdentifier->second )
                    {
                        // This sim track identifier is already in the list, so increment the count of how many hits it relates to.
                        iIdentifierCountPair->second += weight;
                        break;
                    }
                }
                if( iIdentifierCountPair == returnValue.end() ) returnValue.push_back( std::make_pair( *iIdentifier, 1.0 ) );
                // This identifier wasn't found, so add it
            }
        }
    }
    return returnValue;
}

bool QuickTrackAssociatorByHitsImpl::trackingParticleContainsIdentifier( const TrackingParticle* pTrackingParticle, const SimTrackIdentifiers& identifier ) const
{
    // Loop over all of the g4 tracks in the tracking particle
    for( std::vector<SimTrack>::const_iterator iSimTrack=pTrackingParticle->g4Track_begin(); iSimTrack != pTrackingParticle->g4Track_end();
            ++iSimTrack )
    {
        // And see if the sim track identifiers match
        if( iSimTrack->eventId() == identifier.second && iSimTrack->trackId() == identifier.first )
        {
            return true;
        }
    }

    // If control has made it this far then none of the identifiers were found in
    // any of the g4 tracks, so return false.
    return false;
}

template<typename iter> double QuickTrackAssociatorByHitsImpl::getDoubleCount( const TrackerHitAssociator& hitAssociator, iter startIterator, iter endIterator, TrackingParticleRef associatedTrackingParticle ) const
{
    // This method is largely copied from the standard TrackAssociatorByHits. Once I've tested how much difference
    // it makes I'll go through and comment it properly.

    double doubleCount=0.0;
    std::vector < SimHitIdpr > SimTrackIdsDC;

    for( iter iHit=startIterator; iHit != endIterator; iHit++ )
    {
        int idcount=0;

        SimTrackIdsDC.clear();
        hitAssociator.associateHitId( *(getHitFromIter( iHit )), SimTrackIdsDC );
        if( SimTrackIdsDC.size() > 1 )
        {
            for( TrackingParticle::g4t_iterator g4T=associatedTrackingParticle->g4Track_begin(); g4T != associatedTrackingParticle->g4Track_end();
                    ++g4T )
            {
                if( find( SimTrackIdsDC.begin(), SimTrackIdsDC.end(), SimHitIdpr( ( *g4T).trackId(), SimTrackIdsDC.begin()->second ) )
                        != SimTrackIdsDC.end() )
                {
                    idcount++;
                }
            }
        }
        if( idcount > 1 ) {
                  const auto subdetId = getHitFromIter(iHit)->geographicalId().subdetId();
                  const double weight = (subdetId == PixelSubdetector::PixelBarrel || subdetId == PixelSubdetector::PixelEndcap) ?  pixelHitWeight_ : 1.0;
                  doubleCount += weight*(idcount - 1);
                }
    }

    return doubleCount;
}

template<typename iter> double QuickTrackAssociatorByHitsImpl::getDoubleCount( const ClusterTPAssociation& clusterToTPList, iter startIterator, iter endIterator, TrackingParticleRef associatedTrackingParticle ) const
{
    // This code here was written by Subir Sarkar. I'm just splitting it off into a
    // separate method. - Grimes 01/May/2014

    double doubleCount=0;
    std::vector < SimHitIdpr > SimTrackIdsDC;

    for( iter iHit=startIterator; iHit != endIterator; iHit++ )
    {
        int idcount=0;

        std::vector < OmniClusterRef > oClusters=getMatchedClusters( iHit, iHit + 1 );  //only for the cluster being checked
        for( std::vector<OmniClusterRef>::const_iterator it=oClusters.begin(); it != oClusters.end(); ++it )
        {
            auto range = clusterToTPList.equal_range(*it);
            if( range.first != range.second )
            {
                for( auto ip=range.first; ip != range.second; ++ip )
                {
                    const TrackingParticleRef trackingParticle=(ip->second);
                    if( associatedTrackingParticle == trackingParticle )
                    {
                        idcount++;
                    }
                }
            }
        }

        if( idcount > 1 ) {
                  const auto subdetId = getHitFromIter(iHit)->geographicalId().subdetId();
                  const double weight = (subdetId == PixelSubdetector::PixelBarrel || subdetId == PixelSubdetector::PixelEndcap) ?  pixelHitWeight_ : 1.0;
                  doubleCount += weight*(idcount - 1);
                }
    }

    return doubleCount;
}

reco::RecoToSimCollectionSeed QuickTrackAssociatorByHitsImpl::associateRecoToSim( const edm::Handle<edm::View<TrajectorySeed> >& pSeedCollectionHandle_, 
                                                                                  const edm::Handle<TrackingParticleCollection>& trackingParticleCollectionHandle) const
{

    edm::LogVerbatim( "TrackAssociator" ) << "Starting TrackAssociatorByHitsImpl::associateRecoToSim - #seeds=" << pSeedCollectionHandle_->size()
            << " #TPs=" << trackingParticleCollectionHandle->size();

    reco::RecoToSimCollectionSeed returnValue(productGetter_);

    size_t collectionSize=pSeedCollectionHandle_->size();

    for( size_t i=0; i < collectionSize; ++i )
    {
        const TrajectorySeed* pSeed= &( *pSeedCollectionHandle_)[i];

        // The return of this function has first as the index and second as the number of associated hits
        std::vector < std::pair<edm::Ref<TrackingParticleCollection>,double> > trackingParticleQualityPairs=
                  (clusterToTPMap_) ? associateTrack( *clusterToTPMap_, trackingParticleCollectionHandle, nullptr, pSeed->recHits().first, pSeed->recHits().second ) : associateTrack( *hitAssociator_, trackingParticleCollectionHandle, nullptr, pSeed->recHits().first, pSeed->recHits().second );
        for( auto iTrackingParticleQualityPair=
                trackingParticleQualityPairs.begin(); iTrackingParticleQualityPair != trackingParticleQualityPairs.end();
                ++iTrackingParticleQualityPair )
        {
            const edm::Ref<TrackingParticleCollection>& trackingParticleRef=iTrackingParticleQualityPair->first;
            double numberOfSharedHits=iTrackingParticleQualityPair->second;
                        double numberOfValidTrackHits=weightedNumberOfTrackHits(*pSeed);

            if( numberOfSharedHits == 0.0 ) continue; // No point in continuing if there was no association

            //if electron subtract double counting
            if( abs( trackingParticleRef->pdgId() ) == 11 && (trackingParticleRef->g4Track_end() - trackingParticleRef->g4Track_begin()) > 1 )
            {
                if( clusterToTPMap_ ) numberOfSharedHits-=getDoubleCount( *clusterToTPMap_, pSeed->recHits().first, pSeed->recHits().second, trackingParticleRef );
                else numberOfSharedHits-=getDoubleCount( *hitAssociator_, pSeed->recHits().first, pSeed->recHits().second, trackingParticleRef );
            }

            double quality;
            if( absoluteNumberOfHits_ ) quality = numberOfSharedHits;
            else if( numberOfValidTrackHits != 0.0 ) quality = numberOfSharedHits / numberOfValidTrackHits;
            else quality=0;

            if( quality > cutRecoToSim_ && !(threeHitTracksAreSpecial_ && pSeed->nHits() == 3 && numberOfSharedHits < 3.0) )
            {
                returnValue.insert( edm::RefToBase < TrajectorySeed > (pSeedCollectionHandle_, i), std::make_pair( trackingParticleRef, quality ) );
            }
        }
    }

    LogTrace( "TrackAssociator" ) << "% of Assoc Seeds=" << ((double)returnValue.size()) / ((double)pSeedCollectionHandle_->size());
    returnValue.post_insert();
    return returnValue;

}

reco::SimToRecoCollectionSeed QuickTrackAssociatorByHitsImpl::associateSimToReco( const edm::Handle<edm::View<TrajectorySeed> >& pSeedCollectionHandle_, 
                                                                                  const edm::Handle<TrackingParticleCollection>& trackingParticleCollectionHandle) const 
{

    edm::LogVerbatim( "TrackAssociator" ) << "Starting TrackAssociatorByHitsImpl::associateSimToReco - #seeds=" << pSeedCollectionHandle_->size()
            << " #TPs=" << trackingParticleCollectionHandle->size();

    reco::SimToRecoCollectionSeed returnValue(productGetter_);
        if(trackingParticleCollectionHandle->empty())
          return returnValue;

        if(clusterToTPMap_) {
          checkClusterMapProductID(*clusterToTPMap_, trackingParticleCollectionHandle);
        }

    size_t collectionSize=pSeedCollectionHandle_->size();

    for( size_t i=0; i < collectionSize; ++i )
    {
        const TrajectorySeed* pSeed= &( *pSeedCollectionHandle_)[i];

        // The return of this function has first as an edm:Ref to the associated TrackingParticle, and second as the number of associated hits
        std::vector < std::pair<edm::Ref<TrackingParticleCollection>,double> > trackingParticleQualityPairs=
                  (clusterToTPMap_) ? associateTrack( *clusterToTPMap_, trackingParticleCollectionHandle, nullptr, pSeed->recHits().first, pSeed->recHits().second ) : associateTrack( *hitAssociator_, trackingParticleCollectionHandle, nullptr, pSeed->recHits().first, pSeed->recHits().second );
        for( auto iTrackingParticleQualityPair=
                trackingParticleQualityPairs.begin(); iTrackingParticleQualityPair != trackingParticleQualityPairs.end();
                ++iTrackingParticleQualityPair )
        {
            const edm::Ref<TrackingParticleCollection>& trackingParticleRef=iTrackingParticleQualityPair->first;
            double numberOfSharedHits=iTrackingParticleQualityPair->second;
            double numberOfValidTrackHits=weightedNumberOfTrackHits(*pSeed);
            size_t numberOfSimulatedHits=0; // Set a few lines below, but only if required.

            if( numberOfSharedHits == 0.0 ) continue; // No point in continuing if there was no association

            //if electron subtract double counting
            if( abs( trackingParticleRef->pdgId() ) == 11 && (trackingParticleRef->g4Track_end() - trackingParticleRef->g4Track_begin()) > 1 )
            {
                if( clusterToTPMap_ ) numberOfSharedHits-=getDoubleCount( *clusterToTPMap_, pSeed->recHits().first, pSeed->recHits().second, trackingParticleRef );
                else numberOfSharedHits-=getDoubleCount( *hitAssociator_, pSeed->recHits().first, pSeed->recHits().second, trackingParticleRef );
            }

            if( simToRecoDenominator_ == denomsim || (numberOfSharedHits < 3.0 && threeHitTracksAreSpecial_) ) // the numberOfSimulatedHits is not always required, so can skip counting in some circumstances
            {
                // Note that in the standard TrackAssociatorByHits, all of the hits in associatedTrackingParticleHits are checked for
                // various things.  I'm not sure what these checks are for but they depend on the UseGrouping and UseSplitting settings.
                // This associator works as though both UseGrouping and UseSplitting were set to true, i.e. just counts the number of
                // hits in the tracker.
                numberOfSimulatedHits=trackingParticleRef->numberOfTrackerHits();
            }

            double purity = numberOfSharedHits / numberOfValidTrackHits;
            double quality;
            if( absoluteNumberOfHits_ ) quality = numberOfSharedHits;
            else if( simToRecoDenominator_ == denomsim && numberOfSimulatedHits != 0 ) quality= numberOfSharedHits
                    / static_cast<double>( numberOfSimulatedHits );
            else if( simToRecoDenominator_ == denomreco && numberOfValidTrackHits != 0.0 ) quality=purity;
            else quality=0;

            if( quality > qualitySimToReco_ && !(threeHitTracksAreSpecial_ && numberOfSimulatedHits == 3 && numberOfSharedHits < 3.0)
                    && (absoluteNumberOfHits_ || (purity > puritySimToReco_)) )
            {
                returnValue.insert( trackingParticleRef, std::make_pair( edm::RefToBase < TrajectorySeed > (pSeedCollectionHandle_, i), quality ) );
            }
        }
    }

    LogTrace("TrackAssociator") << "% of Assoc TPs=" << ((double)returnValue.size())/((double)trackingParticleCollectionHandle->size());
    returnValue.post_insert();
    return returnValue;
}

double QuickTrackAssociatorByHitsImpl::weightedNumberOfTrackHits(const reco::Track& track) const {
  const reco::HitPattern& p = track.hitPattern();
  const auto pixelHits = p.numberOfValidPixelHits();
  const auto otherHits = p.numberOfValidHits() - pixelHits;
  return pixelHits*pixelHitWeight_ + otherHits;
}

double QuickTrackAssociatorByHitsImpl::weightedNumberOfTrackHits(const TrajectorySeed& seed) const {
  double sum = 0.0;
  for(auto iHit=seed.recHits().first; iHit!=seed.recHits().second; ++iHit) {
    const auto subdetId = getHitFromIter(iHit)->geographicalId().subdetId();
    const double weight = (subdetId == PixelSubdetector::PixelBarrel || subdetId == PixelSubdetector::PixelEndcap) ?  pixelHitWeight_ : 1.0;
    sum += weight;
  }
  return sum;
}