FullVertexFitter.cxx

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/*
  Copyright (C) 2002-2026 CERN for the benefit of the ATLAS collaboration
*/
 
/***************************************************************************
                        FullVertexFitter.cxx  -  Description
 ***************************************************************************/
#include "TrkVertexBilloirTools/FullVertexFitter.h"
#include "TrkDetDescrUtils/GeometryStatics.h"
#include "TrkLinks/LinkToXAODTrackParticle.h" 
#include "TrkParticleBase/LinkToTrackParticleBase.h"
#include "TrkParticleBase/TrackParticleBase.h"
#include "TrkSurfaces/PerigeeSurface.h"
#include "TrkTrack/LinkToTrack.h"
#include "TrkTrack/Track.h"
#include "TrkTrack/TrackCollection.h"
#include "TrkVertexFitterInterfaces/IVertexLinearizedTrackFactory.h"
#include "VxVertex/LinearizedTrack.h"
#include "VxVertex/VxTrackAtVertex.h"
#include <memory>
#include <cmath>
#include <memory>
//xAOD includes 
#include "xAODTracking/Vertex.h" 
#include "xAODTracking/TrackParticle.h" 
 
/* These are some local helper classes only needed for convenience, therefor
within anonymous namespace. They do contain temporary calculations of matrices
and vectors resulting from the Billoir calculation (Note: no transformation
of the perigee parameters is done anymore).*/
namespace
{
    struct BilloirTrack
    {
        BilloirTrack() : perigee ( nullptr ), originalPerigee( nullptr ), linTrack( nullptr ) {}
 
                ~BilloirTrack() = default;
 
                BilloirTrack(const BilloirTrack& arg)
                  : perigee (arg.perigee), // does not own it
                    originalPerigee (arg.originalPerigee), // does not own it
                    linTrack (arg.linTrack->clone()),
                    chi2 (arg.chi2),
                    Di_mat (arg.Di_mat),
                    Ei_mat (arg.Ei_mat),
                    Gi_mat (arg.Gi_mat),
                    Bi_mat (arg.Bi_mat),
                    Ci_inv (arg.Ci_inv),
                    Ui_vec (arg.Ui_vec),
                    BCi_mat(arg.BCi_mat),
                    Dper   (arg.Dper)
                {
                }
 
                BilloirTrack(BilloirTrack&& arg) = default;
        
        Trk::TrackParameters * perigee;
        const Trk::TrackParameters * originalPerigee;
        std::unique_ptr<Trk::LinearizedTrack> linTrack;
        double chi2{};
        AmgMatrix(5,3) Di_mat;
        AmgMatrix(5,3) Ei_mat;
        AmgMatrix(3,3) Gi_mat;
        AmgMatrix(3,3) Bi_mat;     // Bi = Di.T * Wi * Ei
        AmgMatrix(3,3) Ci_inv;     // Ci = (Ei.T * Wi * Ei)^-1
        Amg::Vector3D Ui_vec;     // Ui = Ei.T * Wi * dqi
        AmgMatrix(3,3) BCi_mat;       // BCi = Bi * Ci^-1
        AmgVector(5) Dper;
    };
 
    struct BilloirVertex
    {
                BilloirVertex()
                {
                  A_mat.setZero();
                  T_vec.setZero();
                  BCB_mat.setZero();
                  BCU_vec.setZero();
                };
        double chi2{};
        unsigned int ndf{};
        AmgMatrix(3,3) A_mat;              // T  = sum{Di.T * Wi * Di}
        Amg::Vector3D T_vec;              // A  = sum{Di.T * Wi * dqi}
        AmgMatrix(3,3) BCB_mat;       // BCB = sum{Bi * Ci^-1 * Bi.T}
        Amg::Vector3D BCU_vec;       // BCU = sum{Bi * Ci^-1 * Ui}
    };
}
 
namespace Trk
{
    StatusCode FullVertexFitter::initialize()
    {
        
 
        if ( m_linFactory.retrieve().isFailure() )
        {
            msg(MSG::FATAL)<< "Failed to retrieve tool " << m_linFactory << endmsg;
            return StatusCode::FAILURE;
        }
        
        
            msg(MSG::INFO) << "Retrieved tool " << m_linFactory << endmsg;
        
 
        return StatusCode::SUCCESS;
    }
 
    FullVertexFitter::FullVertexFitter ( const std::string& t, const std::string& n, const IInterface*  p ) : base_class ( t,n,p ),
                                                          m_maxIterations ( 5 ),
                                                          m_maxDchi2PerNdf ( 0.000001 )
 
    {
        declareProperty ( "MaxIterations",        m_maxIterations );
        declareProperty ( "MaxChi2PerNdf",        m_maxDchi2PerNdf );
        declareProperty ( "LinearizedTrackFactory", m_linFactory );
        declareInterface<IVertexFitter> ( this );
    }
 
    FullVertexFitter::~FullVertexFitter() = default;

    std::unique_ptr<xAOD::Vertex> FullVertexFitter::fit ( const EventContext& ctx,
                                                          const std::vector<const Trk::TrackParameters*> & originalPerigees,
                                                          const Amg::Vector3D& firstStartingPoint ) const
    {
                xAOD::Vertex constraint;
                constraint.makePrivateStore();
                constraint.setPosition( firstStartingPoint );
                constraint.setCovariancePosition( AmgSymMatrix(3)(3,3) );
                constraint.setFitQuality( 0.,0.);
               return fit ( ctx, originalPerigees, constraint );
    }

    std::unique_ptr<xAOD::Vertex> FullVertexFitter::fit ( const EventContext& /*ctx*/,
                                                          const std::vector<const Trk::TrackParameters*> & originalPerigees,
                                                          const xAOD::Vertex& firstStartingPoint ) const
    {
        if ( originalPerigees.empty() )
        {
            ATH_MSG_VERBOSE("No tracks to fit in this event.");
            return nullptr;
        }
 
        /* Initialisation of variables */
        double chi2 = 2000000000000.;
        unsigned int nRP = originalPerigees.size();  // Number of tracks to fit
        int ndf = nRP * ( 5-3 ) - 3;            // Number of degrees of freedom
        if ( ndf < 0 ) {ndf = 1;}
 
        /* Determine if we are doing a constraint fit.*/
        bool constraint = false;
        if ( firstStartingPoint.covariancePosition().trace() != 0. )
        {
            constraint = true;
            ndf += 3;
            ATH_MSG_DEBUG("Fitting with constraint.");
            ATH_MSG_VERBOSE(firstStartingPoint.covariancePosition().inverse());
        }
 
        double chi2New=0.;
 
        Amg::Vector3D linPoint ( firstStartingPoint.position() ); // linearization point for track parameters (updated for every iteration)
 
        Amg::Vector3D p0;
 
        std::vector<Amg::Vector3D> mom_at_Origin;
 
        auto fittedVertex = std::make_unique<xAOD::Vertex>();
        fittedVertex->makePrivateStore(); // xAOD::VertexContainer will take ownership of AuxStore when ActualVertex is added to it
 
        std::vector<VxTrackAtVertex> tracksAtVertex;
        std::vector<BilloirTrack> billoirTracks;
 
        /* Iterate fits until the fit criteria are met, or the number of max
        iterations is reached. */
        for ( unsigned int niter=0; niter < m_maxIterations; ++niter )
        {
            ATH_MSG_VERBOSE("Start of iteration " << niter << ", starting point ("
            << linPoint [0] << ", " << linPoint [1] << ", " << linPoint [2]
            << ") and " << originalPerigees.size() << " tracks.");
 
            billoirTracks.clear();
            chi2New = 0.;
 
            AmgMatrix(2,3) D;
            BilloirVertex billoirVertex;
 
            /* Linearize the track parameters wrt. starting point of the fit */
            Amg::Vector3D globalPosition = linPoint;
            Trk::PerigeeSurface perigeeSurface ( globalPosition );
 
            /* Extrapolate the perigees to the startpoint of the fit */
            unsigned int count(0);
            for (const auto *originalPerigee : originalPerigees)
            {
 
                if ( niter == 0 )
                {
                    // need to cast to access parameters() (this is to guarantee that the code knows if it is neutrals or charged parameters)
                    const Trk::TrackParameters* chargedParameters ( nullptr );
                    chargedParameters = dynamic_cast<const Trk::TrackParameters*> ( originalPerigee );
                    if ( chargedParameters==nullptr )
                    {
                        ATH_MSG_ERROR("Track parameters are not charged tracks ... full fit aborted (this will be handled correctly soon)");
                        return nullptr;
                    }
                    
                    
                        p0.x() =  chargedParameters->parameters() [Trk::phi];
                        p0.y() = chargedParameters->parameters() [Trk::theta];
                        p0.z() = chargedParameters->parameters() [Trk::qOverP] ;
                        mom_at_Origin.push_back ( p0 );
                    
                }
 
                // FOR NOW THE ONLY WAY TO CHECK IF EXTRAPOLATION WORKED IS TO CHECK THE RETURN POINTER
                // if it is ZERO then take the original perigee
                // there is a tiny tiny chance that the extrapolation fails because of a non converging runge kutta procedure
                // in all other cases it did not extrapolate because the reference surface of the original perigee
                // is already given to the extrapolation point (or very close nearby)
 
                std::unique_ptr<LinearizedTrack> linTrack ( m_linFactory->linearizedTrack ( originalPerigee, linPoint ) );
                if ( linTrack==nullptr )
                {
                    ATH_MSG_DEBUG("Could not linearize track! Skipping this track!");
                }
                else
                {
                    BilloirTrack locBilloirTrack;
                    
                    locBilloirTrack.originalPerigee = originalPerigee;
                    double d0 = linTrack->expectedParametersAtPCA()[Trk::d0];
                    double z0 = linTrack->expectedParametersAtPCA()[Trk::z0];
                    double phi = linTrack->expectedParametersAtPCA()[Trk::phi];
                    double theta = linTrack->expectedParametersAtPCA()[Trk::theta];
                    double qOverP = linTrack->expectedParametersAtPCA()[Trk::qOverP];
 
                    // calculate f(V_0,p_0)  f_d0 = f_z0 = 0
                    double f_phi = mom_at_Origin[count] ( 0 );
                    double f_theta = mom_at_Origin[count] ( 1 );
                    double f_qOverP = mom_at_Origin[count] ( 2 );
 
                    //calculate Dper[i]
                    locBilloirTrack.Dper[0] = d0;
                    locBilloirTrack.Dper[1] = z0;
                    locBilloirTrack.Dper[2] = phi - f_phi;
                    locBilloirTrack.Dper[3] = theta - f_theta;
                    locBilloirTrack.Dper[4] = qOverP -f_qOverP;
 
                    // the D matrix
                    AmgMatrix(5,3) D_mat;
                    D_mat.setZero();
                    D_mat = linTrack->positionJacobian();
 
                    // calculate the E matrix
                    AmgMatrix(5,3) E_mat;
                    E_mat.setZero();
                    E_mat = linTrack->momentumJacobian();
 
                    // and cache martix operations related to these matrices
                    AmgMatrix(3,5) Dt_W_mat;
                    Dt_W_mat.setZero();
                    AmgMatrix(3,5) Et_W_mat;
                    Et_W_mat.setZero();
                    Dt_W_mat = D_mat.transpose() * ( linTrack->expectedWeightAtPCA() );
                    Et_W_mat = E_mat.transpose() * ( linTrack->expectedWeightAtPCA() );
 
                    // these are the Billoir computed results
                    locBilloirTrack.Di_mat = D_mat;
                    locBilloirTrack.Ei_mat = E_mat;
                    locBilloirTrack.Gi_mat = Et_W_mat*E_mat;
                    locBilloirTrack.Bi_mat = Dt_W_mat * E_mat; // Di.T * Wi * Ei
                    locBilloirTrack.Ui_vec = Et_W_mat * locBilloirTrack.Dper; // Ei.T * Wi * dqi
                    locBilloirTrack.Ci_inv = Et_W_mat * E_mat ; // (Ei.T * Wi * Ei)^-1
                    // we need the inverse matrix here
                    locBilloirTrack.Ci_inv = locBilloirTrack.Ci_inv.inverse().eval();
                        // sum up over all tracks
                        billoirVertex.T_vec       += Dt_W_mat * locBilloirTrack.Dper; // sum{Di.T * Wi * dqi}
                        billoirVertex.A_mat = billoirVertex.A_mat + Dt_W_mat * D_mat ; // sum{Di.T * Wi * Di}
 
                        // remember those results for all tracks
                        locBilloirTrack.BCi_mat = locBilloirTrack.Bi_mat * locBilloirTrack.Ci_inv; // BCi = Bi * Ci^-1
 
                        // and some summed results
                        billoirVertex.BCU_vec += locBilloirTrack.BCi_mat * locBilloirTrack.Ui_vec; // sum{Bi * Ci^-1 * Ui}
                        billoirVertex.BCB_mat =  billoirVertex.BCB_mat + locBilloirTrack.BCi_mat * locBilloirTrack.Bi_mat.transpose() ;// sum{Bi * Ci^-1 * Bi.T}
                        locBilloirTrack.linTrack = std::move(linTrack);
                        billoirTracks.push_back ( std::move(locBilloirTrack) );
                                    count++;
 
                }
            }
 
            // calculate delta (billoirFrameOrigin-position), might be changed by the beam-const
            Amg::Vector3D    V_del     = billoirVertex.T_vec - billoirVertex.BCU_vec;      // V_del = T-sum{Bi*Ci^-1*Ui}
            AmgMatrix(3,3) V_wgt_mat = billoirVertex.A_mat - billoirVertex.BCB_mat;      // V_wgt = A-sum{Bi*Ci^-1*Bi.T}
 
            if ( constraint )
            {
                // add V_del += wgtconst * (billoirFrameOrigin - Vconst) and V_wgt +=wgtconst
                Amg::Vector3D constraintPosInBilloirFrame;
                constraintPosInBilloirFrame.setZero();
                // this will be 0 for first iteration but != 0 from second on
                constraintPosInBilloirFrame[0] = firstStartingPoint.position() [0]-linPoint [0];
                constraintPosInBilloirFrame[1] = firstStartingPoint.position() [1]-linPoint [1];
                constraintPosInBilloirFrame[2] = firstStartingPoint.position() [2]-linPoint [2];
 
                V_del     += firstStartingPoint.covariancePosition().inverse() *constraintPosInBilloirFrame;
                V_wgt_mat += firstStartingPoint.covariancePosition().inverse();
            }
            // cov(delta_V) = V_wgt^-1
            AmgMatrix(3,3) cov_delta_V_mat = V_wgt_mat.inverse().eval() ;
 
            // delta_V = cov_(delta_V) * V_del;
            Amg::Vector3D delta_V = cov_delta_V_mat * V_del;
            /* Momentum Calculation follows here. */
            /* ---------------------------------------------------------------------------------------- */
            std::vector<AmgMatrix(5,5)> cov_delta_P_mat ( nRP );
            std::vector<double> chi2PerTrack;
            chi2PerTrack.clear();
 
            // ===> loop over tracks
            std::vector<BilloirTrack>::iterator BTIter;
            int iRP =0;
            for ( BTIter = billoirTracks.begin(); BTIter != billoirTracks.end() ; ++BTIter )
            {
                // delta_P calculation
                BilloirTrack locP ( ( *BTIter ) );
                Amg::Vector3D delta_P = ( locP.Ci_inv ) * ( ( locP.Ui_vec )- ( locP.Bi_mat.transpose() ) *delta_V );
                mom_at_Origin[iRP] ( 0 ) += delta_P[0];
                if ( fabs ( mom_at_Origin[iRP] ( 0 ) ) > 10*M_PI )
                { // protect while loop
                    if (msgLvl(MSG::WARNING))
                                        {
                                          msg(MSG::WARNING) << " Track direction angles have numerical problems, stop perigee parameter update." << endmsg;
                      msg(MSG::WARNING) << " Phi value: "<<mom_at_Origin[iRP] ( 0 ) <<endmsg;
                                        }
                    return nullptr;
                }
                mom_at_Origin[iRP] ( 1 ) += delta_P[1];
                if ( fabs ( mom_at_Origin[iRP] ( 0 ) ) > 5*M_PI )
                { // protect while loop
                                        if (msgLvl(MSG::WARNING))
                                        {
                                          msg(MSG::WARNING) << " Track direction angles have numerical problems, stop perigee parameter update." << endmsg;
                      msg(MSG::WARNING) << " Theta value: "<<mom_at_Origin[iRP] ( 1 ) <<endmsg;
                                        }
                                        return nullptr;
                }
                mom_at_Origin[iRP] ( 2 ) += delta_P[2];
                //  mom_at_Origin[iRP](0) -= M_PI > M_PI ? M_PI : 0;
                //  mom_at_Origin[iRP](0) += M_PI < -M_PI ? M_PI : 0;
                //correct phi, theta coordinates
                while ( fabs ( mom_at_Origin[iRP] ( 0 ) ) > M_PI ) mom_at_Origin[iRP] ( 0 ) += ( mom_at_Origin[iRP] ( 0 ) > 0 ) ? -2*M_PI : 2*M_PI;
                while ( mom_at_Origin[iRP] ( 1 ) > 2*M_PI ) mom_at_Origin[iRP] ( 1 ) -= 2*M_PI;
                while ( mom_at_Origin[iRP] ( 1 ) < -M_PI ) mom_at_Origin[iRP] ( 1 ) += M_PI;
                if ( mom_at_Origin[iRP] ( 1 ) > M_PI )
                {
                    mom_at_Origin[iRP] ( 1 ) = 2*M_PI - mom_at_Origin[iRP] ( 1 );
                    if ( mom_at_Origin[iRP] ( 0 ) >= 0 )   mom_at_Origin[iRP] ( 0 )  += ( mom_at_Origin[iRP] ( 0 ) >0 ) ? -M_PI : M_PI;
                }
                if ( mom_at_Origin[iRP] ( 1 ) < 0.0 )
                {
                    mom_at_Origin[iRP] ( 1 )  = - mom_at_Origin[iRP] ( 1 );
                    if ( mom_at_Origin[iRP] ( 0 ) >= 0 )  mom_at_Origin[iRP] ( 0 ) += ( mom_at_Origin[iRP] ( 0 ) >0 ) ? -M_PI : M_PI;
                }
 
                //calculate 5x5 cov_delta_P matrix
                // d(d0,z0,phi,theta,qOverP)/d(x,y,z,phi,theta,qOverP)-transformation matrix
                AmgMatrix(5,6) trans_mat;
                trans_mat.setZero();
                trans_mat ( 0,0 ) = locP.Di_mat ( 0,0 ); trans_mat ( 0,1 ) = locP.Di_mat ( 0,1 );
                trans_mat ( 1,0 ) = locP.Di_mat ( 1,0 ); trans_mat ( 1,1 ) = locP.Di_mat ( 1,1 ); trans_mat ( 1,2 ) = 1.;
                trans_mat ( 2,3 ) = 1.; trans_mat ( 3,4 ) = 1.; trans_mat ( 4,5 ) = 1.;
 
                //some intermediate calculations to get 5x5 matrix
                //cov(V,V)
                AmgMatrix(3,3) V_V_mat; V_V_mat.setZero();  V_V_mat = cov_delta_V_mat ;
                //          std::cout<<"V_V_mat = "<<V_V_mat<<std::endl;
                //cov(V,P)
                AmgMatrix(3,3) V_P_mat; V_P_mat.setZero(); V_P_mat =  -cov_delta_V_mat*locP.Gi_mat*locP.Ci_inv ;
                //          std::cout<<"V_P_mat = "<<V_P_mat<<std::endl;
                //cov(P,P)
                AmgMatrix(3,3) P_P_mat; P_P_mat.setZero(); P_P_mat =  locP.Ci_inv + locP.BCi_mat.transpose() *cov_delta_V_mat*locP.BCi_mat ;
                //          std::cout<<"P_P_mat = "<<P_P_mat<<std::endl;
 
                AmgMatrix(6,6) cov_mat;
                cov_mat.setZero();
                cov_mat.block<3,3>(0,3) = V_P_mat;
                cov_mat.block<3,3>(3,0) = V_P_mat.transpose() ;
                cov_mat.block<3,3>(0,0) = V_V_mat ;
                cov_mat.block<3,3>(3,3) = P_P_mat ;
 
                // cov_delta_P calculation
                //          std::cout<<"trans_mat = "<<trans_mat<<std::endl;
                cov_delta_P_mat[iRP] = trans_mat*cov_mat*trans_mat.transpose() ;
                //          std::cout<<"cov_delta_P_mat[iRP] = "<<cov_delta_P_mat[iRP]<<std::endl;
 
                ++iRP;
 
                /* Calculate chi2 per track. */
                ( *BTIter ).chi2= ( ( ( *BTIter ).Dper- ( *BTIter ).Di_mat*delta_V - ( *BTIter ).Ei_mat*delta_P ).transpose() * ( *BTIter ).linTrack->expectedWeightAtPCA() * ( ( *BTIter ).Dper - ( *BTIter ).Di_mat*delta_V - ( *BTIter ).Ei_mat*delta_P ) ) [0];
 
                if ( ( *BTIter ).chi2 < 0 )
                {
                    ATH_MSG_WARNING( "FullVertexFitter::calculate: error in chi2_per_track" );
                    return nullptr;
                }
                chi2New += ( *BTIter ).chi2;
            }
 
            if ( constraint )
            {
                Amg::Vector3D deltaTrk;
                deltaTrk.setZero();
                // last term will also be 0 again but only in the first iteration
                //                  = calc. vtx in billoir frame - (    constraint pos. in billoir frame    )
                deltaTrk[0] = delta_V[0] - ( firstStartingPoint.position() [0] - linPoint [0] );
                deltaTrk[1] = delta_V[1] - ( firstStartingPoint.position() [1] - linPoint [1] );
                deltaTrk[2] = delta_V[2] - ( firstStartingPoint.position() [2] - linPoint [2] );
                chi2New  += ( deltaTrk.transpose() * firstStartingPoint.covariancePosition().inverse() * deltaTrk ) [0];
            }
 
            /* assign new linearization point (= new vertex position in global frame) */
            Amg::Vector3D tmpPos ( linPoint );
            tmpPos[0] += delta_V[0];   tmpPos[1] += delta_V[1];   tmpPos[2] += delta_V[2];
            linPoint = tmpPos;
 
            if ( chi2New < chi2 )
            {
                /* Store the vertex */
                chi2 = chi2New;
                //const AmgMatrix(3,3) * newCovarianceMatrix =  &cov_delta_V_mat ;
                //const AmgMatrix(3,3) newErrorMatrix = newCovarianceMatrix->inverse().eval();
                //fittedVertex = RecVertex ( linPoint.position(), newErrorMatrix, ndf, chi2 );
 
                                //the cov_delta_V_mat is already the inverted form.  -katy 2/2/16
                fittedVertex->setPosition( linPoint );
                                fittedVertex->setCovariancePosition( cov_delta_V_mat );
                                fittedVertex->setFitQuality( chi2, ndf );
 
                // new go through vector and delete entries
                /* // TODO: not needed anymore, tracksAtVertex doesn't store pointers - just the objects themselves <David Shope> (EDM Migration) 03/21/16
                for ( std::vector<Trk::VxTrackAtVertex*>::const_iterator itr = tracksAtVertex.begin();
                        itr != tracksAtVertex.end(); ++itr )
                {
                    delete ( *itr );
                }
                */
 
                tracksAtVertex.clear();
                /* Store the tracks at vertex */
                Amg::Vector3D Vertex(linPoint[0], linPoint[1], linPoint[2]);
                const Trk::PerigeeSurface Surface ( Vertex );
                TrackParameters * refittedPerigee ( nullptr );
                unsigned int iter= 0;
                std::vector<BilloirTrack>::iterator BTIter;
                for ( BTIter = billoirTracks.begin(); BTIter != billoirTracks.end() ; ++BTIter )
                {
                    const AmgMatrix(5,5) * newTrackCovarianceMatrix =  &cov_delta_P_mat[iter] ;
                    //Covariance matrix does not need to be inverted:
                    //                  AmgMatrix(5,5)  newTrackErrorMatrix = (AmgMatrix(5,5)) newTrackCovarianceMatrix->inverse().eval();
                    AmgMatrix(5,5)  newTrackErrorMatrix = (AmgMatrix(5,5)) newTrackCovarianceMatrix->eval();
                    refittedPerigee = new Trk::Perigee ( 0.,0.,mom_at_Origin[iter][0],mom_at_Origin[iter][1],mom_at_Origin[iter][2], 
                                               Surface, std::move(newTrackErrorMatrix) );
                    Trk::VxTrackAtVertex* tmpVxTrkAtVtx = new Trk::VxTrackAtVertex ( ( *BTIter ).chi2, refittedPerigee, ( *BTIter ).originalPerigee );
                    tracksAtVertex.push_back ( *tmpVxTrkAtVtx );
                    iter ++;
                }
            }
        } // end of iteration
        fittedVertex->vxTrackAtVertex() = tracksAtVertex;
        //ATH_MSG_VERBOSE("Final Vertex Fitted: " << fittedVxCandidate->recVertex()); // TODO: can no longer print vertex after converting to xAOD
        return fittedVertex;
    }
 
    std::unique_ptr<xAOD::Vertex> FullVertexFitter::fit ( const EventContext& ctx, const std::vector<const Trk::TrackParameters*>& perigeeList ) const
    {
                Amg::Vector3D tmpVtx(0.,0.,0.);
                return fit ( ctx, perigeeList, tmpVtx );
    }
 
 
        //xAOD interfaced methods. Required to un-block the current situation  
        // with the xAOD tracking design. 
         std::unique_ptr<xAOD::Vertex> FullVertexFitter::fit(const EventContext& ctx, const std::vector<const xAOD::TrackParticle*>& vectorTrk,const Amg::Vector3D& startingPoint)  const
         {
                  xAOD::Vertex constraint;
                  constraint.makePrivateStore();
                  constraint.setPosition( startingPoint );
                  constraint.setCovariancePosition( AmgSymMatrix(3)(3,3) );
                  constraint.setFitQuality( 0.,0.);
                 return fit(ctx, vectorTrk, constraint);
         }//end of the xAOD starting point fit method 
         
             
         std::unique_ptr<xAOD::Vertex> FullVertexFitter::fit(const EventContext& ctx, const std::vector<const xAOD::TrackParticle*>& vectorTrk, const xAOD::Vertex& constraint) const
         { 
           if(vectorTrk.empty()) 
           { 
            msg(MSG::INFO)<<"Empty vector of tracks passed"<<endmsg; 
            return nullptr; 
           } 
            
           //making a list of perigee out of the vector of tracks   
           std::vector<const Trk::TrackParameters*> measuredPerigees; 
            
           for(const auto *i : vectorTrk) 
           { 
            const Trk::TrackParameters * tmpMeasPer = &(i->perigeeParameters()); 
           
            if(tmpMeasPer!=nullptr) measuredPerigees.push_back(tmpMeasPer); 
            else  msg(MSG::INFO)<<"Failed to dynamic_cast this track parameters to perigee"<<endmsg; //TODO: Failed to implicit cast the perigee parameters to track parameters?
           } 
            
            
           std::unique_ptr<xAOD::Vertex> fittedVertex = fit( ctx, measuredPerigees, constraint );
         
           //assigning the input tracks to the fitted vertex through VxTrackAtVertices
           { 
            if( fittedVertex->vxTrackAtVertexAvailable() ) // TODO: I don't think vxTrackAtVertexAvailable() does the same thing as a null pointer check! 
            { 
             if(!fittedVertex->vxTrackAtVertex().empty()) 
             { 
              for(unsigned int i = 0; i <vectorTrk.size(); ++i) 
              { 
         
               LinkToXAODTrackParticle * linkTT = new LinkToXAODTrackParticle; 
               linkTT->setElement(vectorTrk[i]); 
         
               // vxtrackatvertex takes ownership! 
               ( fittedVertex->vxTrackAtVertex() )[i].setOrigTrack(linkTT); 
              }//end of loop for setting orig tracks in. 
             }//end of protection against unsuccessfull updates (no tracks were added) 
            }//end of vector of tracks check 
           }//end of pointer check
 
                   //now set links to xAOD::TrackParticles directly in the xAOD::Vertex
                   unsigned int VTAVsize = fittedVertex->vxTrackAtVertex().size();
                   for (unsigned int i = 0 ; i < VTAVsize ; ++i)
                   {
                     Trk::VxTrackAtVertex* VTAV = &( fittedVertex->vxTrackAtVertex().at(i) );
                     //TODO: Will this pointer really hold 0 if no VxTrackAtVertex is found?
                     if (not VTAV){
                       ATH_MSG_WARNING (" Trying to set link to xAOD::TrackParticle. The VxTrackAtVertex is not found");
                       continue;
                     }
 
                     Trk::ITrackLink* trklink = VTAV->trackOrParticleLink();
 
                     // See if the trklink is to an xAOD::TrackParticle
                     Trk::LinkToXAODTrackParticle* linkToXAODTP = dynamic_cast<Trk::LinkToXAODTrackParticle*>(trklink);
                     if (linkToXAODTP)
                     {
 
                       //Now set the new link to the xAOD vertex
                       fittedVertex->addTrackAtVertex(*linkToXAODTP, VTAV->weight());
 
                     } else {
                       ATH_MSG_WARNING ("Skipping track. Trying to set link to something else than xAOD::TrackParticle. Neutrals not supported.");
                     }
                   } //end of loop
 
           return fittedVertex;
 
         }//end of the xAOD constrained fit method 
 
 
 
}