AFPSiDLinRegTool.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/

 
 
#include "AFP_LocReco/AFPSiDLinRegTool.h"
 
 
AFPSiDLinRegTool::AFPSiDLinRegTool( const std::string& type, 
                                    const std::string& name, 
                                    const IInterface* parent ) : 
  base_class( type, name, parent )
{
}
 
 
StatusCode AFPSiDLinRegTool::initialize()
{
    if(m_trackSelection=="loose" || m_trackSelection=="Loose")
    {
        m_selectionFunction = [](const std::vector<const xAOD::AFPSiHitsCluster*>&) { return true; };
    }
    else if(m_trackSelection=="medium" || m_trackSelection=="Medium")
    {
        m_selectionFunction = [this](const std::vector<const xAOD::AFPSiHitsCluster*>& t) {
            std::vector<int> plane{0, 0, 0, 0};
 
            for (auto&& cluster : t) {
                const int p = cluster->pixelLayerID();
                plane[p]++;
            }
 
            auto HasAnyClusters = [](int n) { return n > 0; };
 
            return std::count_if(plane.begin(), plane.end(), HasAnyClusters) >= m_minimalNumberOfPlanesInTrack;
        };
    }
    else if(m_trackSelection=="tight" || m_trackSelection=="Tight")
    {
        m_selectionFunction = [this](const std::vector<const xAOD::AFPSiHitsCluster*>& t) {
            std::vector<int> plane{0, 0, 0, 0};
 
            for (auto&& cluster : t) {
                const int p = cluster->pixelLayerID();
                plane[p]++;
            }
 
            auto HasMultipleClusters = [](int n) { return n > 1; };
 
            if (std::any_of(plane.begin(), plane.end(), HasMultipleClusters)) { return false; }
 
            // Check number of planes (cluster/plane == 1)
            if (t.size() < m_minimalNumberOfPlanesInTrack) { return false; }
 
            return true;
        };
    }
    else
    {
        ATH_MSG_ERROR("unknown trackSelection "<<m_trackSelection<<", allowed values are \"Loose\", \"Medium\", or \"Tight\" ");
        return StatusCode::FAILURE;
    }
    
    CHECK( m_hitsClusterContainerKey.initialize() );
    
    return StatusCode::SUCCESS;
}
 
 
bool AFPSiDLinRegTool::areNeighbours(const xAOD::AFPSiHitsCluster* a, const xAOD::AFPSiHitsCluster* b) const
{
    // All clusters should be in same station, so no need for checking
 
    const double xa = a->xLocal();
    const double ya = a->yLocal();
 
    const double xb = b->xLocal();
    const double yb = b->yLocal();
 
    const double dx = xa - xb;
    const double dy = ya - yb;
    const double maxDistanceSq = m_allowedDistanceBetweenClustersInTrack * m_allowedDistanceBetweenClustersInTrack;
    return dx * dx + dy * dy <= maxDistanceSq;
}
 
 
std::vector<const xAOD::AFPSiHitsCluster*> AFPSiDLinRegTool::findClusterAroundElement(std::list<const xAOD::AFPSiHitsCluster*>& toJoin, const xAOD::AFPSiHitsCluster* init) const 
{
    // Push initial cluster
    std::vector<const xAOD::AFPSiHitsCluster*> track;
    track.push_back(init);
 
    std::vector<const xAOD::AFPSiHitsCluster*> newNeighbours;
 
    // Repeat while new neighbours are found
    do {
      newNeighbours.clear();
      for (auto&& b : toJoin) {
        bool isNew = false;
        for (auto&& a : track) {
          if (areNeighbours(a, b)) { isNew = true; }
        }
 
        if (isNew) {
          newNeighbours.push_back(b);
          track.push_back(b);
        }
      }
 
      // Clustered clusters are taken out from "clusters" container
      for (auto& t : newNeighbours) {
        toJoin.remove(t);
      }
 
    } while (!newNeighbours.empty());
 
    return track;
}
  
 
std::pair<double, double> AFPSiDLinRegTool::linearRegression(const std::vector<std::pair<double, double>>& YX) const
{
    // here, "x" is Z axis and "y" is X or Y axis
    
    double meanx = 0, meany = 0;
    for (const auto& [y, x] : YX) {
        meany += y;
        meanx += x;
    }
    meanx /= YX.size();
    meany /= YX.size();
    
    double numerator = 0, denominator = 0;
    for (const auto& [y, x] : YX) {
        const double dy = y - meany;
        const double dx = x - meanx;
                
        numerator += dx * dy;
        denominator += dx * dx;
    }
    if (denominator == 0){
      ATH_MSG_WARNING("AFPSiDLinRegTool::linearRegression: denominator is zero");
      return {0.,0.};
    }
    const double slope = numerator / denominator;
    const double position = meany - slope * meanx;
 
    return {position, slope};
}
 
 
StatusCode AFPSiDLinRegTool::reconstructTracks(std::unique_ptr<xAOD::AFPTrackContainer>& outputContainer, const EventContext& ctx) const
{
    SG::ReadHandle<xAOD::AFPSiHitsClusterContainer> clusters( m_hitsClusterContainerKey, ctx );
    if(!clusters.isValid())
    {
        // this is allowed, there might be no AFP data in the input
        return StatusCode::SUCCESS;
    }
    
    // Select clusters in given station
    std::list<const xAOD::AFPSiHitsCluster*> clustersInStation;
    std::copy_if(clusters->begin(), clusters->end(), std::back_inserter(clustersInStation),
                            [this](auto cluster) { return cluster->stationID() == m_stationID; });
 
    ATH_MSG_DEBUG("clusters in " << m_stationID << " size: " << clustersInStation.size());
 
 
    // Loop until all clusters are used
    while (!clustersInStation.empty())
    {
        // Take first element from the list...
        const auto *const initialCluster = clustersInStation.front();
        clustersInStation.pop_front();
 
        // ...and find other clusters around it
        const auto clusterOfClusters = findClusterAroundElement(clustersInStation, initialCluster);
 
        // If track candidate does not pass chosen selection, skip it
        if (!m_selectionFunction(clusterOfClusters)) {
            ATH_MSG_DEBUG("track candidate of size " << clusterOfClusters.size() << " did not pass selection.");
            continue;
        }
        
        auto *track = outputContainer->push_back(std::make_unique<xAOD::AFPTrack>());
 
        // Set stationID
        track->setStationID(m_stationID);
 
        // Add links to clusters
        for (const auto *const cluster : clusterOfClusters) {
            ElementLink<xAOD::AFPSiHitsClusterContainer> clusterLink;
            clusterLink.toContainedElement(*clusters, cluster);
            track->addCluster(clusterLink);
        }
 
        const int nClusters = track->clusters().size();
        
        if(nClusters>0)
        {
            track->setNClusters(nClusters);
        }
        else
        {
            ATH_MSG_DEBUG("track candidate has 0 clusters, erasing ");
            outputContainer->pop_back();
            continue;
        }
        
        // Fit the clusters with a line in X and Y direction along Z
        double x = 0.;
        double y = 0.;
        double xSlope = 0.;
        double ySlope = 0.;
        double chi2  = 0.;
 
        if (nClusters == 1) {
            x = (*(track->clusters().at(0)))->xLocal();
            y = (*(track->clusters().at(0)))->yLocal();
        } 
        else
        {           
            std::vector<std::pair<double, double>> XZ;
            std::vector<std::pair<double, double>> YZ;
 
            for (int i = 0; i < nClusters; i++) {
                const xAOD::AFPSiHitsCluster* cluster = *(track->clusters().at(i));
                                
                XZ.emplace_back(cluster->xLocal(), cluster->zLocal());
                YZ.emplace_back(cluster->yLocal(), cluster->zLocal());
            }
 
            auto linregx = linearRegression(XZ);
            auto linregy = linearRegression(YZ);
 
            std::tie(x, xSlope) = linregx;
            std::tie(y, ySlope) = linregy;
 
            for (int i = 0; i < nClusters; i++) {
                const xAOD::AFPSiHitsCluster* cluster = *(track->clusters().at(i));
 
                const double z  = cluster->zLocal();
                const double ex = cluster->xLocalErr();
                const double ey = cluster->yLocalErr();
 
                const double dx = cluster->xLocal() - (x + xSlope * z);
                const double dy = cluster->yLocal() - (y + ySlope * z);
                                
                chi2 += dx * dx / (ex * ex);
                chi2 += dy * dy / (ey * ey);
            }
        }
 
        // Set track properties
        track->setChi2(chi2);
        track->setXLocal(x);
        track->setXSlope(xSlope);
        track->setYLocal(y);
        track->setYSlope(ySlope);
        track->setZLocal(0.); // because positions of x/y are calculated at z=0
        
        track->setAlgID(xAOD::AFPTrackRecoAlgID::linReg);
        
        std::vector<int> clustersPlane{0, 0, 0, 0};
        for (const auto& cl : track->clusters()) {
            ++clustersPlane[(*cl)->pixelLayerID()];
        }
        auto HasNoClusters = [](int n) { return n == 0; };
        track->setNHoles(std::count_if(clustersPlane.begin(), clustersPlane.end(), HasNoClusters));
        
 
        ATH_MSG_DEBUG("new track with " << track->clusters().size() << " clusters, x: " << track->xLocal()
                        << ", y: " << track->yLocal() << ", sx: " << track->xSlope()
                        << ", sy: " << track->ySlope() << ", chi2: " << track->chi2());
        for (auto&& cluster : track->clusters())
        {
            ATH_MSG_DEBUG('\t' << (*cluster)->xLocal() << " " << (*cluster)->yLocal() << " " << (*cluster)->zLocal());
        }
    }
    
    return StatusCode::SUCCESS;
}