VertexIterativeFitMergingTool.cxx

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/*
  Copyright (C) 2002-2026 CERN for the benefit of the ATLAS collaboration
*/
 
// Local
#include "LeptonTaggers/VertexIterativeFitMergingTool.h"
#include "LeptonTaggers/VarHolder.h"
#include "LeptonTaggers/PromptUtils.h"
 
// C/C++
#include <cmath>
#include <iostream>
#include <sstream>
 
//=============================================================================
Prompt::VertexIterativeFitMergingTool::VertexIterativeFitMergingTool(
  const std::string &name,
  const std::string &type,
  const IInterface  *parent
):
  AthAlgTool       (name, type, parent)
{
  declareInterface<Prompt::IVertexMergingTool>(this);
}
 
//=============================================================================
StatusCode Prompt::VertexIterativeFitMergingTool::initialize()
{
  ATH_CHECK(m_vertexFitterTool.retrieve());
  ATH_CHECK(m_histSvc.retrieve());
 
  ATH_CHECK(makeHist(m_histNvtx2TrkInit,     "nvtx_2trk_init",                   25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histNvtx2TrkPass,     "nvtx_2trk_pass",                   25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histNvtx2TrkUnmerged, "nvtx_2trk_unmerged",               25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histNvtxMerged,        "nvtx_merged",                      25, -0.5, 24.5));
 
  ATH_CHECK(makeHist(m_histNewVtxFitChi2,             "newVtxFit_chi2",                   100, 0.0, 50.0));
  ATH_CHECK(makeHist(m_histNewVtxFitProb,             "newVtxFit_prob",                   100, 0.0,  1.0));
 
  ATH_CHECK(makeHist(m_histNewVtxFitDistToCurr,      "newVtxFit_dist_toCurr",            100, 0.0, 10.0));
  ATH_CHECK(makeHist(m_histNewVtxFitDistToSeed,      "newVtxFit_dist_toSeed",            100, 0.0, 10.0));
  ATH_CHECK(makeHist(m_histNewVtxFitDistToSeedPass, "newVtxFit_dist_toSeed_pass",       100, 0.0, 10.0));
  ATH_CHECK(makeHist(m_histNewVtxFitDistToSeedFail, "newVtxFit_dist_toSeed_fail",       100, 0.0, 10.0));
 
  ATH_CHECK(makeHist(m_histNewVtxFitProbCandOverSeed,           "newVtxFit_prob_candOverSeed",           100, 0.0,  4.0));
  ATH_CHECK(makeHist(m_histNewVtxFitProbCandOverSeedPass,      "newVtxFit_prob_candOverSeed_pass",      100, 0.0,  4.0));
  ATH_CHECK(makeHist(m_histNewVtxFitProbCandOverSeedFail,      "newVtxFit_prob_candOverSeed_fail",      100, 0.0,  4.0));
  ATH_CHECK(makeHist(m_histNewVtxFitProbCandOverSeed3Trk,      "newVtxFit_prob_candOverSeed_3trk",      100, 0.0,  4.0));
  ATH_CHECK(makeHist(m_histNewVtxFitProbCandOverSeed3TrkPass, "newVtxFit_prob_candOverSeed_3trk_pass", 100, 0.0,  4.0));
 
  ATH_CHECK(makeHist(m_histVtx2TrkPairDist,      "Vtx2trkPair_dist",      100, 0.0, 100.0));
  ATH_CHECK(makeHist(m_histVtx2trkPairDistZoom, "Vtx2trkPair_dist_zoom", 100, 0.0,  10.0));
  ATH_CHECK(makeHist(m_histVtx2TrkPairSig1,      "Vtx2trkPair_sig1",      100, 0.0,  20.0));
  ATH_CHECK(makeHist(m_histVtx2TrkPairSig2,      "Vtx2trkPair_sig2",      100, 0.0,  20.0));
 
  ATH_CHECK(makeHist(m_histSelectedTrackCountAll,         "selectedTrack_CountAll",         25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histSelectedTrackCountMatch2Vtx ,  "selectedTrack_CountMatch2Vtx",   25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histSelectedTrackCountWithout2Vtx, "selectedTrack_CountWithout2Vtx", 25, -0.5, 24.5));
 
  ATH_CHECK(makeHist(m_histVtxWithoutLepton2TrkNTrack,        "vtxWithoutLepton2trk_NTrack",         25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histVtxWithoutLepton2TrkNPass,         "vtxWithoutLepton2trk_NPass",          25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histVtxWithoutLepton2TrkNPassUnmerged, "vtxWithoutLepton2trk_NPassUnmerged",  25, -0.5, 24.5));
  ATH_CHECK(makeHist(m_histVtxWithoutLepton2TrkNMerged,       "vtxWithoutLepton2trk_NMerged",        25, -0.5, 24.5));
 
  return StatusCode::SUCCESS;
}
 
Prompt::MergeResultNotOwner Prompt::VertexIterativeFitMergingTool::mergeInitVertices(
  const EventContext& ctx,
  const FittingInput &input,
  const xAOD::TrackParticle *tracklep,
  std::vector<std::unique_ptr<xAOD::Vertex>> &initVtxs,
  const std::vector<const xAOD::TrackParticle *> &selectedTracks
)
{
  //
  // Merge initial (2-track) vertices into new merged vertices with three tracks or more
  //
 
  ATH_MSG_DEBUG("===========================================================================" << std::endl
             << name() << "::mergeInitVertices - start processing");
 
 
  /*
    We store the initial vertices that pass selection in their own list
    with ownership, as this needs to be passed later to the MergeResults.
  */
  MergeResultNotOwner result;
  std::vector<std::unique_ptr<xAOD::Vertex>> vtxsInitPassed;
 
  for(std::unique_ptr<xAOD::Vertex> &vtx: initVtxs) {
    if(passVertexSelection(vtx.get())) {
      ATH_MSG_DEBUG("pushing vertex into result.vtxInitPassed");
      result.vtxsInitPassed.push_back(vtx.get());
      vtxsInitPassed.push_back(std::move(vtx));
    }
  }
 
  const unsigned nvtxInit = initVtxs.size();
  const unsigned nvtxPass = result.vtxsInitPassed.size();
 
  fillTH1(m_histNvtx2TrkInit, nvtxInit);
 
  //
  // Find tracks that do not form 2-track vertex with lepton track
  //
  ATH_MSG_DEBUG(name() << "::mergeInitVertices - processes vertexes without lepton");
 
  std::vector<const xAOD::TrackParticle *> tracksWithoutVertex = getTracksWithoutVertex(
    result.vtxsInitPassed, selectedTracks
  );
 
  // Rank the tracks by pT, will keep the top `m_maxExtraTracks` tracks
  // for fitting vertexes without lepton
  if(tracksWithoutVertex.size() > m_maxExtraTracks) {
    ATH_MSG_DEBUG(" number of tracks without good lepton+track vertex: " << tracksWithoutVertex.size() << std::endl
        << " will only keep the top " << m_maxExtraTracks << " tracks");
 
    std::sort(tracksWithoutVertex.begin(), tracksWithoutVertex.end(), Prompt::SortByIDTrackPt());
 
    tracksWithoutVertex.erase(tracksWithoutVertex.begin() + m_maxExtraTracks, tracksWithoutVertex.end());
  }
 
 
  MergeResultNotOwner resultExtra;
  std::vector<std::unique_ptr<xAOD::Vertex>> twoTrackVtxInit = fit2TrackVertexes(ctx, input, tracksWithoutVertex, Prompt::kTwoTrackVtxWithoutLepton);
 
  ATH_MSG_DEBUG(
    name() << "::mergeInitVertices - will merge vertexes without lepton" << std::endl
    << "   number of selected tracks without good lepton+track vertex: "
    << tracksWithoutVertex        .size() << std::endl
    << "   number of 2-track vertexes without lepton:                  "
    << resultExtra.vtxsInitPassed.size()
  );
 
  //
  // Merge 2-track vertex without lepton only when there are two or more vertices
  //
  mergeIteratively2TrackVtxs(ctx, input, twoTrackVtxInit, resultExtra, Prompt::kIterativeFitVtxWithoutLepton);
 
  if(resultExtra.vtxsInitPassed.size() > 1) {
    //
    // Fill histograms only when there are two or more vertices
    //
    fillTH1(m_histVtxWithoutLepton2TrkNTrack, tracksWithoutVertex.size());
    fillTH1(m_histVtxWithoutLepton2TrkNPass, resultExtra.vtxsInitPassed.size());
    fillTH1(m_histVtxWithoutLepton2TrkNPassUnmerged, resultExtra.vtxsInitPassedNotMerged.size());
    fillTH1(m_histVtxWithoutLepton2TrkNMerged, resultExtra.vtxsNewMerged.size());
  }
 
  ATH_MSG_DEBUG(name() << "::mergeInitVertices - finished merging vertexes without lepton" << std::endl
      << "   number of tracks without good lepton+track vertex:  " << tracksWithoutVertex.size() << std::endl
      << "   number of          2-track vertexes without lepton: " << resultExtra.vtxsInitPassed.size() << std::endl
      << "   number of unmerged 2-track vertexes without lepton: " << resultExtra.vtxsInitPassedNotMerged.size() << std::endl
      << "   number of merged  vertexes without lepton:          " << resultExtra.vtxsNewMerged.size());
 
  //
  // Next, processes 2-track vertexes that contain lepton track
  //
  ATH_MSG_DEBUG("===========================================================================" << std::endl
      << name() << "::mergeInitVertices - process 2-track vertexes with lepton" << std::endl
      << "   lepton track pT="                      << tracklep->pt()                 << std::endl
      << "   number of initial  2-track vertices: " << initVtxs.size()               << std::endl
      << "   number of selected 2-track vertices: " << result.vtxsInitPassed.size() << std::endl
      << "   number of selected ID tracks:        " << selectedTracks.size() );
 
  //
  // Merge 2-track vertexes that contain lepton track
  //
  mergeIteratively2TrackVtxs(ctx, input, vtxsInitPassed, result, Prompt::kIterativeFitVtx);
 
  if(result.vtxsInitPassed.size() > 1) {
    //
    // Fill histograms only when there are at least two vertexes to merge
    //
    fillTH1(m_histNvtx2TrkPass,     nvtxPass);
    fillTH1(m_histNvtx2TrkUnmerged, result.vtxsInitPassedNotMerged.size());
    fillTH1(m_histNvtxMerged,        result.vtxsNewMerged            .size());
  }
 
  //
  // Erase vector of 2-track vertices without lepton that were merged
  //
  resultExtra.vtxsInitPassed.clear();
 
  //
  // Add vertices without lepton track to result
  //
  result.vtxsInitPassedNotMerged.insert(
    result.vtxsInitPassedNotMerged.end(),
    std::make_move_iterator(resultExtra.vtxsInitPassedNotMerged.begin()),
    std::make_move_iterator(resultExtra.vtxsInitPassedNotMerged.end())
  );
 
  result.vtxsNewMerged.insert(
    result.vtxsNewMerged.end(),
    std::make_move_iterator(resultExtra.vtxsNewMerged.begin()),
    std::make_move_iterator(resultExtra.vtxsNewMerged.end())
  );
 
  ATH_MSG_DEBUG("==========================================" << std::endl
    << "mergeInitVertices report" << std::endl
    << " \tnumber of initial  2-track vertices: "
    << initVtxs.size() << std::endl
    << "\tnumber of passed   2-track vertices: "
    << result.vtxsInitPassed.size() << std::endl
    << "\tnumber of unmerged 2-track vertices: "
    << result.vtxsInitPassedNotMerged.size() << std::endl
    << "\tnumber of merged           vertices: "
    << result.vtxsNewMerged.size() << std::endl
    << std::endl
    << "\tnumber of tracks without good lepton+track vertex:  "
    << tracksWithoutVertex.size() << std::endl
    << "\tnumber of          2-track vertexes without lepton: "
    << resultExtra.vtxsInitPassed.size() << std::endl
    << "\tnumber of unmerged 2-track vertexes without lepton: "
    << resultExtra.vtxsInitPassedNotMerged.size() << std::endl
    << "\tnumber of merged  vertexes without lepton:          "
    << resultExtra.vtxsNewMerged.size() << std::endl
    << name() << "::mergeInitVertices - ALL DONE" << std::endl
    << "=========================================="
  );
 
 
  return result;
}
 
bool Prompt::VertexIterativeFitMergingTool::mergeIteratively2TrackVtxs(
  const EventContext& ctx,
  const FittingInput &input,
  std::vector<std::unique_ptr<xAOD::Vertex>> &initVtxs,
  MergeResultNotOwner &result,
  const VtxType vtxType
)
{
  /*
     This function merges iterively vertexes with this algorithm:
     o) Sort 2-track vertexes by sum of track pT
     o) Select the vertex with the highest sum of track pT as the seed vertex
     - Sort all other vertexes by the distance to the seed vertex
     - Add tracks from the closest vertex to the selected vertex
     - Fit new vertex:
     -- if the new vertex passes cuts, select as the new seed vertex
     -- if the new vertex fails cuts, continue with the original seed vertex
     -- Remove this closest vertex from the list
     - Resort remaining tracks by the distance to the seed vertex and repeat
     o) Remove the 2-track vertexes that were merged from the global list and repeat
     */
 
  // Only 0 or 1 2-track vertices - add the vertex to not merged list and return
  if(result.vtxsInitPassed.size() < 2) {
    result.vtxsInitPassedNotMerged = std::move(initVtxs);
 
    ATH_MSG_DEBUG(name() << "::mergeIteratively2TrackVtxs - too few vertexes: nothing more to do");
 
    return false;
  }
 
  //
  // Make 2-track vertex data structures
  //
  std::vector<TwoTrackVtx> vtxs2Track;
 
  for(std::unique_ptr<xAOD::Vertex> &vtx: initVtxs) {
    if(vtx->nTrackParticles() != 2) {
      ATH_MSG_WARNING("mergeIteratively2TrackVtxs - wrong number of tracks: " << vtx->nTrackParticles());
      continue;
    }
 
    if(vtx->nTrackParticles() !=2 ) {
      ATH_MSG_WARNING("mergeIteratively2TrackVtxs - vertex does not contain 2 TrackParticles: ntrack=" << vtx->nTrackParticles());
      continue;
    }
 
    TwoTrackVtx vtx2track;
    vtx2track.trackId0 = vtx->trackParticle(0);
    vtx2track.trackId1 = vtx->trackParticle(1);
 
    if(!vtx2track.trackId0 || !vtx2track.trackId1) {
      ATH_MSG_WARNING("mergeIteratively2TrackVtxs - failed to find TrackParticles for 2-track vertex");
      continue;
    }
 
    vtx2track.vertex        = vtx.get();
    vtx2track.vertexFitProb = Prompt::getVertexFitProb(vtx.get());
    vtx2track.sumTrackPt    = vtx2track.trackId0->pt() + vtx2track.trackId1->pt();
 
    vtxs2Track.push_back(vtx2track);
  }
 
  ATH_MSG_DEBUG(name() << "::mergeIteratively2TrackVtxs - start processing with " << vtxs2Track.size() << " input vertexes ");
 
  if(vtxs2Track.size() < 2) {
    ATH_MSG_WARNING("mergeIteratively2TrackVtxs - logic error: found only " << vtxs2Track.size() << " 2-track vertex");
    return false;
  }
 
  //
  // Sort 2-track vertexes by ID track pT
  //
  std::sort(vtxs2Track.begin(), vtxs2Track.end(), Prompt::SortTwoTrackVtxBySumTrackPt());
 
  ATH_MSG_DEBUG(name() << "::mergeIteratively2TrackVtxs - number of 2 track passed vertexes=" << vtxs2Track.size());
 
  for(const TwoTrackVtx &vtx: vtxs2Track) {
    ATH_MSG_DEBUG("Input vertex with 2 tracks sum pT=" << vtx.sumTrackPt << "\n    " << vtxAsStr(vtx.vertex, true));
  }
 
  //
  // Plot distances between all unique pairs of 2-track vertexes
  //
  plotVertexDistances(vtxs2Track);
 
  //
  // Iterative fit vertices for merging:
  //
  std::vector<TwoTrackVtx>::iterator currVit = vtxs2Track.begin();
 
  /*
    Seed new vertex with 2-track vertex containing
    highest pT ID track (non-lepton track)
  */
  while(currVit != vtxs2Track.end()) {
    xAOD::Vertex* seedVtx = currVit->vertex;
 
    /*
      We potentially generate a new vertex here,
      so it has to be a unique_ptr. If the vertex could not be merged,
      then the newMergedVtx will remain a nullptr.
    */
    std::unique_ptr<xAOD::Vertex> newMergedVtx = nullptr;
    getNewMergedVertex(
      ctx, seedVtx, newMergedVtx, input, currVit, vtxs2Track, vtxType
    );
 
    // Check to see if the new merged vertex is not a nullptr
    // (i.e., if the vertex could be merged)
    if(newMergedVtx) {
      // Remove 2-track vertexes that were merged
      removeMerged2TrackVertexes(newMergedVtx.get(), vtxs2Track);
 
      // Reset current vertex iterator to beginning
      currVit = vtxs2Track.begin();
 
      ATH_MSG_DEBUG(name() << "::mergeIteratively2TrackVtxs - new merged vertex:\n" << vtxAsStr(newMergedVtx.get(), false));
 
      // Save new merged vertex (it is new because it is distinct from seed vertex)
      result.vtxsNewMerged.push_back(std::move(newMergedVtx));
    }
    else {
      //
      // This vertex could not be merged - try next one
      //
      ++currVit;
 
      ATH_MSG_DEBUG(name() << "::mergeIteratively2TrackVtxs - could not merge 2-track vertex:\n" << vtxAsStr(seedVtx, false));
    }
  }
 
  //
  // Record unmerged two-track vertexes
  //
  for(std::unique_ptr<xAOD::Vertex> &vtxUniquePtr: initVtxs) {
    for(TwoTrackVtx &vtx: vtxs2Track) {
      if (vtxUniquePtr.get() == vtx.vertex){
        result.vtxsInitPassedNotMerged.push_back(std::move(vtxUniquePtr));
 
        ATH_MSG_DEBUG("Unmerged " << vtxAsStr(vtx.vertex, true));
      }
    }
  }
 
  ATH_MSG_DEBUG(name() << "::mergeIteratively2TrackVtxs - finished processing:" << std::endl
  << "   number of unmerged 2-track vertexes=" << vtxs2Track           .size() << std::endl
  << "   number of merged           vertexes=" << result.vtxsNewMerged.size() );
 
  for(TwoTrackVtx &vtx: vtxs2Track) {
    ATH_MSG_DEBUG("Unmerged " << vtxAsStr(vtx.vertex, true));
  }
 
  for(const std::unique_ptr<xAOD::Vertex>& vtx: result.vtxsNewMerged) {
    ATH_MSG_DEBUG("Merged " << vtxAsStr(vtx.get(), true));
  }
 
  return true;
}
 
void Prompt::VertexIterativeFitMergingTool::getNewMergedVertex(
  const EventContext& ctx,
  xAOD::Vertex* seedVtx,
  std::unique_ptr<xAOD::Vertex> &newMergedVtx,
  const FittingInput &input,
  std::vector<TwoTrackVtx>::iterator &currVit,
  std::vector<TwoTrackVtx> &vtxs2Track,
  const VtxType vtxType
) {
  // Generate a list of 2-track vertices other than the seed vertex
  std::vector<TwoTrackVtx> others;
 
  for(std::vector<TwoTrackVtx>::iterator vit = vtxs2Track.begin(); vit != vtxs2Track.end(); ++vit) {
    if(vit != currVit) {
      others.push_back(*vit);
    }
  }
 
  // Sort other vertices by distance to the seed vertex
  std::sort(others.begin(), others.end(), Prompt::SortTwoTrackVtxByDistToSeed(seedVtx));
 
  // Call recursive function to fit seed+closest vertex pairs
  xAOD::Vertex* mergedVtx = fitSeedVertexCluster(
    ctx, input, seedVtx, vtxType, others
  );
 
  /*
    If the returned vertex is the seed vertex, that means that the
    vertex could not be merged. We do not want to return it because
    then we would assign two unique_ptrs to the same underlying
    vertex object. Instead, we leave newMergedVtx as a nullptr
 
    If they are different, then we do in fact have a new vertex,
    and we are safe to encapsulate it in a unique_ptr.
  */
  if (mergedVtx == seedVtx){
    newMergedVtx.reset(nullptr);
  } else {
    newMergedVtx.reset(mergedVtx);
  }
}
 
//=============================================================================
xAOD::Vertex* Prompt::VertexIterativeFitMergingTool::fitSeedVertexCluster(
  const EventContext& ctx,
  const FittingInput &input,
  xAOD::Vertex* seedVtx,
  const VtxType vtxType,
  std::vector<TwoTrackVtx> &others
)
{
  //
  // Reached end of the recursive loop
  //
  if(others.empty()) {
    return seedVtx;
  }
 
  //
  // Re-sort other vertices by distance to the seed vertex - needed because seed position changes when vertices are merged
  //
  std::sort(others.begin(), others.end(), Prompt::SortTwoTrackVtxByDistToSeed(seedVtx));
 
  // Take closest vertex
  xAOD::Vertex* currVtx = others.front().vertex;
 
  if(!currVtx) {
    ATH_MSG_WARNING("VertexIterativeFitMergingTool::fitSeedVertexCluster - current vertex is null pointer");
    return seedVtx;
  }
 
  //
  // Remove closest vertex from the list
  //
  others.erase(others.begin());
 
  /*
    Found nearby vertex - fit merged vertex
  */
  std::unique_ptr<xAOD::Vertex> candVtx =fitSeedPlusOtherVertex(
    ctx, input, seedVtx, currVtx, vtxType
  ); 
 
  if(!candVtx) {
    //
    // Failed to fit new vertex - continue with current seed vertex
    //
 
    ATH_MSG_DEBUG("fitSeedVertexCluster - NEW MERGED VERTEX FIT FAILED" << std::endl
          << "---------------------------------------------------------------------------");
 
    return fitSeedVertexCluster(ctx, input, std::move(seedVtx), vtxType, others);
  }
 
  const double probCand = getVertexFitProb(candVtx.get());
  const double probSeed = getVertexFitProb(seedVtx);
 
  double probCandOverSeed = -1.0;
 
  if(probSeed > 0.0) {
    probCandOverSeed = probCand/probSeed;
  }
 
  const double distToSeed = getDistance(seedVtx, candVtx.get());
  const double distToCurr = getDistance(currVtx, candVtx.get());
 
  fillTH1(m_histNewVtxFitChi2, candVtx->chiSquared());
  fillTH1(m_histNewVtxFitProb, probCand);
 
  fillTH1(m_histNewVtxFitDistToSeed,       distToSeed);
  fillTH1(m_histNewVtxFitDistToCurr,       distToCurr);
  fillTH1(m_histNewVtxFitProbCandOverSeed, probCandOverSeed);
 
  if(seedVtx->nTrackParticles() > 2) {
    fillTH1(m_histNewVtxFitProbCandOverSeed3Trk, probCandOverSeed);
  }
 
  std::stringstream str;
 
 
    str << "   dist to seed=" << distToSeed << ", probCandOverSeed=" << probCandOverSeed << std::endl
  << "   seed: "        << vtxAsStr(seedVtx, false) << std::endl
  << "   curr: "        << vtxAsStr(currVtx, true)
  << "   cand: "        << vtxAsStr(candVtx.get(), true)
  << "fitSeedVertexCluster - finished" << std::endl
  << "---------------------------------------------------------------------------" << std::endl;
 
 
 
  if(!(passVertexSelection(candVtx.get()) && probCandOverSeed > m_minCandOverSeedFitProbRatio)) {
    //
    // New fitted merged vertex failed selection
    //
 
    ATH_MSG_DEBUG("fitSeedVertexCluster - FAIL NEW MERGED VERTEX\n" << str.str());
 
 
    fillTH1(m_histNewVtxFitDistToSeedFail,       distToSeed);
    fillTH1(m_histNewVtxFitProbCandOverSeedFail, probCandOverSeed);
 
    //
    // Continue with current seed vertex
    //
    return fitSeedVertexCluster(ctx, input, seedVtx, vtxType, others);
  }
 
  fillTH1(m_histNewVtxFitDistToSeedPass,       distToSeed);
  fillTH1(m_histNewVtxFitProbCandOverSeedPass, probCandOverSeed);
 
  if(seedVtx->nTrackParticles() > 2) {
    fillTH1(m_histNewVtxFitProbCandOverSeed3TrkPass, probCand/probSeed);
  }
 
  //
  // Succesfully fitted new vertex
  //
 
  ATH_MSG_DEBUG("fitSeedVertexCluster - PASS NEW MERGED VERTEX" << str.str());
 
 
  return fitSeedVertexCluster(ctx, input, candVtx, vtxType, others);
}
 
// this signature is only called recursively, if a merged vertex candidate has been
// found.  Ensures that we release the final merged candidate to hand it 
// back to the original caller. 
xAOD::Vertex* Prompt::VertexIterativeFitMergingTool::fitSeedVertexCluster(
  const EventContext& ctx,
  const FittingInput &input,
      std::unique_ptr<xAOD::Vertex> & seedVtx,
  const VtxType vtxType,
  std::vector<TwoTrackVtx> &others
){
  // remember the seed vertex before the call 
  xAOD::Vertex* originalSeed = seedVtx.get(); 
  // call the original signature 
  xAOD::Vertex* iterationResult = fitSeedVertexCluster(ctx, input,seedVtx.get(), vtxType,others);
  // if the iteration has finished (it is returning the seed), release the seed vertex. 
  // This will be re-captured by the top level caller after exiting the recursion stack.
  if (iterationResult == originalSeed) return seedVtx.release();
  // otherwise, return the iteration result. The seed vertex will be deleted 
  // when the call stack unwinds.  
  else return iterationResult; 
}
 
unsigned Prompt::VertexIterativeFitMergingTool::removeMerged2TrackVertexes(
  const xAOD::Vertex *mergedVtx,
  std::vector<TwoTrackVtx> &vtxs
) const {
  if(!mergedVtx) {
    ATH_MSG_WARNING("VertexIterativeFitMergingTool::removeMerged2TrackVertexes - merged vertex is null pointer");
    return 0;
  }
 
  unsigned icount = 0;
  std::vector<TwoTrackVtx>::iterator vit = vtxs.begin();
 
  while(vit != vtxs.end()) {
    int iCountMatchedTrack = 0;
 
    for(unsigned k = 0; k < mergedVtx->nTrackParticles(); ++k) {
      const xAOD::TrackParticle *track = mergedVtx->trackParticle(k);
 
      if(!track) {
        ATH_MSG_WARNING("removeMerged2TrackVertexes - merged vertex contains null TrackParticle pointer");
        continue;
      }
 
      if(vit->trackId0 == track) { iCountMatchedTrack++; }
      if(vit->trackId1 == track) { iCountMatchedTrack++; }
    }
 
    if(iCountMatchedTrack == 2) {
      //
      // Found 2-track vertex that was merged - remove this vertex
      //
      vit = vtxs.erase(vit);
      icount++;
 
      ATH_MSG_DEBUG("removeMerged2TrackVertexes - removed merged 2-track vertex");
    }
    else {
      ++vit;
 
      ATH_MSG_DEBUG("removeMerged2TrackVertexes - skip unmerged 2-track vertex");
    }
  }
 
  ATH_MSG_DEBUG(
    name() << "::removeMerged2TrackVertexes - merged vertex ntrack=" << mergedVtx->nTrackParticles()
    << ", removed " << icount << " merged 2-track vertexes"
  );
 
  return icount;
}
 
//=============================================================================
void Prompt::VertexIterativeFitMergingTool::plotVertexDistances(
  const std::vector<TwoTrackVtx> &others
) {
  for(std::vector<TwoTrackVtx>::const_iterator fit = others.begin(); fit != others.end(); ++fit) {
    for(std::vector<TwoTrackVtx>::const_iterator sit = fit+1; sit != others.end(); ++sit) {
      const double dist = Prompt::getDistance(fit->vertex, sit->vertex);
      const double sig1 = Prompt::getNormDist(fit->vertex->position(), sit->vertex->position(), fit->vertex->covariance(), msg(MSG::WARNING));
      const double sig2 = Prompt::getNormDist(fit->vertex->position(), sit->vertex->position(), sit->vertex->covariance(), msg(MSG::WARNING));
 
      fillTH1(m_histVtx2TrkPairDist,     dist);
      fillTH1(m_histVtx2trkPairDistZoom, dist);
      fillTH1(m_histVtx2TrkPairSig1,     sig1);
      fillTH1(m_histVtx2TrkPairSig2,     sig2);
    }
  }
}
 
//=============================================================================
std::vector<const xAOD::TrackParticle *> Prompt::VertexIterativeFitMergingTool::getTracksWithoutVertex(
  const std::vector<xAOD::Vertex*> &passVtxs,
  const std::vector<const xAOD::TrackParticle *> &selectedTracks
)
{
  //
  // Plot unmatched tracks
  //
  std::vector<const xAOD::TrackParticle *> tracksWithoutVertex;
 
  unsigned iCountDoMatch = 0;
 
  for(const xAOD::TrackParticle *track: selectedTracks) {
    bool match = false;
 
    for(const xAOD::Vertex *vtx: passVtxs) {
      for(unsigned k = 0; k < vtx->nTrackParticles(); ++k) {
        const xAOD::TrackParticle *vtxTrack = vtx->trackParticle(k);
 
        if(vtxTrack == track) {
          match = true;
          break;
        }
      }
    }
 
    if(match) {
      iCountDoMatch++;
    }
    else {
      tracksWithoutVertex.push_back(track);
    }
  }
 
  fillTH1(m_histSelectedTrackCountAll,         selectedTracks.size());
  fillTH1(m_histSelectedTrackCountMatch2Vtx,   iCountDoMatch);
  fillTH1(m_histSelectedTrackCountWithout2Vtx, tracksWithoutVertex.size());
 
  return tracksWithoutVertex;
}
 
//=============================================================================
bool Prompt::VertexIterativeFitMergingTool::passVertexSelection(const xAOD::Vertex *vtx) const
{
  //
  // Check whether vertex passes quality cuts
  //
  if(!vtx) {
    ATH_MSG_WARNING("passVertexSelection - input vertex is null pointer");
    return false;
  }
 
  if(!(vtx->numberDoF() > 0 && vtx->chiSquared() >= 0)) {
    return false;
  }
 
  const double fitProb = Prompt::getVertexFitProb(vtx);
 
  ATH_MSG_DEBUG("passVertexSelection - vertex pointer=" << vtx << " chi2/ndof=" << vtx->chiSquared() << "/" << vtx->numberDoF() << ", prob=" << fitProb);
 
  return fitProb > m_minFitProb;
}
 
//=============================================================================
std::unique_ptr<xAOD::Vertex> Prompt::VertexIterativeFitMergingTool::fitSeedPlusOtherVertex(
  const EventContext& ctx,
  const FittingInput &input,
  const xAOD::Vertex *seedVtx,
  const xAOD::Vertex *otherVtx,
  const VtxType vtxType
)
{
  //
  // Fit two 2-track vertexes
  //
  if(!seedVtx) {
    ATH_MSG_WARNING("fitSeedPlusOtherVertex - null seed Vertex pointer");
    return 0;
  }
 
  if(!otherVtx) {
    ATH_MSG_WARNING("fitSeedPlusOtherVertex - null other Vertex pointer");
    return 0;
  }
 
  if(otherVtx->nTrackParticles() != 2) {
    ATH_MSG_WARNING("fitSeedPlusOtherVertex - other Vertex does not have 2 tracks: ntrack=" << otherVtx->nTrackParticles());
    return 0;
  }
 
  //
  // Collect tracks from the seed vertex
  //
  std::vector<const xAOD::TrackParticle *> tracks;
 
  for(unsigned k = 0; k < seedVtx->nTrackParticles(); ++k) {
    const xAOD::TrackParticle *track = seedVtx->trackParticle(k);
 
    if(track) {
      tracks.push_back(track);
    }
    else {
      ATH_MSG_WARNING("fitSeedPlusOtherVertex - seed vertex contains TrackParticle null pointer");
    }
  }
 
  //
  // Collect tracks from other vertices
  //
  for(unsigned k = 0; k < otherVtx->nTrackParticles(); ++k) {
    const xAOD::TrackParticle *track = otherVtx->trackParticle(k);
 
    if(track) {
      tracks.push_back(track);
    }
    else {
      ATH_MSG_WARNING("fitSeedPlusOtherVertex - other vertex contains TrackParticle null pointer");
    }
  }
 
  //
  // Fit new vertex
  //
  std::unique_ptr<xAOD::Vertex> secVtx = m_vertexFitterTool->fitVertexWithSeed(
    ctx, input, tracks, seedVtx->position(), vtxType
  );
 
  if(!secVtx) {
    ATH_MSG_WARNING("fitSeedPlusOtherVertex - failed to fit vertex");
    return 0;
  }
 
  return secVtx;
}
 
std::vector<std::unique_ptr<xAOD::Vertex>> Prompt::VertexIterativeFitMergingTool::fit2TrackVertexes(
  const EventContext& ctx,
  const FittingInput &input,
  std::vector<const xAOD::TrackParticle *> &selectedTracks,
  const VtxType vtxType
)
{
  //
  // Fit all possible combinations of two 2-track vertexes
  //
  std::vector<std::unique_ptr<xAOD::Vertex>> passVtxs;
 
  if(selectedTracks.size() < 2) {
    ATH_MSG_DEBUG("fit2TrackVertexeses - 0 or 1 input tracks - nothing to do");
    return passVtxs;
  }
 
  ATH_MSG_DEBUG(name() << "::fit2TrackVertexes - start with " << selectedTracks.size() << " tracks");
 
  //
  // Sort tracks by decreasing pT
  //
  std::sort(selectedTracks.begin(), selectedTracks.end(), SortTracksByPt());
 
  unsigned icount = 0;
 
  for(std::vector<const xAOD::TrackParticle *>::const_iterator it1 = selectedTracks.begin(); it1 != selectedTracks.end(); ++it1) {
    for(std::vector<const xAOD::TrackParticle *>::const_iterator it2 = it1 + 1; it2 != selectedTracks.end(); ++it2) {
      const xAOD::TrackParticle *track1 = *it1;
      const xAOD::TrackParticle *track2 = *it2;
 
      if(!track1 || !track2) {
        ATH_MSG_WARNING("fit2TrackVertexeses - logic error: TrackParticle null pointer");
        continue;
      }
 
      std::vector<const xAOD::TrackParticle *> fit_tracks = {track1, track2};
 
      //
      // Fit new vertex
      //
      std::unique_ptr<xAOD::Vertex> vtx = m_vertexFitterTool->fitVertexWithPrimarySeed(
        ctx, input, fit_tracks, vtxType
      );
 
      icount++;
 
      if(!vtx) {
        ATH_MSG_WARNING("fit2TrackVertexeses - failed to fit vertex");
        continue;
      }
 
      if(passVertexSelection(vtx.get())) {
        ATH_MSG_DEBUG("fit2TrackVertexeses - pass vertex: " << vtxAsStr(vtx.get(), true));
        passVtxs.push_back(std::move(vtx));
      }
    }
  }
 
  ATH_MSG_DEBUG(name() << "::fit2TrackVertexes - finished processing: " << std::endl
        << "   number of input tracks:            " << selectedTracks.size() << std::endl
        << "   number of 2-track combinations:    " << icount                 << std::endl
        << "   number of passed 2-track vertexes: " << passVtxs      .size() << std::endl
        << name() << "::fit2TrackVertexes - all is done" );
 
  return passVtxs;
}
 
//=============================================================================
StatusCode Prompt::VertexIterativeFitMergingTool::makeHist(TH1 *&h, const std::string &key, int nbin, double xmin, double xmax)
{
  //
  // Initiliase histogram pointer. If configured to run in validation mode, then create and register histogram
  //
  h = 0;
 
  if(m_outputStream.empty() || key.empty()) {
    return StatusCode::SUCCESS;
  }
 
  const std::string hname    = name() + "_" + key;
  const std::string hist_key = "/"+m_outputStream+"/"+hname;
 
  h = new TH1D(hname.c_str(), hname.c_str(), nbin, xmin, xmax);
  h->SetDirectory(0);
 
  return m_histSvc->regHist(hist_key, h);
}