From d56055729552a5cc2496038bef86ae0461a1c003 Mon Sep 17 00:00:00 2001
From: Felix Schlepper <felix.schlepper@cern.ch>
Date: Tue, 24 Jun 2025 21:29:36 +0200
Subject: [PATCH] ITS: redefine seeding vertex label

Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>
---
 .../include/ITStracking/VertexerTraits.h      | 20 ++++++++++++-------
 .../ITS/tracking/src/VertexerTraits.cxx       | 18 ++++++++---------
 2 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/Detectors/ITSMFT/ITS/tracking/include/ITStracking/VertexerTraits.h b/Detectors/ITSMFT/ITS/tracking/include/ITStracking/VertexerTraits.h
index 6554e53fa2ee8..a842f04abfc62 100644
--- a/Detectors/ITSMFT/ITS/tracking/include/ITStracking/VertexerTraits.h
+++ b/Detectors/ITSMFT/ITS/tracking/include/ITStracking/VertexerTraits.h
@@ -101,19 +101,25 @@ class VertexerTraits
   virtual bool usesMemoryPool() const noexcept { return true; }
   void setMemoryPool(std::shared_ptr<BoundedMemoryResource>& pool) { mMemoryPool = pool; }
 
-  template <typename T = o2::MCCompLabel>
-  static std::pair<T, float> computeMain(const bounded_vector<T>& elements)
+  static std::pair<o2::MCCompLabel, float> computeMain(const bounded_vector<o2::MCCompLabel>& elements)
   {
-    T elem;
+    // we only care about the source&event of the tracks, not the trackId
+    auto composeVtxLabel = [](const o2::MCCompLabel& lbl) -> o2::MCCompLabel {
+      return {o2::MCCompLabel::maxTrackID(), lbl.getEventID(), lbl.getSourceID(), lbl.isFake()};
+    };
+    std::unordered_map<o2::MCCompLabel, size_t> frequency;
+    for (const auto& element : elements) {
+      ++frequency[composeVtxLabel(element)];
+    }
+    o2::MCCompLabel elem{};
     size_t maxCount = 0;
-    for (auto& element : elements) {
-      size_t count = std::count(elements.begin(), elements.end(), element);
+    for (const auto& [key, count] : frequency) {
       if (count > maxCount) {
         maxCount = count;
-        elem = element;
+        elem = key;
       }
     }
-    return std::make_pair(elem, static_cast<float>(maxCount) / elements.size());
+    return std::make_pair(elem, static_cast<float>(maxCount) / static_cast<float>(elements.size()));
   }
 
  protected:
diff --git a/Detectors/ITSMFT/ITS/tracking/src/VertexerTraits.cxx b/Detectors/ITSMFT/ITS/tracking/src/VertexerTraits.cxx
index 51cd98aa1366d..11304e17d234a 100644
--- a/Detectors/ITSMFT/ITS/tracking/src/VertexerTraits.cxx
+++ b/Detectors/ITSMFT/ITS/tracking/src/VertexerTraits.cxx
@@ -506,18 +506,18 @@ void VertexerTraits::computeVertices(const int iteration)
 
       if (beamDistance2 < nsigmaCut && o2::gpu::GPUCommonMath::Abs(mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[2]) < mVrtParams[iteration].maxZPositionAllowed) {
         atLeastOneFound = true;
-        vertices.emplace_back(o2::math_utils::Point3D<float>(mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[0],
-                                                             mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[1],
-                                                             mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[2]),
-                              mTimeFrame->getTrackletClusters(rofId)[iCluster].getRMS2(),          // Symm matrix. Diagonal: RMS2 components,
-                                                                                                   // off-diagonal: square mean of projections on planes.
-                              mTimeFrame->getTrackletClusters(rofId)[iCluster].getSize(),          // Contributors
-                              mTimeFrame->getTrackletClusters(rofId)[iCluster].getAvgDistance2()); // In place of chi2
+        auto& vertex = vertices.emplace_back(o2::math_utils::Point3D<float>(mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[0],
+                                                                            mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[1],
+                                                                            mTimeFrame->getTrackletClusters(rofId)[iCluster].getVertex()[2]),
+                                             mTimeFrame->getTrackletClusters(rofId)[iCluster].getRMS2(),          // Symm matrix. Diagonal: RMS2 components,
+                                                                                                                  // off-diagonal: square mean of projections on planes.
+                                             mTimeFrame->getTrackletClusters(rofId)[iCluster].getSize(),          // Contributors
+                                             mTimeFrame->getTrackletClusters(rofId)[iCluster].getAvgDistance2()); // In place of chi2
 
         if (iteration) {
-          vertices.back().setFlags(Vertex::UPCMode);
+          vertex.setFlags(Vertex::UPCMode);
         }
-        vertices.back().setTimeStamp(mTimeFrame->getTrackletClusters(rofId)[iCluster].getROF());
+        vertex.setTimeStamp(mTimeFrame->getTrackletClusters(rofId)[iCluster].getROF());
         if (mTimeFrame->hasMCinformation()) {
           bounded_vector<o2::MCCompLabel> labels(mMemoryPool.get());
           for (auto& index : mTimeFrame->getTrackletClusters(rofId)[iCluster].getLabels()) {