Move instead of copy when merging hits in the o2-sim hit merger

This removes a deep copy of every hit when the hit merger joins the sub-events of an event.

- mergeAndAdjustHits copied each hit into the merged container; it now reserves the total size and moves them.
- The track and track-reference merges now reserve their output.
- TPC HitGroup declared a defaulted destructor, which suppresses its implicit move; the line is removed so HitGroup moves its five vectors.
- The printf debug output in the merge loops is dropped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/Base/include/DetectorsBase/Detector.h

@@ -457,22 +457,25 @@ class DetImpl : public o2::base::Detector
   {
     auto entries = hitbuffervector.size();
 
-    auto targetdata = new T;  // used to collect data inside a single container
+    T targetdata;             // used to collect data inside a single container
     T* filladdress = nullptr; // pointer used for final ROOT IO
     if (entries == 1) {
       filladdress = hitbuffervector[0].get();
       // nothing to do; we can directly do IO from the existing buffer
     } else {
       // here we need to do merging and index adjustment
       int nprimTot = 0;
+      size_t nhits = 0;
       for (auto entry = 0; entry < entries; entry++) {
         nprimTot += nprimaries[entry];
+        nhits += hitbuffervector[entry] ? hitbuffervector[entry]->size() : 0;
       }
+      targetdata.reserve(nhits);
       // offset for pimary track index
       int idelta0 = 0;
       // offset for secondary track index
       int idelta1 = nprimTot;
-      filladdress = targetdata;
+      filladdress = &targetdata;
       for (int entry = entries - 1; entry >= 0; --entry) {
         // proceed in the order of subevent Ids
         int index = subevtsOrdered[entry];
@@ -487,8 +490,8 @@ class DetImpl : public o2::base::Detector
           for (auto& hit : *incomingdata) {
             hit.SetTrackID(offsetTrackIndex(hit.GetTrackID(), nprim, idelta0, idelta1));
           }
-          // this could be further generalized by using a policy for T
-          std::copy(incomingdata->begin(), incomingdata->end(), std::back_inserter(*targetdata));
+          // move rather than copy, since hits may own memory themselves (e.g. TPC HitGroup)
+          targetdata.insert(targetdata.end(), std::make_move_iterator(incomingdata->begin()), std::make_move_iterator(incomingdata->end()));
         }
         // adjust offsets for next subevent
         idelta0 += nprim;
@@ -500,10 +503,7 @@ class DetImpl : public o2::base::Detector
     targetbr->SetAddress(&filladdress);
     targetbr->Fill();
     targetbr->ResetAddress();
-    targetdata->clear();
-    hitbuffervector.clear();
     hitbuffervector = L(); // swap with empty vector to release mem
-    delete targetdata;
   }
 
   void mergeHitEntries(TTree& origin, TTree& target, std::vector<int> const& trackoffsets, std::vector<int> const& nprimaries, std::vector<int> const& subevtsOrdered) final

Detectors/TPC/simulation/include/TPCSimulation/Point.h

@@ -115,8 +115,6 @@ class HitGroup : public o2::BaseHit
   {
   }
 
-  ~HitGroup() = default;
-
   void addHit(float x, float y, float z, float time, float e)
   {
 #ifdef HIT_AOS

run/O2HitMerger.h

@@ -489,14 +489,18 @@ class O2HitMerger : public fair::mq::Device
     const auto entries = vectorOfSubEventMCTracks.size();
 
     if (entries > 1) {
+      size_t ntracks = 0;
+      for (auto tracks : vectorOfSubEventMCTracks) {
+        ntracks += tracks->size();
+      }
+      targetdata->reserve(ntracks);
       //
       // loop over subevents to store the primary events
       //
       int nprimTot = 0;
       for (int entry = entries - 1; entry >= 0; --entry) {
         int index = nsubevents[entry];
         nprimTot += nprimaries[index];
-        printf("merge %d %5d %5d %5d \n", entry, index, nsubevents[entry], nsubevents[index]);
         for (int i = 0; i < nprimaries[index]; i++) {
           auto& track = (*vectorOfSubEventMCTracks[index])[i];
           if (track.isTransported()) { // reset daughters only if track was transported, it will be fixed below
@@ -595,6 +599,11 @@ class O2HitMerger : public fair::mq::Device
       incomingdata = vectorOfT[0];
     } else {
       targetdata = std::make_unique<T>();
+      size_t nentries = 0;
+      for (auto data : vectorOfT) {
+        nentries += data->size();
+      }
+      targetdata->reserve(nentries);
       // loop over subevents
       Int_t nprimTot = 0;
       for (int entry = 0; entry < entries; entry++) {
@@ -726,7 +735,6 @@ class O2HitMerger : public fair::mq::Device
       std::vector<int> subevOrdered((int)(nsubevents.size()));
       for (int entry = entries - 1; entry >= 0; --entry) {
         subevOrdered[nsubevents[entry] - 1] = entry;
-        printf("HitMerger entry: %d nprimry: %5d trackoffset: %5d \n", entry, nprimaries[entry], trackoffsets[entry]);
       }
 
       // This is a hook that collects some useful statistics/properties on the event