Do not offset an invalid track index when merging sub-events

This fixes a problem in the sub-event merging and adds a unit test.

- Merging offsets every track index, and the offset was applied
  unconditionally.
- An index already flagged invalid therefore came out as offset - 1 and
  pointed at an unrelated track.
- The MCTrack mother index in O2HitMerger has always guarded against this,
  hits and track references did not.
- Both now use Detector::offsetTrackIndex, which leaves a negative index alone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

Detectors/Base/include/DetectorsBase/Detector.h

@@ -171,6 +171,17 @@ class Detector : public FairDetector
     return iter != indexmapping.end() ? iter->second : -1;
   }
 
+  // the index a hit or a track reference gets when the sub-events of one event
+  // are concatenated; an index already flagged invalid carries no track to
+  // shift and stays invalid
+  static int offsetTrackIndex(int trackID, int nprimaries, int primaryOffset, int secondaryOffset)
+  {
+    if (trackID < 0) {
+      return trackID;
+    }
+    return trackID + (trackID < nprimaries ? primaryOffset : secondaryOffset);
+  }
+
   // interfaces to attach properly encoded hit information to a FairMQ message
   // and to decode it
   virtual void attachHits(fair::mq::Channel&, fair::mq::Parts&) = 0;
@@ -398,10 +409,7 @@ class DetImpl : public o2::base::Detector
           if (incomingdata) {
             // fix the trackIDs for this data
             for (auto& hit : *incomingdata) {
-              const auto oldID = hit.GetTrackID();
-              // offset depends on whether the trackis a primary or secondary
-              Int_t offset = (oldID < nprim) ? idelta0 : idelta1;
-              hit.SetTrackID(oldID + offset);
+              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));
@@ -465,10 +473,7 @@ class DetImpl : public o2::base::Detector
         if (incomingdata) {
           // fix the trackIDs for this data
           for (auto& hit : *incomingdata) {
-            const auto oldID = hit.GetTrackID();
-            // offset depends on whether the trackis a primary or secondary
-            int offset = (oldID < nprim) ? idelta0 : idelta1;
-            hit.SetTrackID(oldID + offset);
+            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));

Detectors/Base/test/testStack.cxx

@@ -170,3 +170,13 @@ BOOST_AUTO_TEST_CASE(Stack_does_not_reuse_index_map_of_previous_event)
 
   BOOST_CHECK_EQUAL(det.mHits[0].GetTrackID(), -1);
 }
+
+// An invalid index must not be offset when sub-events are merged
+BOOST_AUTO_TEST_CASE(Offsetting_keeps_an_invalid_index_invalid)
+{
+  const int nprimaries = 5, primaryOffset = 10, secondaryOffset = 100;
+
+  BOOST_CHECK_EQUAL(o2::base::Detector::offsetTrackIndex(3, nprimaries, primaryOffset, secondaryOffset), 13);
+  BOOST_CHECK_EQUAL(o2::base::Detector::offsetTrackIndex(7, nprimaries, primaryOffset, secondaryOffset), 107);
+  BOOST_CHECK_EQUAL(o2::base::Detector::offsetTrackIndex(-1, nprimaries, primaryOffset, secondaryOffset), -1);
+}

run/O2HitMerger.h

@@ -627,9 +627,7 @@ class O2HitMerger : public fair::mq::Device
 
   void updateTrackIdWithOffset(TrackReference& ref, Int_t nprim, Int_t idelta0, Int_t idelta1)
   {
-    Int_t cId = ref.getTrackID();
-    Int_t ioffset = (cId < nprim) ? idelta0 : idelta1;
-    ref.setTrackID(cId + ioffset);
+    ref.setTrackID(o2::base::Detector::offsetTrackIndex(ref.getTrackID(), nprim, idelta0, idelta1));
   }
 
   void initHitTreeAndOutFile(std::string prefix, int detID)