Write tracked V0s, cascades and 3-bodies in collision order (#15838)

* Write tracked V0s, cascades and 3-bodies in collision order

This fixes the row order of the tracked strangeness tables written by the AOD producer.

- The tracked V0, cascade and 3-body rows were written in strangeness-tracker order, which is not always collision order.
- Analyses slicing them by collision then abort with "TraCascIndices index fIndexCollisions is not sorted".
- The rows are now written in the per-collision order already built in prepareStrangenessTracking.
- The track index of a row no longer comes from a running counter, so a skipped strange track cannot shift the rows after it.

https://its.cern.ch/jira/browse/O2-7197


* Keep strange tracks in decay order also with one vertexer thread

This makes the order of the tracked strangeness tables within a collision the same in MC as in data.

- SVertexer sorted the strange tracks by decay only when running with more than one thread; MC reconstruction runs it with one.
- The strange tracks are now sorted by decay in all cases.
- The AOD producer groups them by collision with a stable sort, so the decay order within a collision is kept.

https://its.cern.ch/jira/browse/O2-7197

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

Author: sawenzel

Detectors/AOD/src/AODProducerWorkflowSpec.cxx

@@ -1527,7 +1527,7 @@ void AODProducerWorkflowDPL::prepareStrangenessTracking(const o2::globaltracking
   std::exclusive_scan(mVertexStrLUT.begin(), mVertexStrLUT.end(), mVertexStrLUT.begin(), 0);
 
   // sort by collision ID
-  std::sort(mCollisionStrTrk.begin(), mCollisionStrTrk.end(), [](const auto& a, const auto& b) { return a.first < b.first; });
+  std::stable_sort(mCollisionStrTrk.begin(), mCollisionStrTrk.end(), [](const auto& a, const auto& b) { return a.first < b.first; });
   mStrTrkIndices.clear();
   mStrTrkIndices.resize(mCollisionStrTrk.size(), -1);
 }
@@ -1536,7 +1536,6 @@ template <typename V0C, typename CC, typename D3BC>
 void AODProducerWorkflowDPL::fillStrangenessTrackingTables(const o2::globaltracking::RecoContainer& recoData, V0C& v0Curs, CC& cascCurs, D3BC& d3BodyCurs)
 {
   int itsTableIdx = -1;
-  int sTrkID = 0;
   int nV0 = 0;
   int nCasc = 0;
   int nD3Body = 0;
@@ -1555,7 +1554,10 @@ void AODProducerWorkflowDPL::fillStrangenessTrackingTables(const o2::globaltrack
   cascCurs.reserve(nCasc);
   d3BodyCurs.reserve(nD3Body);
 
-  for (const auto& sTrk : recoData.getStrangeTracks()) {
+  // Write the rows grouped by collision, the order analyses slice these tables in
+  auto sTracks = recoData.getStrangeTracks();
+  for (const auto& collStrTrk : mCollisionStrTrk) {
+    const auto& sTrk = sTracks[collStrTrk.second];
     auto ITSIndex = GIndex{sTrk.mITSRef, GIndex::ITS};
     auto item = mGIDToTableID.find(ITSIndex);
     if (item != mGIDToTableID.end()) {
@@ -1565,7 +1567,7 @@ void AODProducerWorkflowDPL::fillStrangenessTrackingTables(const o2::globaltrack
       continue;
     }
     if (sTrk.mPartType == dataformats::kStrkV0) {
-      v0Curs(mStrTrkIndices[sTrkID++],
+      v0Curs(mStrTrkIndices[collStrTrk.second],
              itsTableIdx,
              sTrk.mDecayRef,
              sTrk.mDecayVtx[0],
@@ -1577,7 +1579,7 @@ void AODProducerWorkflowDPL::fillStrangenessTrackingTables(const o2::globaltrack
              sTrk.mTopoChi2,
              sTrk.getAverageClusterSize());
     } else if (sTrk.mPartType == dataformats::kStrkCascade) {
-      cascCurs(mStrTrkIndices[sTrkID++],
+      cascCurs(mStrTrkIndices[collStrTrk.second],
                itsTableIdx,
                sTrk.mDecayRef,
                sTrk.mDecayVtx[0],
@@ -1589,7 +1591,7 @@ void AODProducerWorkflowDPL::fillStrangenessTrackingTables(const o2::globaltrack
                sTrk.mTopoChi2,
                sTrk.getAverageClusterSize());
     } else {
-      d3BodyCurs(mStrTrkIndices[sTrkID++],
+      d3BodyCurs(mStrTrkIndices[collStrTrk.second],
                  itsTableIdx,
                  sTrk.mDecayRef,
                  sTrk.mDecayVtx[0],

Detectors/Vertexing/src/SVertexer.cxx

@@ -225,8 +225,8 @@ void SVertexer::produceOutput(o2::framework::ProcessingContext& pc)
 
     std::vector<int> sortIdx(strTracksTmp.size());
     std::iota(sortIdx.begin(), sortIdx.end(), 0);
-    // if mNTreads > 1 we need to sort tracks, clus and MCLabs by their mDecayRef
-    if (mNThreads > 1 && mNStrangeTracks > 1) {
+    // sort tracks, clus and MCLabs by their mDecayRef, also with one thread, so that they follow the vertex order
+    if (mNStrangeTracks > 1) {
       std::sort(sortIdx.begin(), sortIdx.end(), [&strTracksTmp](int i1, int i2) { return strTracksTmp[i1].mDecayRef < strTracksTmp[i2].mDecayRef; });
     }