DPL Analysis: Update mc histograms example to use generic grouping (#235)

Author: aalkin

Tutorials/src/mcHistograms.cxx

@@ -90,10 +90,12 @@ struct AccessMcTruth {
 
 // Loop over MCColisions and get corresponding collisions (there can be more than one)
 // For each of them get the corresponding tracks
-// NOTE This does not work with the Run 3 data model at present. You will receive an exception about an unsorted index. Please use the next tutorial example.
+// Note the use of "SmallGroups" template, that allows to handle both Run 2, where
+// we have exactly 1-to-1 correspondence between collisions and mc collisions, and
+// Run 3, where we can have 0, 1, or more collisions for a given mc collision
 struct LoopOverMcMatched {
   OutputObj<TH1F> etaDiff{TH1F("etaDiff", ";eta_{MC} - eta_{Rec}", 100, -2, 2)};
-  void process(aod::McCollision const& mcCollision, soa::Join<aod::McCollisionLabels, aod::Collisions> const& collisions,
+  void process(aod::McCollision const& mcCollision, soa::SmallGroups<soa::Join<aod::McCollisionLabels, aod::Collisions>> const& collisions,
                soa::Join<aod::Tracks, aod::McTrackLabels> const& tracks, aod::McParticles const& mcParticles)
   {
     // access MC truth information with mcCollision() and mcParticle() methods
@@ -111,41 +113,11 @@ struct LoopOverMcMatched {
   }
 };
 
-// WORKAROUND of previous example for Run 3 data model (see remarks above)
-struct LoopOverMcMatchedRun3 {
-  OutputObj<TH1F> etaDiff{TH1F("etaDiff", ";eta_{MC} - eta_{Rec}", 100, -2, 2)};
-  void process(aod::McCollisions const& mcCollisions, soa::Join<aod::McCollisionLabels, aod::Collisions> const& allCollisions,
-               soa::Join<aod::Tracks, aod::McTrackLabels> const& tracks, aod::McParticles const& allMcParticles)
-  {
-    for (auto& mcCollision : mcCollisions) {
-
-      // Do grouping by hand
-      auto collisions = allCollisions.select(aod::mccollisionlabel::mcCollisionId == mcCollision.globalIndex());
-      auto mcParticles = allMcParticles.sliceBy(aod::mcparticle::mcCollisionId, mcCollision.globalIndex());
-
-      // access MC truth information with mcCollision() and mcParticle() methods
-      LOGF(info, "MC collision at vtx-z = %f with %d mc particles and %d reconstructed collisions", mcCollision.posZ(), mcParticles.size(), collisions.size());
-      for (auto& collision : collisions) {
-        LOGF(info, "  Reconstructed collision at vtx-z = %f", collision.posZ());
-
-        // NOTE this will be replaced by a improved grouping in the future
-        auto groupedTracks = tracks.sliceBy(aod::track::collisionId, collision.globalIndex());
-        LOGF(info, "  which has %d tracks", groupedTracks.size());
-        for (auto& track : groupedTracks) {
-          etaDiff->Fill(track.mcParticle().eta() - track.eta());
-        }
-      }
-    }
-  }
-};
-
 WorkflowSpec defineDataProcessing(ConfigContext const& cfgc)
 {
   return WorkflowSpec{
     adaptAnalysisTask<VertexDistribution>(cfgc),
     adaptAnalysisTask<AccessMcData>(cfgc),
     adaptAnalysisTask<AccessMcTruth>(cfgc),
-    // adaptAnalysisTask<LoopOverMcMatched>(cfgc),
-    adaptAnalysisTask<LoopOverMcMatchedRun3>(cfgc),
-  };
+    adaptAnalysisTask<LoopOverMcMatched>(cfgc)};
 }