Generators: Embedd into correct event when using collision-context

This fixes a problem with embedding and when option `--embedIntoFile`
is used. Here, we had an index problem in the primary generator class, which
was not taking into account the actual event index according the collision context.
This could lead to wrong vertices or using wrong MCHeader information for the embedding decision.

Author: sawenzel

Generators/include/Generators/PrimaryGenerator.h

@@ -80,6 +80,9 @@ class PrimaryGenerator : public FairPrimaryGenerator
   /** Public embedding methods **/
   Bool_t embedInto(TString fname);
 
+  /// sets the embedding index
+  void setEmbedIndex(int idx) { mEmbedIndex = idx; }
+
   void setExternalVertexForNextEvent(double x, double y, double z);
 
   // sets the vertex mode; if mode is kCCDB, a valid MeanVertexObject pointer must be given at the same time

Generators/src/PrimaryGenerator.cxx

@@ -81,7 +81,7 @@ Bool_t PrimaryGenerator::GenerateEvent(FairGenericStack* pStack)
   /** generate event **/
 
   /** normal generation if no embedding **/
-  if (!mEmbedTree) {
+  if (!mEmbedTree || mEmbedIndex < 0) {
     fixInteractionVertex(); // <-- always fixes vertex outside of FairROOT
     auto ret = FairPrimaryGenerator::GenerateEvent(pStack);
     if (ret) {
@@ -91,17 +91,18 @@ Bool_t PrimaryGenerator::GenerateEvent(FairGenericStack* pStack)
   }
 
   /** this is for embedding **/
-
-  /** setup interaction vertex **/
-  mEmbedTree->GetEntry(mEmbedIndex);
-  setInteractionVertex(mEmbedEvent);
-
-  /** notify event generators **/
-  auto genList = GetListOfGenerators();
-  for (int igen = 0; igen < genList->GetEntries(); ++igen) {
-    auto o2gen = dynamic_cast<Generator*>(genList->At(igen));
-    if (o2gen) {
-      o2gen->notifyEmbedding(mEmbedEvent);
+  if (mEmbedIndex >= 0) {
+    /** setup interaction vertex **/
+    mEmbedTree->GetEntry(mEmbedIndex);
+    setInteractionVertex(mEmbedEvent);
+
+    /** notify event generators **/
+    auto genList = GetListOfGenerators();
+    for (int igen = 0; igen < genList->GetEntries(); ++igen) {
+      auto o2gen = dynamic_cast<Generator*>(genList->At(igen));
+      if (o2gen) {
+        o2gen->notifyEmbedding(mEmbedEvent);
+      }
     }
   }
 

run/O2PrimaryServerDevice.h

@@ -46,6 +46,7 @@
 #include <chrono>
 #include <CCDB/BasicCCDBManager.h>
 #include <TRandom3.h>
+#include <regex>
 
 namespace o2
 {
@@ -135,6 +136,17 @@ class O2PrimaryServerDevice final : public fair::mq::Device
 
       auto embedinto_filename = conf.getEmbedIntoFileName();
       if (!embedinto_filename.empty()) {
+        // determine the sim prefix from the embedding filename
+        // the filename should be an MCHeader file ... so it should match SOME_PATH/prefix_MCHeader.root
+        std::regex re(R"((.*/)?([^/]+)_MCHeader\.root$)");
+        std::smatch match;
+
+        if (std::regex_search(embedinto_filename, match, re)) {
+          std::cout << "Extracted embedding prefix : " << match[2] << '\n';
+          mEmbeddIntoPrefix = match[2];
+        } else {
+          LOG(fatal) << "Embedding asked but no suitable embedding prefix extractable from " << embedinto_filename;
+        }
         mPrimGen->embedInto(embedinto_filename);
       }
 
@@ -197,6 +209,19 @@ class O2PrimaryServerDevice final : public fair::mq::Device
             auto& vertex = vertices.at(collisionindex);
             LOG(info) << "Setting vertex " << vertex << " for event " << mEventCounter << " for prefix " << mSimConfig.getOutPrefix() << " from CollContext";
             mPrimGen->setExternalVertexForNextEvent(vertex.X(), vertex.Y(), vertex.Z());
+
+            // set correct embedding index for PrimaryGenerator ... based on collision context for embedding
+            auto& collisionParts = mCollissionContext->getEventParts()[collisionindex];
+            int background_index = -1; // -1 means no embedding taking place for this signal
+
+            // find the part that corresponds to the event embeded into
+            for (auto& part : collisionParts) {
+              if (mCollissionContext->getSimPrefixes()[part.sourceID] == mEmbeddIntoPrefix) {
+                background_index = part.entryID;
+                LOG(info) << "Setting embedding index to " << background_index;
+              }
+            }
+            mPrimGen->setEmbedIndex(background_index);
           }
         }
         mPrimGen->GenerateEvent(mStack);
@@ -696,6 +721,7 @@ class O2PrimaryServerDevice final : public fair::mq::Device
   // some information specific to use case when we have a collision context
   o2::steer::DigitizationContext* mCollissionContext = nullptr; //!
   std::unordered_map<int, int> mEventID_to_CollID;              //!
+  std::string mEmbeddIntoPrefix;                                //! sim prefix of background events
 
   TRandom3 mSeedGenerator; //! specific random generator for seed generation for work chunks
 };