Optional InteractionSampler sampling overwrite

Allow to overwrite InteractionSampler by means of virtualizing
sampling function.

Used in CollisionContextTool do allow putting collisions at fixed N intervals (export mode)
for the purpose of systematic studies / debugging.

- To use this feature say `export ALICEO2_ENFORCE_TRIVIAL_BC_SAMPLER="2:5"
  to put 5 collisions into every 2nd bunch-crossing (within the bunch filling scheme)

Author: sawenzel

DataFormats/simulation/include/SimulationDataFormat/InteractionSampler.h

@@ -67,7 +67,7 @@ class InteractionSampler
   void print() const;
 
  protected:
-  int simulateInteractingBC();
+  virtual int simulateInteractingBC();
   void nextCollidingBC(int n);
 
   o2::math_utils::RandomRing<10000> mBCJumpGenerator;  // generator of random jumps in BC
@@ -89,7 +89,7 @@ class InteractionSampler
 
   static constexpr float DefIntRate = 50e3; ///< default interaction rate
 
-  ClassDefNV(InteractionSampler, 1);
+  ClassDef(InteractionSampler, 1);
 };
 
 //_________________________________________________
@@ -113,6 +113,23 @@ inline void InteractionSampler::nextCollidingBC(int n)
   mIR.bc = mInteractingBCs[mCurrBCIdx];
 }
 
+// Special case of InteractionSampler without actual sampling.
+// Engineers interaction sequence by putting one in each N-th BC with multiplicity mult.
+class FixedSkipBC_InteractionSampler : public InteractionSampler
+{
+
+ public:
+  FixedSkipBC_InteractionSampler(int every_n, int mult) : mEveryN{every_n}, mMultiplicity{mult}, InteractionSampler() {}
+
+ protected:
+  int simulateInteractingBC() override;
+
+ private:
+  int mEveryN;       // the skip number ---> fills every N-th BC in the bunch filling scheme
+  int mMultiplicity; // how many events to put if bc is filled
+  ClassDef(FixedSkipBC_InteractionSampler, 1);
+};
+
 } // namespace steer
 } // namespace o2
 

DataFormats/simulation/src/InteractionSampler.cxx

@@ -130,6 +130,24 @@ int InteractionSampler::simulateInteractingBC()
   return ncoll;
 }
 
+//_________________________________________________
+int FixedSkipBC_InteractionSampler::simulateInteractingBC()
+{
+  // Returns number of collisions assigned to selected BC
+
+  nextCollidingBC(mEveryN);  // we jump regular intervals
+  int ncoll = mMultiplicity; // well defined pileup
+
+  // assign random time withing a bunch
+  for (int i = ncoll; i--;) {
+    mTimeInBC.push_back(mCollTimeGenerator.getNextValue());
+  }
+  if (ncoll > 1) { // sort in DECREASING time order (we are reading vector from the end)
+    std::sort(mTimeInBC.begin(), mTimeInBC.end(), [](const float a, const float b) { return a > b; });
+  }
+  return ncoll;
+}
+
 //_________________________________________________
 void InteractionSampler::setBunchFilling(const std::string& bcFillingFile)
 {

DataFormats/simulation/src/SimulationDataLinkDef.h

@@ -24,6 +24,7 @@
 #pragma link off all functions;
 
 #pragma link C++ class o2::steer::InteractionSampler + ;
+#pragma link C++ class o2::steer::FixedSkipBC_InteractionSampler + ;
 #pragma link C++ class o2::sim::StackParam + ;
 #pragma link C++ class o2::conf::ConfigurableParamHelper < o2::sim::StackParam> + ;
 #pragma link C++ class o2::MCTrackT < double> + ;

Steer/src/CollisionContextTool.cxx

@@ -302,17 +302,34 @@ int main(int argc, char* argv[])
   for (int id = 0; id < ispecs.size(); ++id) {
     auto mode = ispecs[id].syncmode;
     if (mode == InteractionLockMode::NOLOCK) {
-      o2::steer::InteractionSampler sampler;
-      sampler.setInteractionRate(ispecs[id].interactionRate);
+      auto sampler = std::make_unique<o2::steer::InteractionSampler>();
+
+      // for debug purposes: allows to instantiate trivial sampler
+      if (const char* env = getenv("ALICEO2_ENFORCE_TRIVIAL_BC_SAMPLER")) {
+        std::string spec(env);
+        std::regex re(R"((\d+):(\d+))");
+        std::smatch match;
+        int every_n = 1, mult = 1;
+        if (std::regex_match(spec, match, re)) {
+          every_n = std::stoi(match[1]);
+          mult = std::stoi(match[2]);
+        } else {
+          LOG(error) << "ALICEO2_ENFORCE_TRIVIAL_BC_SAMPLER format invalid, expected NUMBER_1:NUMBER_2";
+          exit(1);
+        }
+        sampler.reset(new o2::steer::FixedSkipBC_InteractionSampler(every_n, mult));
+      }
+
+      sampler->setInteractionRate(ispecs[id].interactionRate);
       if (!options.bcpatternfile.empty()) {
-        setBCFillingHelper(sampler, options.bcpatternfile);
+        setBCFillingHelper(*sampler, options.bcpatternfile);
       }
       o2::InteractionTimeRecord record;
       // this loop makes sure that the first collision is within the range of orbits asked (if noEmptyTF is enabled)
       do {
-        sampler.setFirstIR(o2::InteractionRecord(options.firstBC, orbitstart));
-        sampler.init();
-        record = sampler.generateCollisionTime();
+        sampler->setFirstIR(o2::InteractionRecord(options.firstBC, orbitstart));
+        sampler->init();
+        record = sampler->generateCollisionTime();
       } while (options.noEmptyTF && usetimeframelength && record.orbit >= orbitstart + orbits_total);
       int count = 0;
       do {
@@ -325,7 +342,7 @@ int main(int argc, char* argv[])
         std::pair<o2::InteractionTimeRecord, std::vector<o2::steer::EventPart>> insertvalue(record, parts);
         auto iter = std::lower_bound(collisions.begin(), collisions.end(), insertvalue, [](std::pair<o2::InteractionTimeRecord, std::vector<o2::steer::EventPart>> const& a, std::pair<o2::InteractionTimeRecord, std::vector<o2::steer::EventPart>> const& b) { return a.first < b.first; });
         collisions.insert(iter, insertvalue);
-        record = sampler.generateCollisionTime();
+        record = sampler->generateCollisionTime();
         count++;
       } while ((ispecs[id].mcnumberasked > 0 && count < ispecs[id].mcnumberasked)); // TODO: this loop should probably be replaced by a condition with usetimeframelength and number of orbits
 
@@ -360,7 +377,7 @@ int main(int argc, char* argv[])
       }
 
       // keep bunch filling information produced by these samplers
-      bunchFillings.push_back(sampler.getBunchFilling());
+      bunchFillings.push_back(sampler->getBunchFilling());
 
     } else {
       // we are in some lock/sync mode and modify existing collisions