External generator example for simple cocktails/gap gens

Providing a custom generator example that can serve as
a template for generators using multiple internal o2::eventgen::Generator
generators (cocktails, gap triggers, ...).

MC/config/examples/external/generator/SimpleCocktail.C

@@ -0,0 +1,80 @@
+// Example of an implementation of an event generator
+// that alternates between 2 gun generators. Serves as example
+// to construct any meta-generator (such as cocktails) consisting
+// or using a pool of underlying o2::eventgen::Generators.
+
+// Test is using #o2-sim-dpl-eventgen --nEvents 10 --generator external --configKeyValues "GeneratorExternal.fileName=${O2DPG_ROOT}/MC/config/examples/external/generator/SimpleCocktail.C;GeneratorExternal.funcName=getSimpleGap()"
+
+namespace o2
+{
+namespace eventgen
+{
+
+
+/// A very simple gap generator alternating between 2 different particle guns
+class SimpleGap : public Generator
+{
+public:
+  SimpleGap() {
+    // put 2 different generators in the cocktail of generators
+    gens.push_back(new o2::eventgen::BoxGenerator(22,10,-5,5,0,10,0,360));
+    gens.push_back(new o2::eventgen::BoxGenerator(11,10,-5,5,0,10,0,360));
+  }
+
+  ~SimpleGap() = default;
+
+  Bool_t Init() override
+  {
+    // init all sub-gens
+    for (auto gen : gens) {
+      gen->Init();
+    }
+    addSubGenerator(0, "Gun 1"); // name the generators
+    addSubGenerator(1, "Gun 2");
+    return Generator::Init();
+  }
+
+  Bool_t generateEvent() override
+  {
+    // here we call the individual gun generators in turn
+    // (but we could easily call all of them to have cocktails)
+    currentindex++;
+    currentgen = gens[currentindex % 2];
+    currentgen->generateEvent();
+    // notify the sub event generator
+    notifySubGenerator(currentindex % 2);
+    return true;
+  }
+
+  // We override this function to import the particles from the
+  // underlying generators into **this** generator instance
+  Bool_t importParticles() override
+  {
+    mParticles.clear(); // clear container of mother class
+    currentgen->importParticles();
+    std::copy(currentgen->getParticles().begin(), currentgen->getParticles().end(), std::back_insert_iterator(mParticles));
+
+    // we need to fix particles statuses --> need to enforce this on the importParticles level of individual generators
+    for (auto& p : mParticles) {
+      auto st = o2::mcgenstatus::MCGenStatusEncoding(p.GetStatusCode(), p.GetStatusCode()).fullEncoding;
+      p.SetStatusCode(st);
+      p.SetBit(ParticleStatus::kToBeDone, true);
+    }
+
+    return true;
+  }
+
+private:
+  int currentindex = -1;
+  o2::eventgen::BoxGenerator* currentgen = nullptr;
+  std::vector<o2::eventgen::BoxGenerator*> gens;
+};
+
+} // namespace eventgen
+} // namespace o2
+
+/** generator instance and settings **/
+
+FairGenerator* getSimpleGap() {
+  return new o2::eventgen::SimpleGap();
+}