fix: Poke at Pythia8 to make it reproducible (#3392)

CMakeLists.txt

@@ -213,6 +213,7 @@ set(_acts_nlohmanjson_version 3.2.0)
 set(_acts_onnxruntime_version 1.12.0)
 set(_acts_root_version 6.20)
 set(_acts_tbb_version 2020.1)
+set(_acts_pythia8_version 8.309)
 
 # recommended dependency version. if there is an opportunity to reach
 # this version we will try so.
@@ -436,15 +437,7 @@ if(ACTS_BUILD_EXAMPLES_HEPMC3)
   find_package(HepMC3 ${_acts_hepmc3_version} REQUIRED CONFIG)
 endif()
 if(ACTS_BUILD_EXAMPLES_PYTHIA8)
-  find_package(Pythia8 REQUIRED)
-
-  if(DEFINED CMAKE_CXX_STANDARD)
-    if(${CMAKE_CXX_STANDARD} GREATER_EQUAL 20)
-      message(WARNING "ACTS is configured to build with C++20."
-        "As of version 309, Pythia8 does not compile under C++20."
-        "You can find a patch file at: https://gist.github.com/paulgessinger/7a823b4015d4511cf829472291c8ddcf")
-    endif()
-  endif()
+  find_package(Pythia8 ${_acts_pythia8_version} REQUIRED)
 endif()
 # other dependencies
 if(ACTS_BUILD_DOCS)

Examples/Algorithms/GeneratorsPythia8/ActsExamples/Generators/Pythia8ProcessGenerator.cpp

@@ -23,16 +23,35 @@
 
 namespace ActsExamples {
 
-namespace {
-struct FrameworkRndmEngine : public Pythia8::RndmEngine {
-  RandomEngine& rng;
+struct Pythia8RandomEngineWrapper : public Pythia8::RndmEngine {
+  RandomEngine* rng{nullptr};
+
+  struct {
+    std::size_t numUniformRandomNumbers = 0;
+    double first = std::numeric_limits<double>::quiet_NaN();
+    double last = std::numeric_limits<double>::quiet_NaN();
+  } statistics;
+
+  Pythia8RandomEngineWrapper() = default;
 
-  FrameworkRndmEngine(RandomEngine& rng_) : rng(rng_) {}
   double flat() override {
-    return std::uniform_real_distribution<double>(0.0, 1.0)(rng);
+    if (rng == nullptr) {
+      throw std::runtime_error(
+          "Pythia8RandomEngineWrapper: no random engine set");
+    }
+
+    double value = std::uniform_real_distribution<double>(0.0, 1.0)(*rng);
+    if (statistics.numUniformRandomNumbers == 0) {
+      statistics.first = value;
+    }
+    statistics.last = value;
+    statistics.numUniformRandomNumbers++;
+    return value;
   }
+
+  void setRandomEngine(RandomEngine& rng_) { rng = &rng_; }
+  void clearRandomEngine() { rng = nullptr; }
 };
-}  // namespace
 
 Pythia8Generator::Pythia8Generator(const Config& cfg, Acts::Logging::Level lvl)
     : m_cfg(cfg),
@@ -49,11 +68,31 @@ Pythia8Generator::Pythia8Generator(const Config& cfg, Acts::Logging::Level lvl)
   m_pythia8->settings.mode("Beams:frameType", 1);
   m_pythia8->settings.parm("Beams:eCM",
                            m_cfg.cmsEnergy / Acts::UnitConstants::GeV);
+
+  m_pythia8RndmEngine = std::make_shared<Pythia8RandomEngineWrapper>();
+
+#if PYTHIA_VERSION_INTEGER >= 8310
+  m_pythia8->setRndmEnginePtr(m_pythia8RndmEngine);
+#else
+  m_pythia8->setRndmEnginePtr(m_pythia8RndmEngine.get());
+#endif
+
+  RandomEngine rng{m_cfg.initializationSeed};
+  m_pythia8RndmEngine->setRandomEngine(rng);
   m_pythia8->init();
+  m_pythia8RndmEngine->clearRandomEngine();
 }
 
 // needed to allow unique_ptr of forward-declared Pythia class
-Pythia8Generator::~Pythia8Generator() = default;
+Pythia8Generator::~Pythia8Generator() {
+  ACTS_INFO("Pythia8Generator produced "
+            << m_pythia8RndmEngine->statistics.numUniformRandomNumbers
+            << " uniform random numbers");
+  ACTS_INFO(
+      "                 first = " << m_pythia8RndmEngine->statistics.first);
+  ACTS_INFO(
+      "                  last = " << m_pythia8RndmEngine->statistics.last);
+}
 
 std::pair<SimVertexContainer, SimParticleContainer>
 Pythia8Generator::operator()(RandomEngine& rng) {
@@ -64,13 +103,10 @@ Pythia8Generator::operator()(RandomEngine& rng) {
 
   // pythia8 is not thread safe and generation needs to be protected
   std::lock_guard<std::mutex> lock(m_pythia8Mutex);
-// use per-thread random engine also in pythia
-#if PYTHIA_VERSION_INTEGER >= 8310
-  m_pythia8->rndm.rndmEnginePtr(std::make_shared<FrameworkRndmEngine>(rng));
-#else
-  FrameworkRndmEngine rndmEngine(rng);
-  m_pythia8->rndm.rndmEnginePtr(&rndmEngine);
-#endif
+  // use per-thread random engine also in pythia
+
+  m_pythia8RndmEngine->setRandomEngine(rng);
+
   {
     Acts::FpeMonitor mon{0};  // disable all FPEs while we're in Pythia8
     m_pythia8->next();
@@ -157,6 +193,9 @@ Pythia8Generator::operator()(RandomEngine& rng) {
   std::pair<SimVertexContainer, SimParticleContainer> out;
   out.first.insert(vertices.begin(), vertices.end());
   out.second.insert(particles.begin(), particles.end());
+
+  m_pythia8RndmEngine->clearRandomEngine();
+
   return out;
 }
 

Examples/Algorithms/GeneratorsPythia8/ActsExamples/Generators/Pythia8ProcessGenerator.hpp

@@ -27,6 +27,8 @@ class Pythia;
 
 namespace ActsExamples {
 
+struct Pythia8RandomEngineWrapper;
+
 class Pythia8Generator : public EventGenerator::ParticlesGenerator {
  public:
   struct Config {
@@ -48,6 +50,8 @@ class Pythia8Generator : public EventGenerator::ParticlesGenerator {
     bool labelSecondaries = true;
     /// The spatial threshold to consider a particle originating from a vertex
     double spatialVertexThreshold = 1.0 * Acts::UnitConstants::um;
+    /// Random seed for the initialization stage of Pythia8
+    unsigned int initializationSeed = 42;
   };
 
   Pythia8Generator(const Config& cfg, Acts::Logging::Level lvl);
@@ -69,6 +73,7 @@ class Pythia8Generator : public EventGenerator::ParticlesGenerator {
   Config m_cfg;
   std::unique_ptr<const Acts::Logger> m_logger;
   std::unique_ptr<::Pythia8::Pythia> m_pythia8;
+  std::shared_ptr<Pythia8RandomEngineWrapper> m_pythia8RndmEngine;
   std::mutex m_pythia8Mutex;
 };
 

Examples/Python/src/Pythia8.cpp

@@ -46,7 +46,11 @@ void addPythia8(Context& ctx) {
       .def_readwrite("printShortEventListing",
                      &Gen::Config::printShortEventListing)
       .def_readwrite("printLongEventListing",
-                     &Gen::Config::printLongEventListing);
+                     &Gen::Config::printLongEventListing)
+      .def_readwrite("labelSecondaries", &Gen::Config::labelSecondaries)
+      .def_readwrite("spatialVertexThreshold",
+                     &Gen::Config::spatialVertexThreshold)
+      .def_readwrite("initializationSeed", &Gen::Config::initializationSeed);
 
   patchClassesWithConfig(p8);
 }

Examples/Python/tests/root_file_hashes.txt

@@ -1,4 +1,4 @@
-test_pythia8__pythia8_particles.root: 6e00f6ab534ef5888861bef3ca5bb47d45171237e875eef0d119df7bda8cba46
+test_pythia8__pythia8_particles.root: a8b53c12e771091bc2f5ace0198e9f6c5ac1909e2e6d0e3e4945e8174048ef62
 test_fatras__particles_simulation.root: bc970873fef0c2efd86ed5413623802353d2cd04abea72de14e8cdfc0e40076f
 test_fatras__hits.root: 6e4beb045fa1712c4d14c280ba33c3fa13e4aff9de88d55c3e32f62ad226f724
 test_geant4__particles_simulation.root: 3c9c6265183b04c9d62ed5f2d0709c6dd74e33fbb53ac0aeb3274f6600257fc1

cmake/FindPythia8.cmake

@@ -14,8 +14,13 @@ find_path(Pythia8_INCLUDE_DIR
   PATHS /opt/pythia8 /usr/local
   DOC "The Pythia8 include directory")
 
+file(READ "${Pythia8_INCLUDE_DIR}/Pythia8/Pythia.h" Pythia8_VERSION_FILE)
+string(REGEX MATCH "#define PYTHIA_VERSION (8\.[0-9]+)" _ ${Pythia8_VERSION_FILE})
+set(Pythia8_VERSION ${CMAKE_MATCH_1})
+
 find_package_handle_standard_args(Pythia8
-  REQUIRED_VARS Pythia8_LIBRARY Pythia8_INCLUDE_DIR)
+  REQUIRED_VARS Pythia8_LIBRARY Pythia8_INCLUDE_DIR
+  VERSION_VAR Pythia8_VERSION)
 
 add_library(Pythia8 SHARED IMPORTED)
 set_property(TARGET Pythia8 PROPERTY IMPORTED_LOCATION ${Pythia8_LIBRARY})