refactor: Stable RNG for smeared digitization in Examples

Examples/Algorithms/Digitization/src/DigitizationAlgorithm.cpp

@@ -143,8 +143,7 @@ ProcessCode DigitizationAlgorithm::execute(const AlgorithmContext& ctx) const {
   measurementParticlesMap.reserve(simHits.size());
   measurementSimHitsMap.reserve(simHits.size());
 
-  // Setup random number generator
-  auto rng = m_cfg.randomNumbers->spawnGenerator(ctx);
+  RandomSeed eventSeed = m_cfg.randomNumbers->generateSeed(ctx);
 
   // Some statistics
   std::size_t skippedHits = 0;
@@ -153,6 +152,10 @@ ProcessCode DigitizationAlgorithm::execute(const AlgorithmContext& ctx) const {
   // Thus we need to store the cell data from the simulation.
   CellsMap cellsMap;
 
+  // Count the number of hits per particle and module. Cleared before each
+  // module.
+  std::map<SimBarcode, std::uint32_t> particleOnModuleHitCount;
+
   ACTS_DEBUG("Starting loop over modules ...");
   for (const auto& simHitsGroup : groupByModule(simHits)) {
     // Manual pair unpacking instead of using
@@ -182,6 +185,8 @@ ProcessCode DigitizationAlgorithm::execute(const AlgorithmContext& ctx) const {
       ACTS_VERBOSE("Digitizer found for module " << moduleGeoId);
     }
 
+    particleOnModuleHitCount.clear();
+
     // Run the digitizer. Iterate over the hits for this surface inside the
     // visitor so we do not need to lookup the variant object per-hit.
     std::visit(
@@ -194,6 +199,24 @@ ProcessCode DigitizationAlgorithm::execute(const AlgorithmContext& ctx) const {
             const auto& simHit = *h;
             const auto simHitIdx = simHits.index_of(h);
 
+            const auto hitIndex =
+                particleOnModuleHitCount[simHit.particleId()]++;
+
+            if (hitIndex > 0) {
+              ACTS_DEBUG("more than one hit for particle "
+                         << simHit.particleId() << " on module " << moduleGeoId
+                         << " - still forwarding it to the digitizer");
+            }
+
+            RandomSeed moduleSeed = (moduleGeoId.value() & 0xFFFFFFFF) ^
+                                    ((moduleGeoId.value() >> 32) & 0xFFFFFFFF);
+            RandomSeed particleSeed =
+                (simHit.particleId().value() & 0xFFFFFFFF) ^
+                ((simHit.particleId().value() >> 32) & 0xFFFFFFFF);
+            RandomSeed hitSeed =
+                eventSeed + moduleSeed + particleSeed + hitIndex;
+            RandomEngine rng(hitSeed);
+
             DigitizedParameters dParameters;
 
             if (simHit.depositedEnergy() < m_cfg.minEnergyDeposit) {

Examples/Framework/src/Framework/RandomNumbers.cpp

@@ -20,7 +20,7 @@ RandomEngine RandomNumbers::spawnGenerator(
 }
 
 RandomSeed RandomNumbers::generateSeed(const AlgorithmContext& context) const {
-  return m_cfg.seed + context.eventNumber;
+  return m_cfg.seed + static_cast<RandomSeed>(context.eventNumber);
 }
 
 }  // namespace ActsExamples

Tests/UnitTests/Examples/Io/Json/JsonDigitizationConfigTests.cpp

@@ -141,7 +141,7 @@ BOOST_AUTO_TEST_CASE(GaussianSmearing) {
     if (s.forcePositive[i]) {
       ref = std::abs(ref);
     }
-    CHECK_CLOSE_REL(par[i], ref, 0.15);
+    CHECK_CLOSE_ABS(par[i], ref, 0.15);
   }
 }