From 0496b2086ffc1f100f1fda6269ba154d5255a2f1 Mon Sep 17 00:00:00 2001
From: swenzel <sandro.wenzel@cern.ch>
Date: Mon, 17 Feb 2025 17:58:57 +0100
Subject: [PATCH] Fix mother and daughter indices in generator cocktails

Particles in a generated event carry indices to refer to mother and dauther particles.
These indices need to be adjusted when we combine multiple events into a cocktail.
---
 Generators/src/GeneratorHybrid.cxx | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Generators/src/GeneratorHybrid.cxx b/Generators/src/GeneratorHybrid.cxx
index f968a9c4b3513..729d69527c384 100644
--- a/Generators/src/GeneratorHybrid.cxx
+++ b/Generators/src/GeneratorHybrid.cxx
@@ -390,6 +390,26 @@ bool GeneratorHybrid::importParticles()
     for (auto subIndex : subGenIndex) {
       LOG(info) << "Importing particles for task " << subIndex;
       auto subParticles = gens[subIndex]->getParticles();
+
+      // The particles carry mother and daughter indices, which are relative
+      // to the sub-generator. We need to adjust these indices to reflect that particles
+      // are now embedded into a cocktail.
+      auto offset = mParticles.size();
+      for (auto& p : subParticles) {
+        for (int i = 0; i < 2; ++i) {
+          if (p.GetMother(i) != -1) {
+            const auto newindex = p.GetMother(i) + offset;
+            p.SetMother(i, newindex);
+          }
+        }
+        if (p.GetNDaughters() > 0) {
+          for (int i = 0; i < 2; ++i) {
+            const auto newindex = p.GetDaughter(i) + offset;
+            p.SetDaughter(i, newindex);
+          }
+        }
+      }
+
       mParticles.insert(mParticles.end(), subParticles.begin(), subParticles.end());
       // fetch the event Header information from the underlying generator
       gens[subIndex]->updateHeader(&mMCEventHeader);