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Workaround for HepMC3 bug #15093

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Merged
sawenzel merged 1 commit into from
Feb 24, 2026
+35 −7
Merged

Workaround for HepMC3 bug #15093

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42 changes: 35 additions & 7 deletions Generators/src/GeneratorHepMC.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,7 @@
#include <fairlogger/Logger.h>
#include "FairPrimaryGenerator.h"
#include <cmath>
#include <sstream>

namespace o2
{
Expand Down Expand Up @@ -420,6 +421,34 @@ void GeneratorHepMC::updateHeader(o2::dataformats::MCEventHeader* eventHeader)
auto pdfInfo = mEvent->pdf_info();
auto hiInfo = mEvent->heavy_ion();

// Workaround for a bug in HepMC3 (3.3.1 on 23/02/2026): GenHeavyIon::from_string() for the "v0"
// format skips reading user_cent_estimate, but to_string() always writes it.
// This shifts all subsequent fields by one, causing a istringstream failure and and heavy_ion()
// to return null even when the attribute is present and well-formed.
// For now we use this manual parser in case the infos are available
if (!hiInfo) {
auto attStr = mEvent->attribute_as_string("GenHeavyIon");
if (!attStr.empty() && attStr[0] == 'v') {
std::istringstream is(attStr);
std::string version;
is >> version;
if (version == "v0") {
auto hi = std::make_shared<HepMC3::GenHeavyIon>();
double spectNeutrons, spectProtons, eccentricity, userCentEst;
is >> hi->Ncoll_hard >> hi->Npart_proj >> hi->Npart_targ >> hi->Ncoll >> spectNeutrons >> spectProtons // deprecated v0 fields
>> hi->N_Nwounded_collisions >> hi->Nwounded_N_collisions >> hi->Nwounded_Nwounded_collisions >> hi->impact_parameter >> hi->event_plane_angle >> eccentricity // deprecated v0 field
>> hi->sigma_inel_NN >> hi->centrality >> userCentEst // GenHeavyIon::to_string always writes this, but GenHeavyIon::from_string skips it for v0 (HepMC3 bug to fix)
>> hi->Nspec_proj_n >> hi->Nspec_targ_n >> hi->Nspec_proj_p >> hi->Nspec_targ_p;
if (!is.fail()) {
LOG(debug) << "GenHeavyIon: using manual v0 parser (workaround for HepMC3 from_string bug)";
hiInfo = hi;
} else {
LOG(warn) << "GenHeavyIon: manual v0 parser also failed on: [" << attStr << "]";
}
}
}
}

// Set default cross-section
if (xSection) {
eventHeader->putInfo<float>(Key::xSection, xSection->xsec());
Expand Down Expand Up @@ -457,8 +486,9 @@ void GeneratorHepMC::updateHeader(o2::dataformats::MCEventHeader* eventHeader)

// Set heavy-ion information
if (hiInfo) {
eventHeader->putInfo<int>(Key::impactParameter,
hiInfo->impact_parameter);
eventHeader->SetB(hiInfo->impact_parameter); // sets the impact parameter to the FairMCEventHeader field for quick access in the AO2D
eventHeader->putInfo<float>(Key::impactParameter,
hiInfo->impact_parameter);
eventHeader->putInfo<int>(Key::nPart,
hiInfo->Npart_proj + hiInfo->Npart_targ);
eventHeader->putInfo<int>(Key::nPartProjectile, hiInfo->Npart_proj);
Expand All @@ -471,11 +501,9 @@ void GeneratorHepMC::updateHeader(o2::dataformats::MCEventHeader* eventHeader)
hiInfo->Nwounded_N_collisions);
eventHeader->putInfo<int>(Key::nCollNWoundedNwounded,
hiInfo->Nwounded_Nwounded_collisions);
eventHeader->putInfo<int>(Key::planeAngle,
hiInfo->event_plane_angle);
eventHeader->putInfo<int>(Key::sigmaInelNN,
hiInfo->sigma_inel_NN);
eventHeader->putInfo<int>(Key::centrality, hiInfo->centrality);
eventHeader->putInfo<float>(Key::planeAngle, hiInfo->event_plane_angle);
eventHeader->putInfo<float>(Key::sigmaInelNN, hiInfo->sigma_inel_NN);
eventHeader->putInfo<float>(Key::centrality, hiInfo->centrality);
eventHeader->putInfo<int>(Key::nSpecProjectileProton, hiInfo->Nspec_proj_p);
eventHeader->putInfo<int>(Key::nSpecProjectileNeutron, hiInfo->Nspec_proj_n);
eventHeader->putInfo<int>(Key::nSpecTargetProton, hiInfo->Nspec_targ_p);
Expand Down