TrigComboHypoTool.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
 
#include "TrigComboHypoTool.h"
#include "GaudiKernel/SystemOfUnits.h"
#include "TrigCompositeUtils/Combinators.h"
#include "TrigCompositeUtils/TrigCompositeUtils.h"
 
#include <Math/Vector4D.h>    // for LorentzVector
#include <Math/Vector4Dfwd.h> // PtEtaPhiM typedef
#include <Math/Vector2D.h>    // for DisplacementVector
#include <Math/Vector2Dfwd.h> // Polar2DVectorF typedef
 
#include "xAODTrigMissingET/TrigMissingETContainer.h"
#include "FourMomUtils/xAODP4Helpers.h"
 
#include <algorithm>
#include <cmath>
 
constexpr float invGeV = 1. / Gaudi::Units::GeV;
 
using namespace TrigCompositeUtils;
 
// Translate strings into enum values
const std::map<std::string, TrigComboHypoTool::ComboHypoVars> VarMap = {
  {"dR",   TrigComboHypoTool::ComboHypoVars::DR},
  {"invm", TrigComboHypoTool::ComboHypoVars::INVM},
  {"dphi", TrigComboHypoTool::ComboHypoVars::DPHI},
  {"mT",   TrigComboHypoTool::ComboHypoVars::MT},
  {"deta", TrigComboHypoTool::ComboHypoVars::DETA}
};
 
TrigComboHypoTool::TrigComboHypoTool(const std::string& type, 
                     const std::string& name, 
                     const IInterface*  parent)
  : ComboHypoToolBase(type, name, parent)
{}
 
 
bool TrigComboHypoTool::VarInfo::validate(std::string& errmsg) const {
  if (legA==0){
    errmsg = "legA ID not set!";
    return false;
  }
  if (legB==0){
    errmsg="legB ID not set!";
    return false;
  }
  if ((!useMin) && (!useMax)){
    errmsg="Trying to configure the Tool without setting at least one of UseMin or UseMax!";
    return false;
  }
  if (legA==legB && (legA_is_MET || legB_is_MET)) {
    errmsg = "Cannot specify the same MET leg for both sides!";
    return false;
  }
  return true;
}
 
 
StatusCode TrigComboHypoTool::initialize()
{
  ATH_MSG_DEBUG("Variable   = " << m_varTag_vec );
  ATH_MSG_DEBUG("UseCut min = " << m_useMin_vec );
  ATH_MSG_DEBUG("UseCut max = " << m_useMax_vec );
  ATH_MSG_DEBUG("varCut min = " << m_varMin_vec );
  ATH_MSG_DEBUG("varCut max = " << m_varMax_vec );
  ATH_MSG_DEBUG("LegA       = " << m_legA_vec );
  ATH_MSG_DEBUG("LegB       = " << m_legB_vec );
 
  ATH_CHECK( m_monTool_vec.retrieve() );
 
  if (m_legA_vec.size() != m_legB_vec.size()) {
    ATH_MSG_ERROR("Trying to configure the Tool with legA and legB vectors of different size!");
    return StatusCode::FAILURE;
  }
  if (m_useMin_vec.size() != m_useMax_vec.size()) {
    ATH_MSG_ERROR("Trying to configure the Tool with UseMin and UseMax vectors of different size!");
    return StatusCode::FAILURE;
  }
  if (m_legA_vec.size() != m_useMax_vec.size()) {
    ATH_MSG_ERROR("Trying to configure the Tool with legA/B and UseMax/Min vectors of different size!");
    return StatusCode::FAILURE;
  }
  if (m_varTag_vec.size() != m_useMax_vec.size()) {
    ATH_MSG_ERROR("Trying to configure the Tool with varTag and UseMax/Min(LegA/B) vectors of different size!");
    return StatusCode::FAILURE;
  }
 
  for (size_t i=0; i<m_varTag_vec.size(); ++i){
    VarInfo info;
    info.index = i;
    if(!m_monTool_vec.empty()) {
      info.monToolName = m_monTool_vec[i].name();
    }
    if (VarMap.find(m_varTag_vec[i]) == VarMap.end()){
      ATH_MSG_ERROR("The variable is not present in the ComboHypoVars list");
      return StatusCode::FAILURE;
    }
    info.varTag = (m_varTag_vec[i]);
    info.var = VarMap.at(m_varTag_vec[i]);
    //
    info.useMin = m_useMin_vec[i];
    if(info.useMin) {info.varMin=m_varMin_vec[i];}
    info.useMax = m_useMax_vec[i];
    if(info.useMax) {info.varMax=m_varMax_vec[i];}
    //
    info.legA = m_legA_vec[i];
    info.legA_is_MET = m_isLegA_MET_vec[i];
    info.legB = m_legB_vec[i];
    info.legB_is_MET = m_isLegB_MET_vec[i];
    info.legsAreEqual = info.legA==info.legB;
 
    std::string validmsg{""};
    if(!info.validate(validmsg)) {
      ATH_MSG_ERROR(validmsg);
      return StatusCode::FAILURE;
    }
 
    m_varInfo_vec.push_back(std::move(info));
  }
  ATH_MSG_DEBUG("Initialization completed successfully");
  
  return StatusCode::SUCCESS;
}
 
 
StatusCode TrigComboHypoTool::decide(Combo::LegDecisionsMap& passingLegs, const EventContext& /*context*/) const {
 
  // if no combinations passed, then exit 
  if (passingLegs.empty()) {
    return StatusCode::SUCCESS;
  }
 
  ATH_MSG_DEBUG("Looking for legs from " << decisionId() << " in the map. Map contains features for " << passingLegs.size() << " legs, which may be data for many chains.");
  for(const auto& legpair : passingLegs) {
    ATH_MSG_DEBUG("  Leg " << legpair.first << " has " << legpair.second.size() << " features");
  }
 
  // select the leg decisions from the map with this ID:
  std::vector<Combination> legDecisions;
  ATH_CHECK(selectLegs(passingLegs, legDecisions));
 
  // Track if we have at least 2 objects on the target legs that can be used for variable computation
  bool hasViableLegs{true};
  // Determine the functional leg multiplicities for combinations to generate
  std::vector<size_t> legMultiplicityForComputation(legMultiplicity().size(),0);
  if (m_skipLegCheck) {
    // Handle the case where there is exactly one leg and hence the chain ID is used
    // This implies a multiplicity of 2
    legMultiplicityForComputation[0] = 2;
    hasViableLegs = legDecisions[0].size() >= 2;
  } else {
    for (const VarInfo& varInfo : m_varInfo_vec){
      ATH_MSG_DEBUG("Var " << varInfo.varTag << " needs legs " << varInfo.legA << ", " << varInfo.legB);
      
      // Assess the leg decisions and extract the relevant ones
      if (passingLegs.contains(varInfo.legA) && passingLegs.contains(varInfo.legB)) {
        size_t goodLegA{false}, goodLegB{false};
        int32_t iLegA = getIndexFromLeg(varInfo.legA);
        int32_t iLegB = getIndexFromLeg(varInfo.legB);
        if ((iLegA<0) or (iLegB<0)){
          ATH_MSG_ERROR("TrigComboHypoTool::decide: Index into array is negative");
          return StatusCode::FAILURE;
        }
        goodLegA = !passingLegs[varInfo.legA].empty();
        legMultiplicityForComputation[iLegA] = std::max<size_t>(1,legMultiplicityForComputation[iLegA]);
        ATH_MSG_DEBUG("Leg " << varInfo.legA << " has " << passingLegs[varInfo.legA].size() << " features --> " << (goodLegA ? "pass" : "fail"));
        if(varInfo.legB != varInfo.legA) {
          goodLegB = !passingLegs[varInfo.legB].empty();
          ATH_MSG_DEBUG("Leg " << varInfo.legB << " has " << passingLegs[varInfo.legB].size() << " features --> " << (goodLegB ? "pass" : "fail"));
          legMultiplicityForComputation[iLegB] = std::max<size_t>(1,legMultiplicityForComputation[iLegB]);
        } else {
          goodLegB = goodLegA = passingLegs[varInfo.legA].size() >= 2;
          ATH_MSG_DEBUG("Leg " << varInfo.legA << " has " << passingLegs[varInfo.legA].size() << " features --> " << (goodLegB ? "pass" : "fail"));
          // If we do a computation on the same leg, we need to generate a pair of objects here
          legMultiplicityForComputation[iLegA] = std::max<size_t>(2,legMultiplicityForComputation[iLegA]);
        }
        hasViableLegs &= (goodLegA && goodLegB);
        if (!hasViableLegs) {
          ATH_MSG_DEBUG("Did not find at least 2 features on the target legs to compute " << varInfo.varTag);
        }
      } else {
        ATH_MSG_DEBUG(
          "Insufficient passing legs to compute " << varInfo.varTag
          << ", intended on (" << varInfo.legA << ", " << varInfo.legB << ")"
        );
        hasViableLegs = false;
      }
    }
  }
 
  if (!hasViableLegs) {
    ATH_MSG_DEBUG("This ComboHypoTool cannot run in this event, this chain **REJECTS** this event.");
    eraseFromLegDecisionsMap(passingLegs);
    if (msgLvl(MSG::DEBUG)) printDebugInformation(passingLegs);
    return StatusCode::SUCCESS;
  }
 
  // Create and initialise the combinations generator for the requirements of this chain, given the objects available in this event.
  // Extract the features on legs not used for the decision, so they stay in the navigation
  Combination extraLegs;
  HLT::NestedUniqueCombinationGenerator nucg;
  for (size_t legindex = 0; size_t legmult : legMultiplicityForComputation){
    size_t out_of = legDecisions[legindex].size();
    if(legmult==0) {
      extraLegs.insert(extraLegs.end(),legDecisions[legindex].cbegin(),legDecisions[legindex].cend());
    } else {
      nucg.add({out_of, legmult});
      ATH_MSG_DEBUG("For leg index " << legindex << " we will be choosing any " << legmult << " Decision Objects out of " << out_of);
    }
    ++legindex;
  }
 
  // Iterate over all variable computations
  std::vector<Combination> passingCombinations;
  std::vector<float> values;
  values.reserve(m_varInfo_vec.size());
  size_t warnings = 0, iterations = 0;
  // Correct for the legs on which we compute with 2 features
  auto get_index_offset = [&legMultiplicityForComputation](size_t legindex) {
    size_t offset{0};
    for (auto iLeg=legMultiplicityForComputation.cbegin(); iLeg!=legMultiplicityForComputation.cbegin()+legindex; ++iLeg) {
      offset += (*iLeg)-1;
    }
    return offset;
  };
  do {
    bool lastDecision(true);
    const std::vector<size_t> combination = nucg();
    ++nucg;
    ++iterations;
    values.clear();
 
    // This collects all the features contributing to any variable computation
    Combination combinationToRecord;
    for (auto iVarInfo = m_varInfo_vec.cbegin(); iVarInfo!=m_varInfo_vec.cend() && lastDecision; ++iVarInfo){
      // Just the features for the current variable evaluation
      Combination combinationToCheck;
 
      size_t legA_index = 0;
      size_t legB_index = 0;
 
      // For 1-leg chain, the legID is invalid
      if (!m_skipLegCheck) {
        legA_index = getIndexFromLeg(iVarInfo->legA);
        legB_index = getIndexFromLeg(iVarInfo->legB);
      }
 
      ATH_MSG_DEBUG(
        "Computing " << iVarInfo->varTag << " on legs "
        << iVarInfo->legA << " (" << legA_index << "), "
        << iVarInfo->legB << " (" << legB_index << ")"
      );
      if(iVarInfo->legA==iVarInfo->legB) {
        // 2 objects on 1 leg
        // Due to multiplicity checks, a computation like 'dRAA' never overlaps with one like 'dRAB'
        Combination featurePair = {legDecisions[legA_index][combination.at(legA_index+get_index_offset(legA_index))],legDecisions[legA_index][combination.at(legA_index+get_index_offset(legA_index)+1)]};
        combinationToCheck.insert(combinationToCheck.end(),featurePair.cbegin(),featurePair.cend());
        combinationToRecord.insert(combinationToRecord.end(),featurePair.cbegin(),featurePair.cend());
      } else {
        // 1 object each on 2 legs
        Combination featurePair = {legDecisions[legA_index][combination.at(legA_index+get_index_offset(legA_index))],legDecisions[legB_index][combination.at(legB_index+get_index_offset(legB_index))]};
        combinationToCheck.insert(combinationToCheck.end(),featurePair.cbegin(),featurePair.cend());
        combinationToRecord.insert(combinationToRecord.end(),featurePair.cbegin(),featurePair.cend());
      }
 
      try {
        lastDecision = executeAlgStep(combinationToCheck, *iVarInfo, values);
        ATH_MSG_DEBUG("Combination " << (iterations - 1) << " decided to be " <<  (lastDecision ? "passing" : "failing") << " " << iVarInfo->varTag);
      } catch (std::exception& e) {
        ATH_MSG_ERROR(e.what());
        return StatusCode::FAILURE;
      }
 
      if ((iterations >= m_combinationsThresholdWarn && warnings == 0) or (iterations >= m_combinationsThresholdBreak)) {
        ATH_MSG_WARNING("Have so far processed " << iterations << " combinations for " << decisionId() << " in this event, " << passingCombinations.size() << " passing.");
        ++warnings;
        if (iterations >= m_combinationsThresholdBreak) {
          ATH_MSG_WARNING("Too many combinations! Breaking the loop at this point.");
          break;
        }
      }
    }
 
    // Assess the collective decision on the combination
    if (lastDecision) {
      combinationToRecord.insert(combinationToRecord.end(),extraLegs.cbegin(),extraLegs.cend());
      passingCombinations.push_back(std::move(combinationToRecord));
      if (m_modeOR == true and m_enableOverride) {
        break;
      }
    } else { // the combination failed
      if (m_modeOR == false and m_enableOverride) {
        break;
      }
    }
 
    // Monitoring of variables for only accepted events
    if(lastDecision && !m_monTool_vec.empty()) {
      for (const VarInfo& varInfo : m_varInfo_vec) {
        float value = values[varInfo.index];
        auto varOfAccepted  = Monitored::Scalar(m_varTag_vec[varInfo.index]+"OfAccepted", value );//varInfo->monToolName+"OfAccepted", value );
        auto monitorIt      = Monitored::Group (m_monTool_vec[varInfo.index], varOfAccepted);
        ATH_MSG_VERBOSE( varInfo.varTag << " = " << value << " is in range " << varInfo.rangeStr() << ".");
        ATH_MSG_VERBOSE("m_varTag_vec = "<< m_varTag_vec<<", values = "<<values << ", valIndex = "<< varInfo.index <<", monToolName = " << varInfo.monToolName << ", monToolVec = "<< m_monTool_vec);
      }
    }
  } while (nucg);
 
  if (m_modeOR) {
 
    ATH_MSG_DEBUG("Passing " << passingCombinations.size() << " combinations out of " << iterations << ", " 
      << decisionId() << (passingCombinations.size() ? " **ACCEPTS**" : " **REJECTS**") << " this event based on OR logic.");
 
    if (m_enableOverride) {
      ATH_MSG_DEBUG("Note: stopped after the first successful combination due to the EnableOverride flag.");  
    }
 
  } else {  // modeAND
 
    const bool passAll = (passingCombinations.size() == iterations);
 
    ATH_MSG_DEBUG("Passing " << passingCombinations.size() << " combinations out of " << iterations << ", " 
      << decisionId() << (passAll ? " **ACCEPTS**" : " **REJECTS**") << " this event based on AND logic.");
 
    if (m_enableOverride) {
      ATH_MSG_DEBUG("Note: stopped after the first failed combination due to the EnableOverride flag.");  
    }
 
    if (not passAll) {
      passingCombinations.clear();
    }
 
  }
 
  if (not passingCombinations.empty()) { // need partial erasure of the decsions (only those not present in any combination)
    updateLegDecisionsMap(passingCombinations, passingLegs);
  } else { // need complete erasure of input decisions
    eraseFromLegDecisionsMap(passingLegs);
  }
 
  if (msgLvl(MSG::DEBUG)) printDebugInformation(passingLegs);
  return StatusCode::SUCCESS;
}
 
 
bool TrigComboHypoTool::executeAlgStep(const Combination& combination, const VarInfo& varInfo, std::vector<float> &vals) const {
  ATH_MSG_DEBUG("Executing selection " << varInfo.index << " of " << m_varInfo_vec.size() << ": " << varInfo.rangeStr());
 
  std::pair<KineInfo,KineInfo> kinepair;
  if(!fillPairKinematics(kinepair, combination, varInfo)) {
    ATH_MSG_ERROR("Failed to extract kinematics of feature pair!");
    return false;
  }
 
  if(msgLvl(MSG::VERBOSE)) {
    float eta_check, phi_check, pt_check;
    std::tie(eta_check,phi_check,pt_check) = kinepair.first;
    msg() << MSG::VERBOSE << "  Test filled legA kinematics: pt " << pt_check*invGeV << ", eta " << eta_check << ", phi " << phi_check << endmsg;
 
    std::tie(eta_check,phi_check,pt_check) = kinepair.second;
    msg() << MSG::VERBOSE << "  Test filled legB kinematics: pt " << pt_check*invGeV << ", eta " << eta_check << ", phi " << phi_check << endmsg;
  }
 
  // apply the cut
  float value = compute(kinepair,varInfo.var);
  if(!m_monTool_vec.empty()) {
    auto varOfProcessed = Monitored::Scalar(m_varTag_vec[varInfo.index]+"OfProcessed"  , value );
    auto monitorIt      = Monitored::Group (m_monTool_vec[varInfo.index], varOfProcessed);
  }
  vals.push_back(value);
  bool pass = varInfo.test(value);
 
  ATH_MSG_DEBUG("  Found a combination with " << value);
  if(!pass) {
    ATH_MSG_DEBUG("  Combination failed var cut: " << varInfo.varTag << " = " << value << " not in range " << varInfo.rangeStr());
  }
  return pass;
}
 

bool testLegId(const Combo::LegDecision& d, uint32_t targetleg) {
  auto combId = HLT::Identifier(d.first);
  if(!TrigCompositeUtils::isLegId(combId)) return false;
  return combId.numeric() == targetleg;
}
 
 
bool TrigComboHypoTool::fillLegDecisions_sameLeg(std::pair<Combo::LegDecision,Combo::LegDecision>& legpair, const Combination& combination, uint32_t leg) const {
  Combination leg_features;
  if(m_skipLegCheck) {
    // If there is only one leg, the decision IDs don't have a leg ID
    std::copy(combination.begin(),combination.end(),std::back_inserter(leg_features));
  } else {
    // Extract the features matching the legs
    // We take all of them, so as to be able to check if there is any ambiguity
    auto isMyLeg = [&leg](const Combo::LegDecision& d) { return testLegId(d,leg); };
    std::copy_if(combination.begin(),combination.end(),std::back_inserter(leg_features),isMyLeg);
  }
 
  if (leg_features.size()==2) {
    legpair.first = leg_features[0];
    legpair.second = leg_features[1];
  } else {
    ATH_MSG_ERROR(leg_features.size() << " Decision Objects supplied on leg " << leg
                  << ", must be 2 for same-leg topo selection!");
    return false;
  }
 
  return true;
}
 
 
bool TrigComboHypoTool::fillLegDecisions_diffLeg(std::pair<Combo::LegDecision,Combo::LegDecision>& legpair, const Combination& combination, uint32_t legA, uint32_t legB) const {
  // Extract the features matching the legs
  // We take all of them, so as to be able to check if there is any ambiguity
  auto isLegA = [&legA](const Combo::LegDecision& d) { return testLegId(d,legA); };
  auto isLegB = [&legB](const Combo::LegDecision& d) { return testLegId(d,legB); };
  Combination legA_features, legB_features;
 
  std::copy_if(combination.begin(),combination.end(),std::back_inserter(legA_features),isLegA);
  if(legA_features.size()!=1) {
    ATH_MSG_ERROR(legA_features.size() << " Decision Objects supplied on leg " << legA
                  << ", must be 1 for different-leg topo selection!");
    return false;
  }
 
  std::copy_if(combination.begin(),combination.end(),std::back_inserter(legB_features),isLegB);
  if (legB_features.size()!=1) {
    ATH_MSG_ERROR(legB_features.size() << " Decision Objects supplied on leg " << legB
                  << ", must be 1 for different-leg topo selection!");
    return false;
  }
 
  legpair.first = legA_features[0];
  legpair.second = legB_features[0];
 
  return true;
}
 
 
bool TrigComboHypoTool::fillPairKinematics(std::pair<KineInfo,KineInfo>& kinepair, const Combination& combination, const VarInfo& varInfo) const {
    ATH_MSG_VERBOSE("  Decision objects available = "<< combination);
    // Check that there are enough features
    size_t nFeatures(combination.size());
    if (nFeatures < 2){
      ATH_MSG_ERROR("Number of Decision Objects passed is less than 2! Sum over decision objects on all legs = " << combination.size() );
      return false;
    }
    std::pair<Combo::LegDecision,Combo::LegDecision> legpair;
    if (varInfo.legsAreEqual) {fillLegDecisions_sameLeg(legpair,combination,varInfo.legA);}
    else {fillLegDecisions_diffLeg(legpair,combination,varInfo.legA,varInfo.legB);}
    ATH_MSG_VERBOSE("    Fill leg A kinematics");
    if(!fillKineInfo(kinepair.first,legpair.first,varInfo.legA_is_MET)) {
      ATH_MSG_ERROR("Failed to extract requisite kinematic info from leg " << varInfo.legA << "!");
      return false;
    }
    ATH_MSG_VERBOSE("    Fill leg B kinematics");
    if(!fillKineInfo(kinepair.second,legpair.second,varInfo.legB_is_MET)) {
      ATH_MSG_ERROR("Failed to extract requisite kinematic info from leg " << varInfo.legB << "!");
      return false;
    }
    return true;
  }
 
 
bool TrigComboHypoTool::fillKineInfo(TrigComboHypoTool::KineInfo& kinematics, Combo::LegDecision decision, bool isMET) const {
  float eta, phi, pt;
  if (isMET) {
    auto pLink = TrigCompositeUtils::findLink<xAOD::TrigMissingETContainer>( *decision.second, featureString() ).link;
    if (!pLink.isValid()){
      ATH_MSG_ERROR("link for MET not valid");
      return false;
    }
    ROOT::Math::XYVectorF metv((*pLink)->ex(),(*pLink)->ey());
    eta = FLOATDEFAULT;
    phi = metv.phi();
    pt  = metv.r();
  } else {
    auto pLink = TrigCompositeUtils::findLink<xAOD::IParticleContainer>( *decision.second, featureString() ).link;
    if (!pLink.isValid()){
      ATH_MSG_ERROR("link for IParticle not valid");
      return false;
    }
    eta = (*pLink)->p4().Eta();
    phi = (*pLink)->p4().Phi();
    pt  = (*pLink)->p4().Pt();
  }
  ATH_MSG_VERBOSE("      Filled kinematics with pt " << pt*invGeV << ", eta " << eta << ", phi " << phi);
  kinematics = std::make_tuple(eta,phi,pt);
  return true;
}
 
 
float TrigComboHypoTool::compute(const std::pair<KineInfo,KineInfo>& kinepair, ComboHypoVars var) const {
  const auto& [legA_kine,legB_kine] = kinepair;
  const auto& [eta1,phi1,pt1] = legA_kine;
  const auto& [eta2,phi2,pt2] = legB_kine;
 
  ATH_MSG_DEBUG("    Leg A has pt " << pt1*invGeV << ", eta " << eta1 << ", phi " << phi1);
  ATH_MSG_DEBUG("    Leg B has pt " << pt2*invGeV << ", eta " << eta2 << ", phi " << phi2);
 
  float value(0);
  switch(var) {
    case ComboHypoVars::DR:
      {
        value = xAOD::P4Helpers::deltaR(eta1,phi1,eta2,phi2);
        break;
      }
    case ComboHypoVars::DPHI:
      {
        value = std::fabs(xAOD::P4Helpers::deltaPhi(phi1,phi2));
        break;
      }
    case ComboHypoVars::INVM:
      {
        ROOT::Math::PtEtaPhiMVector p1(pt1,eta1,phi1,0.), p2(pt2,eta2,phi2,0.);
        value = (p1+p2).M()*invGeV; // Convert to GeV
        break;
      }
    case ComboHypoVars::MT:
      {
    //Transverse mass defined for semi-visible decays in hadron colliders. See PDG 49.6, section on kinematics
    value = std::sqrt(2*pt1*pt2*(1-std::cos(xAOD::P4Helpers::deltaPhi(phi1,phi2) ) ) )*invGeV; // Convert to GeV
        break;
      }
    case ComboHypoVars::DETA:
      {
        value = std::fabs(eta2-eta1);
        break;
      }
    default:
      {
        ATH_MSG_ERROR("Undefined variable requested -- should never happen!");
      }
  }
  return value;
}