ComboHypo.cxx

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/*
  Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
*/
 
#include "DecisionHandling/ComboHypo.h"
#include "TrigCompositeUtils/HLTIdentifier.h"
#include "TrigSteeringEvent/TrigRoiDescriptorCollection.h"
#include "AthViews/View.h"
#include <sstream>
 
using namespace TrigCompositeUtils;
 
ComboHypo::ComboHypo(const std::string& name, ISvcLocator* pSvcLocator) 
: ::AthReentrantAlgorithm(name, pSvcLocator)
{}
 
 
ComboHypo::~ComboHypo()
{}
 
 
StatusCode ComboHypo::initialize() {
 
  ATH_CHECK( m_outputs.initialize() );
  ATH_CHECK( m_inputs.initialize() );
  ATH_CHECK( m_inputs.size() == m_outputs.size() );
 
  if (m_hypoTools.size()>0) {
    ATH_CHECK(m_hypoTools.retrieve());
  }
 
  for (auto& tool: m_hypoTools ) {
    ATH_CHECK(tool->setLegMultiplicity(m_multiplicitiesReqMap));
  }
  
  // find max inputs size
  auto maxMultEl = std::max_element( m_multiplicitiesReqMap.begin(), m_multiplicitiesReqMap.end(),  
    []( const Combo::MultiplicityReqMap::value_type& a, const Combo::MultiplicityReqMap::value_type& b ){
      return a.second.size() < b.second.size();
    });
  
  const size_t maxMult = maxMultEl->second.size();
 
  if (msgLvl(MSG::DEBUG)){
    ATH_MSG_DEBUG( "with these inputs:");
    for (const auto& inp : m_inputs) {
      ATH_MSG_DEBUG("-- "<< inp.key());
    }
    ATH_MSG_DEBUG( "with this multiplicity map:");
    for ( const auto& m : m_multiplicitiesReqMap ) {
      ATH_MSG_DEBUG("-- " << m.first
        << " multiplicities: " << m.second 
        <<", input-collection-indicies: " << m_legToInputCollectionMap[m.first]);
    }
  }
 
  ATH_CHECK( m_multiplicitiesReqMap.size() != 0 );
 
  bool errorOccured = false;
  if (m_checkMultiplicityMap) {
    for ( const auto& [key, value] : m_multiplicitiesReqMap ) {
      // Check the size of the multiplicities-required-per-leg vector is the same as the input-collection-index-per-leg vector
      Combo::LegMap::const_iterator it_input = m_legToInputCollectionMap.find(key);
      if (it_input == m_legToInputCollectionMap.end()) {
        ATH_MSG_ERROR(key << " was not registered in the LegToInputCollectionMap");
        errorOccured = true;
      } else if (value.size() != it_input->second.size()) {
        ATH_MSG_ERROR("Inconsistent configuration vector sizes for " << key << " Multiplicities Required size:" << value.size() 
          << ", Input Collections Index size:" << it_input->second.size());
        errorOccured = true;
      }
      // TimM Jult 21: What is this check doing?
      if ( value.size() > maxMult )  {
        errorOccured =  true;
        ATH_MSG_ERROR( "Chain " << key
          << " configured with input multiplicity " << value.size() << " like this: " << value
          << " which is lower than for this chain " << maxMultEl->first << " " << maxMult);
      }
    }
  }
 
  return ( errorOccured ? StatusCode::FAILURE : StatusCode::SUCCESS );
}
 
 
StatusCode ComboHypo::copyDecisions( const Combo::LegDecisionsMap & passingLegs, const EventContext& context ) const {
 DecisionIDContainer passing;
  for (auto const& element : passingLegs) {
    passing.insert(element.first);
  }
  
  ATH_MSG_DEBUG( "Copying "<<passing.size()<<" positive decision IDs to outputs");
 
  for ( size_t input_counter = 0; input_counter < m_inputs.size(); ++input_counter ) {
    // new output decisions
    SG::WriteHandle<DecisionContainer> outputHandle = createAndStore(m_outputs.at(input_counter), context ); 
    auto outDecisions = outputHandle.ptr();    
    auto inputHandle = SG::makeHandle( m_inputs.at(input_counter), context );
    if ( inputHandle.isValid() ) {
 
      for (const Decision* inputDecision : *inputHandle) {
        auto thisEL = TrigCompositeUtils::decisionToElementLink(inputDecision, context);
 
        // from all positive decision in the input only the ones that survived counting are passed over
        const DecisionIDContainer& common = passedDecisionIDs(inputDecision, passing);
 
        // check if this EL is in the combination map for the passing decIDs:
        ATH_MSG_DEBUG("Searching this element in the map: ("<<thisEL.dataID() << " , " << thisEL.index()<<")");
        DecisionIDContainer finalIds;   
        for (const DecisionID c : common){
          const HLT::Identifier cID = HLT::Identifier(c);
          // add the decID only if this candidate passed the combination selection
          const std::vector<ElementLink<DecisionContainer>>& Comb=passingLegs.at(c);
          if(std::find(Comb.begin(), Comb.end(), thisEL) == Comb.end()) {
            continue;
          }
          ATH_MSG_DEBUG("  Adding "<< cID <<" because EL is found in the passingLegs map");
          finalIds.insert( cID.numeric() ); // all Ids used by the Filter, including legs
          if (TrigCompositeUtils::isLegId ( cID )){
            const HLT::Identifier mainChain = TrigCompositeUtils::getIDFromLeg( cID );
            finalIds.insert( mainChain.numeric() );
            ATH_MSG_DEBUG("  Adding "<< mainChain <<" consequently");
          }
        }
 
        Decision* newDec = newDecisionIn( outDecisions, inputDecision, comboHypoAlgNodeName(), context );
        ATH_MSG_DEBUG("New decision (Container Index:" << input_counter << ", Element Index:"<< newDec->index() <<") has "
          << (TrigCompositeUtils::findLink<TrigRoiDescriptorCollection>(newDec, initialRoIString())).isValid()
          << " valid initialRoI, "<< TrigCompositeUtils::getLinkToPrevious(newDec).size() <<" previous decisions and "<<finalIds.size()<<" decision IDs") ;   
        insertDecisionIDs( finalIds, newDec );
 
      }
    }
 
    if (msgLvl(MSG::DEBUG)) {
      ATH_MSG_DEBUG("Output Handle " << m_outputs.at(input_counter).key() << " with " << outputHandle->size() <<" Decision objects");
      for (const Decision* d : *outputHandle){
        DecisionIDContainer objDecisions;      
        decisionIDs( d, objDecisions );
        ATH_MSG_DEBUG("  Decision object #" << d->index() << " with " << objDecisions.size()<<" positive decision IDs");
        for (const TrigCompositeUtils::DecisionID id : objDecisions ) {
          ATH_MSG_DEBUG("   --- Passes: " << HLT::Identifier( id ));
        }  
      }
    }
  }
 
  return StatusCode::SUCCESS;
}
 
 
StatusCode ComboHypo::execute(const EventContext& context ) const {
  ATH_MSG_DEBUG( "Executing " << name() << "..." );
  
  // it maps decidionID to the combinations (list of dec object) that passed that ID
  Combo::LegDecisionsMap dmap;
  ATH_CHECK( fillDecisionsMap( dmap, context ) );
 
  //this is added for saving good combinations for the hypocombo tools
  Combo::LegDecisionsMap passingLegs;
 
 
  // loop over all chains in the mult-map
  for ( const auto& m : m_multiplicitiesReqMap ) { 
    const HLT::Identifier chainId = HLT::Identifier(m.first);
    const std::vector<int>& multiplicityPerLeg = m.second;
    const DecisionID requiredDecisionID = chainId.numeric();
 
    DecisionIDContainer allDecisionIds;
    allDecisionIds.insert(requiredDecisionID);
 
    bool overallDecision = true;
 
    std::vector< SG::SGKeySet > legFeatureHashes; 
    legFeatureHashes.resize( multiplicityPerLeg.size() );
    SG::sgkey_t passthroughCounter = 0; 
 
    Combo::LegDecisionsMap thisChainCombMap;
 
    // This map records the history for any given feature.
    // E.g. for a key corresponding to a 4th Step muon, the entries in the payload std::set will be the 3rd step, 2nd step and 1st step
    // features from within the same ROI.
    //
    // Using this information the combo hypo is able to deduce that a 4th-step-muon and its prior 2nd-step-muon should not be considered as two distinct candidates.
    //
    // Such a check is needed when implementing tag-and-probe style chains as when performing the probe processing we will be comparing 1st, 2nd, 3rd,. etc. 
    // step features on the probe leg to final-step features on the tag leg.
    SG::SGKeyMap<std::set<uint32_t>> priorFeaturesMap;
 
    // Check multiplicity of each leg of this chain
    for ( size_t legIndex = 0; legIndex <  multiplicityPerLeg.size(); ++legIndex ) {
      const size_t requiredMultiplicity =  multiplicityPerLeg.at( legIndex );
 
      HLT::Identifier legId = chainId;
      // If there is only one leg, then we just use the chain's name.
      if (multiplicityPerLeg.size() > 1) {
        ATH_MSG_DEBUG(chainId << " has multiplicityPerLeg.size() > 1, so we use legXXX_HLT_YYY, instead of HLT_YYY");
        legId = TrigCompositeUtils::createLegName(chainId, legIndex);
      }
 
      const DecisionID requiredDecisionIDLeg = legId.numeric();
      ATH_MSG_DEBUG("Container " << legIndex << ", looking at leg : " << legId );
 
      Combo::LegDecisionsMap::const_iterator it = dmap.find(requiredDecisionIDLeg);
      if ( it == dmap.end() ) {
        continue;
      }
 
      // Check the total number of decision objects we have available on this leg from which to satisfy its multiplicity requirement
      const size_t nLegDecisionObjects = it->second.size();
 
      ATH_MSG_DEBUG( "Will attempt to meet the required multiplicity of " << requiredMultiplicity  << " for leg " << legId 
        << " with " << nLegDecisionObjects << " Decision Objects in leg " << legIndex << " of " << legId);
 
      // We extract unique passing features on the leg, if there are no features yet - then the L1 ROI is used as a fall-back feature
      // A special behaviour is that if this fall-back activates, and the L1 ROI is a full-scan ROI, then the leg is assumed valid. 
      // This is regardless of whether or not other legs also use the same FS ROI. Regardless of if the leg's required multiplicity.
      // This keeps the leg alive until the actual FS reconstruction may occur. At which point, the following ComboHypo will begin
      // to cut on the actual reconstructed physics objects.
      //
      // The behaviour may also be kept even after we have started to process a leg through HypoAlgs. This is done by the HypoAlg
      // setting the "feature" to be the same as the initialRoI. The initialRoI must still be a FullScan ROI for this to work.
      //
      // Finally, the same behaviour may also be triggered by the HypoAlg adding an an int32_t decoration called "noCombo" with value 1
      // to the Decision Object.
 
      for (const ElementLink<DecisionContainer>& dEL : it->second){
        SG::sgkey_t featureKey = 0; // The container hash of the DecisionObject's most-recent feature, and its initial ROI
        SG::sgkey_t roiKey = 0;
        Decision::index_type featureIndex = 0, roiIndex = 0; // The container index of the DecisionObject's most-recent feature, and its initial ROI
        bool roiIsFullscan = false; // Will be set to true if the DecisionObject's initial ROI is flagged as FullScan
        bool objectRequestsNoMultiplicityCheck = false; // Will be set to true if the object has been flagged as independently satisfying all requirements on a leg
        ATH_CHECK( extractFeatureAndRoI(it->first, dEL, featureKey, featureIndex, roiKey, roiIndex, roiIsFullscan, objectRequestsNoMultiplicityCheck, priorFeaturesMap) );
        const bool theFeatureIsTheROI = (SG::sgkeyEqual (featureKey, roiKey) and featureIndex == roiIndex); // The user explicitly set the feature === the RoI
        const bool thereIsNoFeatureYet = (featureKey == 0 and roiKey != 0); // The leg has not yet started to process
        if (objectRequestsNoMultiplicityCheck or (roiIsFullscan and (theFeatureIsTheROI or thereIsNoFeatureYet))) {
          // This passthroughCounter integer is being to generate unique "hash" values to allow Jets or FS ROI to meet the multiplicity requirements of this leg
          for (size_t i = 0; i < requiredMultiplicity; ++i) {
            legFeatureHashes.at( legIndex ).insert( ++passthroughCounter );
            ATH_MSG_DEBUG("  -- Add feature hash '" << passthroughCounter << "' to leg " << legIndex 
              << ". (Note: unique passing hash generated from " << (objectRequestsNoMultiplicityCheck ? "an object requesting NO multiplicity checks" : "an FullScan ROI") << ")");
          }
        } else {
          const SG::sgkey_t uniquenessHash = (featureKey != 0 ? (featureKey + featureIndex) : (roiKey + roiIndex)); 
          legFeatureHashes.at( legIndex ).insert( uniquenessHash );
          ATH_MSG_DEBUG("  -- Add feature hash '" << uniquenessHash << "' to leg " << legIndex << ".");
        }
      }
 
      // save combinations of all legs for the tools
      thisChainCombMap.insert (*it);
      allDecisionIds.insert(requiredDecisionIDLeg);
      
    } // end loop over legIndex
 
    // Up-cast any features which are actually earlier-step versions of other features (see priorFeaturesMap above)
    // to be considered as equivalent to the later step version, note that this is for combo uniqueness comparison purposes only.
    for (SG::SGKeySet& legHashes : legFeatureHashes) {
      // We will continue to up-cast a single feature at a time in each leg's set of features, until no more upcast opportunities are available.
      size_t emergencyBreak = 0;
      while (true) {
        bool somethingChanged = false;
        for (const SG::sgkey_t legHash : legHashes) {
          for (auto const& [key, payloadSet] : priorFeaturesMap) {
            if (payloadSet.count(legHash) == 1) {
              ATH_MSG_DEBUG("Feature hash=" << legHash << " identified as a prior feature of hash=" << key 
                << ", we will up-cast this hash to the later version for ComboHypo uniqueness comparison purposes.");
              legHashes.erase(legHash);
              legHashes.insert(key);
              // CAUTION we have mutated a set we're iterating over. We must now break out of the loop over legHashes. This requires two breaks.
              // This also lets the upcast cascade, i.e. what we insert as 'key' here could trigger another up-cast when considered as 'legHash' in the next iteration of the while(true) loop.
              somethingChanged = true;
              break;
            }
          } // End inner for loop (over priorFeaturesMap)
          if (somethingChanged) {
            break; // Break out of the outer loop when something changes, this avoids a continued invalid iteration over a mutated container.
          }
        } // End outer for loop (over legHashes)
        if (!somethingChanged or ++emergencyBreak == 500) {
          if (emergencyBreak == 500) {
            ATH_MSG_WARNING("ComboHypo emergency loop break activated!");
          }
          break; // Break out of the while(true) loop when all elements of legHashes have been checked and none were upcast
        }
      }
    } // Loop over the different legs
 
    // Remove any duplicated features which are shared between legs.
    // Keep the feature only in the leg which can afford to lose the least number of object, given its multiplicity requirement.
    // FIXME: The  order in which we iterate over allFeatureHashes can
    //        affect the trigger results.  But OK not to use SGKeySet here,
    //        since any duplicates will already have been removed
    //        in legFeatureHashes.
    std::set<SG::sgkey_t> allFeatureHashes;
    for (const SG::SGKeySet& legHashes : legFeatureHashes) {
      allFeatureHashes.insert(legHashes.begin(), legHashes.end());
    }
    for (const SG::sgkey_t featureHash : allFeatureHashes) {
      size_t legsWithHash = 0; 
      size_t keepLegIndex = 0; 
      int32_t keepLegMargin = std::numeric_limits<int32_t>::max(); 
      for (size_t legIndex = 0; legIndex <  multiplicityPerLeg.size(); ++legIndex) {
        if (legFeatureHashes.at(legIndex).count(featureHash) == 0) {
          continue;
        }
        ++legsWithHash;
        const int32_t requiredMultiplicity = multiplicityPerLeg.at(legIndex);
        const int32_t currentMultiplicity = legFeatureHashes.at(legIndex).size();
        const int32_t safetyMargin = currentMultiplicity - requiredMultiplicity; // Signed, if -ve then the chain has already been failed by the leg at legIndex
        if (safetyMargin < keepLegMargin) {
          keepLegMargin = safetyMargin;
          keepLegIndex = legIndex;
        }
      }
      if (legsWithHash == 1) {
        continue;
      }
      // If a feature is found on multiple legs, then remove it from all but the leg which can afford to lose it the least
      for (size_t legIndex = 0; legIndex <  multiplicityPerLeg.size(); ++legIndex) {
        if (legIndex == keepLegIndex) {
          ATH_MSG_DEBUG("Keeping feature hash '" << featureHash << "', on leg " << legIndex << ". This leg can least afford to lose it. "
            << "Leg has " << legFeatureHashes.at(legIndex).size() 
            << " features, and a multiplicity requirement of " << multiplicityPerLeg.at(legIndex));
          continue;
        }
        if (legFeatureHashes.at(legIndex).erase(featureHash)) {
          ATH_MSG_DEBUG("Removed duplicate feature hash '" << featureHash << "', from leg " << legIndex << ". Leg now has " << legFeatureHashes.at(legIndex).size() 
            << " remaining features, and a multiplicity requirement of " << multiplicityPerLeg.at(legIndex));
        }
      }
    }
 
    //check that the multiplicity of unique features is high enough
    for (size_t legIndex = 0; legIndex <  multiplicityPerLeg.size(); ++legIndex) {
      const size_t requiredMultiplicity = multiplicityPerLeg.at(legIndex);
      const size_t currentMultiplicity = legFeatureHashes.at(legIndex).size();
      if (currentMultiplicity < requiredMultiplicity) {
        ATH_MSG_DEBUG("Leg " << legIndex << " fails multiplicity check. Required unique features:" << requiredMultiplicity << ", found unique features: " << currentMultiplicity);
        overallDecision = false;
        break;
      }
    }
 
    //Overall chain decision
    ATH_MSG_DEBUG( "Chain " << chainId <<  ( overallDecision ? " is accepted" : " is rejected")  <<" after multiplicity requirements" );
    if ( overallDecision == true ) {
      for (auto decID: allDecisionIds) {
        // saving the good combinations
        passingLegs.insert (thisChainCombMap.begin(), thisChainCombMap.end());
        ATH_MSG_DEBUG("  Passing " << HLT::Identifier(decID)<<" after multiplicity test");
      }
    }      
  }
 
  if  (passingLegs.size()!=0){
    // launching the tools:
    for ( auto& tool: m_hypoTools ) {
      ATH_MSG_DEBUG( "Calling  tool "<<tool->name());
      ATH_CHECK( tool->decide( passingLegs, context ) );
    }
  }
 
  // this is only for debug:
  if (msgLvl(MSG::DEBUG)){
    for (auto const& [id, decisions] : passingLegs) {
      ATH_MSG_DEBUG("" << (decisions.empty() ? "failing " : "passing "+std::to_string(decisions.size())+" decisions ") << id );
    }
  }
 
  // need to pass all combinations, since not all element pass the decID
  ATH_CHECK( copyDecisions( passingLegs, context ) );
 
  return StatusCode::SUCCESS;
}
 
 
StatusCode ComboHypo::extractFeatureAndRoI(const HLT::Identifier& chainLegId,
  const ElementLink<DecisionContainer>& dEL,
  SG::sgkey_t& featureKey,
  Decision::index_type& featureIndex,
  SG::sgkey_t& roiKey,
  Decision::index_type& roiIndex,
  bool& roiIsFullscan,
  bool& objectRequestsNoMultiplicityCheck,
  SG::SGKeyMap<std::set<uint32_t>>& priorFeaturesMap) const 
{
  // Return collections for the findLinks call. 
  // While we will be focusing on the most recent feature, for tag-and-probe we need to keep a record of the features from the prior steps too.
 
  // Construct a sub-graph following just this leg back through the nav
  DecisionIDContainer chainLegIdSet = {chainLegId.numeric()};
  TrigCompositeUtils::NavGraph subGraph;
  recursiveGetDecisions((*dEL), subGraph, chainLegIdSet, /*enforceDecisionOnStartNode =*/ true);
 
  if (subGraph.finalNodes().size() != 1) {
    ATH_MSG_ERROR("We are only expecting to search from a single navigation node in extractFeatureAndRoI");
    return StatusCode::FAILURE;
  }
  const NavGraphNode* start = *(subGraph.finalNodes().begin());
 
  std::vector<SG::sgkey_t> keys;
  std::vector<uint32_t> clids; // We don't care about the class ID. This part gets ignored.
  std::vector<Decision::index_type> indicies;
  std::vector<const Decision*> sources;
 
  std::set<const Decision*> fullyExploredFrom; // This is a cache which typelessFindLinks will use to avoid re-visiting already explored regions of the graph
  // Note: This call to typelessFindLinks is exploring from a NavGraphNode* rather than a Decision*,
  // this indicates that the search is restricted to a sub-graph (specifically, only following one chain-leg)
  const bool foundFeature = typelessFindLinks(start, featureString(), keys, clids, indicies, sources, TrigDefs::allFeaturesOfType, &fullyExploredFrom);
 
  const Decision* featureSource = nullptr;
  // The "most recent" feature (from the step just run) is the one we find first. Hence it's at index 0
  if (foundFeature) {
    featureKey = keys.at(0);
    featureIndex = indicies.at(0);
    featureSource = sources.at(0);
  }
 
  objectRequestsNoMultiplicityCheck = (featureSource and featureSource->hasDetail<int32_t>("noCombo") and featureSource->getDetail<int32_t>("noCombo") == 1);
 
  if (foundFeature and priorFeaturesMap.count(featureKey + featureIndex) == 0) {
    const std::string* key_str = evtStore()->keyToString(featureKey);
    ATH_MSG_DEBUG("Note: Will use feature hash " << featureKey + featureIndex << ", for " << (key_str ? *key_str : "UNKNOWN") << " index=" << featureIndex);
    // Use the deep-search data to look further back than .at(0)
    // Here's where we keep the record of the features in previous steps. Step ordering is unimportant, we can use a set.
    if (keys.size() > 1) {
      for (size_t i = 1; i < keys.size(); ++i) { // Skip the 1st entry, this will be equal to featureKey and featureIndex from typelessFindLink above.
        // featureKey + featureIndex should be considered as equivalent to a per-feature hash (featureKey is a real hash, featureIndex is an offset index)
        if (featureKey + featureIndex == keys.at(i) + indicies.at(i)) {
          continue; // Do not add the case where a feature is re-attached to more than one step.
        }
        priorFeaturesMap[featureKey + featureIndex].insert(keys.at(i) + indicies.at(i));
      }
    } else { // Exactly one feature. Make a note of this by inserting an empty set, such that we don't do this search again.
      priorFeaturesMap.insert( std::pair<uint32_t, std::set<uint32_t>>(featureKey + featureIndex, std::set<uint32_t>()) );
    }
  }
 
  // Try and get seeding ROI data too.
  uint32_t roiClid{0}; // Unused
  const Decision* roiSource{nullptr}; // Unused
  const bool foundROI = typelessFindLink(subGraph, initialRoIString(), roiKey, roiClid, roiIndex, roiSource);
  if (foundROI) {
    ElementLink<TrigRoiDescriptorCollection> roiEL(roiKey, roiIndex);
    ATH_CHECK( roiEL.isValid() );
    roiIsFullscan = (*(roiEL))->isFullscan();
    if (!foundFeature) {
      const std::string* roi_str = evtStore()->keyToString(roiKey);
      ATH_MSG_DEBUG("Note: Located fallback-ROI, if used this will have feature hash =" << roiKey + roiIndex << ", for " << (roi_str ? *roi_str : "UNKNOWN") << " index=" << roiIndex);
    }
  }
 
  if (!foundFeature && !foundROI) {
    ATH_MSG_WARNING("Did not find the feature or initialRoI for " << dEL.dataID() << " index " << dEL.index());
  }
 
  return StatusCode::SUCCESS;
}
 
 
StatusCode ComboHypo::fillDecisionsMap( Combo::LegDecisionsMap &  dmap, const EventContext& context) const {
  for ( size_t inputContainerIndex = 0; inputContainerIndex < m_inputs.size(); ++inputContainerIndex ) {   
    auto inputHandle = SG::makeHandle( m_inputs.at(inputContainerIndex), context );
    if ( !inputHandle.isValid() ) {
      ATH_MSG_ERROR( "No input ReadHandle from " << inputHandle.key() );
      return StatusCode::FAILURE;
    }
    ATH_MSG_DEBUG( "-- Found ReadHandle from " << inputHandle.key() <<" with "<< inputHandle->size() << " elements:"  );
    for ( const Decision* decision : *inputHandle ) {
      ATH_MSG_DEBUG( "-- -- Input Decision #"<< decision->index() <<" with "<< decisionIDs( decision ).size() << " active IDs. Populating the multiplicity map:" );
      for ( const DecisionID id: decisionIDs( decision ) ) {
        HLT::Identifier chainID = HLT::Identifier(id);
        int32_t chainLeg = 0; // Assume initially that the chain is not multi-leg, and update these two values if it is.
        if (isLegId(id)) {
          chainID = getIDFromLeg(id);
          chainLeg = getIndexFromLeg(id);
        }
 
        // We need to check if we are configured to accept DecisionObjects passing 'chainID' ...
        Combo::LegMap::const_iterator it = m_legToInputCollectionMap.find(chainID.name());
        if (it == m_legToInputCollectionMap.end()) {
          ATH_MSG_VERBOSE("-- -- -- Ignoring the DecsionID " << id << " on leg " << chainLeg << " as it does not correspond to any of the " << m_legToInputCollectionMap.size() << " chains this Alg is processing.");
          continue;
        }
 
        // ... and if so we need to further check that we are accepting passing IDs for chainLeg on the current inputContainerIndex
        const std::vector<int>& legToInputCollectionIndex = it->second;
        const size_t requiredInputContainerIndex = static_cast<size_t>(legToInputCollectionIndex.at(chainLeg));
        if (requiredInputContainerIndex != inputContainerIndex) {
          ATH_MSG_VERBOSE("-- -- -- Ignoring the DecisionID " << id << " on leg " << chainLeg << " as we are only permitted to accept passing objects on leg #" << chainLeg << " of " << chainID.name()
            << " which come from input collection index " << requiredInputContainerIndex << " (which is " << m_inputs.at(requiredInputContainerIndex).key() << ")"
            << ". Not the current index " << inputContainerIndex << " (which is " << m_inputs.at(inputContainerIndex).key() << ")");    
          continue;
        }
 
        ATH_MSG_DEBUG( " ++++ " << HLT::Identifier( id ) );
        dmap[id].push_back( TrigCompositeUtils::decisionToElementLink(decision, context) );
      }
    }
  }
 
  if (msgLvl(MSG::DEBUG)){
    ATH_MSG_DEBUG( "Decision map filled :" );
    size_t legCount = 0;
    for (const auto& entry: dmap){
      ATH_MSG_DEBUG("leg ["<<legCount<<"]: ");
      const std::vector<ElementLink<DecisionContainer>>& decisions = entry.second;
      ATH_MSG_DEBUG(" ++++ " << HLT::Identifier( entry.first ) <<" Number Decisions: "<< decisions.size());
      for (const ElementLink<DecisionContainer>& d : decisions){
        ATH_MSG_DEBUG("     Decision: (ContainerKey:"<<d.dataID()<<", DecisionElementIndex:"<<d.index()<<")");
      }
      legCount++;
    }
  }
 
 
  return StatusCode::SUCCESS;
}