[PWGHF] TrackIndexSkimCreator: hoist collision-invariant slices out of the 2/3-prong loops (AliceO2Group#16666)

Author: ktf

PWGHF/TableProducer/trackIndexSkimCreator.cxx

@@ -83,6 +83,7 @@
 #include <cstdlib>
 #include <iterator> // std::distance
 #include <numeric>
+#include <optional>
 #include <string>  // std::string
 #include <utility> // std::forward
 #include <vector>  // std::vector
@@ -2400,6 +2401,9 @@ struct HfTrackIndexSkimCreator {
 
       // first loop over positive tracks
       const auto groupedTrackIndicesPos1 = positiveFor2And3Prongs->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache);
+      const auto groupedTrackIndicesNeg1 = negativeFor2And3Prongs->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache);
+      std::optional<decltype(positiveSoftPions->sliceByCached(aod::track::collisionId, 0, cache))> groupedTrackIndicesSoftPionsPos;
+      std::optional<decltype(negativeSoftPions->sliceByCached(aod::track::collisionId, 0, cache))> groupedTrackIndicesSoftPionsNeg;
       int lastFilledD0 = -1; // index to be filled in table for D* mesons
       for (auto trackIndexPos1 = groupedTrackIndicesPos1.begin(); trackIndexPos1 != groupedTrackIndicesPos1.end(); ++trackIndexPos1) {
         const auto trackPos1 = trackIndexPos1.template track_as<TTracks>();
@@ -2418,7 +2422,6 @@ struct HfTrackIndexSkimCreator {
         }
 
         // first loop over negative tracks
-        const auto groupedTrackIndicesNeg1 = negativeFor2And3Prongs->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache);
         for (auto trackIndexNeg1 = groupedTrackIndicesNeg1.begin(); trackIndexNeg1 != groupedTrackIndicesNeg1.end(); ++trackIndexNeg1) {
           const auto trackNeg1 = trackIndexNeg1.template track_as<TTracks>();
 
@@ -3198,8 +3201,10 @@ struct HfTrackIndexSkimCreator {
                                                                                                                                                                                                                         // if D* enabled and pt of the D0 is larger than the minimum of the D* one within 20% (D* and D0 momenta are very similar, always within 20% according to PYTHIA8)
             // second loop over positive tracks
             if (TESTBIT(whichHypo2Prong[kN2ProngDecays], 0) && (!config.applyKaonPidIn3Prongs || TESTBIT(trackIndexNeg1.isIdentifiedPid(), ChannelKaonPid))) { // only for D0 candidates; moreover if kaon PID enabled, apply to the negative track
-              auto groupedTrackIndicesSoftPionsPos = positiveSoftPions->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache);
-              for (auto trackIndexPos2 = groupedTrackIndicesSoftPionsPos.begin(); trackIndexPos2 != groupedTrackIndicesSoftPionsPos.end(); ++trackIndexPos2) {
+              if (!groupedTrackIndicesSoftPionsPos) {
+                groupedTrackIndicesSoftPionsPos.emplace(positiveSoftPions->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache));
+              }
+              for (auto trackIndexPos2 = groupedTrackIndicesSoftPionsPos->begin(); trackIndexPos2 != groupedTrackIndicesSoftPionsPos->end(); ++trackIndexPos2) {
                 if (trackIndexPos2 == trackIndexPos1) {
                   continue;
                 }
@@ -3235,8 +3240,10 @@ struct HfTrackIndexSkimCreator {
 
             // second loop over negative tracks
             if (TESTBIT(whichHypo2Prong[kN2ProngDecays], 1) && (!config.applyKaonPidIn3Prongs || TESTBIT(trackIndexPos1.isIdentifiedPid(), ChannelKaonPid))) { // only for D0bar candidates; moreover if kaon PID enabled, apply to the positive track
-              auto groupedTrackIndicesSoftPionsNeg = negativeSoftPions->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache);
-              for (auto trackIndexNeg2 = groupedTrackIndicesSoftPionsNeg.begin(); trackIndexNeg2 != groupedTrackIndicesSoftPionsNeg.end(); ++trackIndexNeg2) {
+              if (!groupedTrackIndicesSoftPionsNeg) {
+                groupedTrackIndicesSoftPionsNeg.emplace(negativeSoftPions->sliceByCached(aod::track::collisionId, collision.globalIndex(), cache));
+              }
+              for (auto trackIndexNeg2 = groupedTrackIndicesSoftPionsNeg->begin(); trackIndexNeg2 != groupedTrackIndicesSoftPionsNeg->end(); ++trackIndexNeg2) {
                 if (trackIndexNeg1 == trackIndexNeg2) {
                   continue;
                 }