Try to make random sampling for unbinding reproducible

src/config_parser.cpp

@@ -71,6 +71,7 @@ bool Parameter_t::TrySingleValueParameter(string ParameterName, stringstream &Pa
   TrySetPar(PeriodicBoundaryOn);
   TrySetPar(SnapshotHasIdBlock);
   TrySetPar(MaxPhysicalSofteningHalo);
+  TrySetPar(RandomSeedFactor);
   TrySetPar(TracerParticleBitMask);
 
 #undef TrySetPar
@@ -265,9 +266,9 @@ void Parameter_t::BroadCast(MpiWorker_t &world, int root)
   _SyncAtom(MaxSnapshotIndex, MPI_INT);
   _SyncReal(BoxSize);
   _SyncReal(SofteningHalo);
-  _SyncReal(MaxPhysicalSofteningHalo);
+  _SyncReal(MaxPhysicalSofteningHalo);  
   _SyncVecBool(IsSet);
-
+  _SyncAtom(RandomSeedFactor, MPI_HBT_INT);
   _SyncVec(SnapshotDirBase, MPI_CHAR);
   _SyncVec(SnapshotFormat, MPI_CHAR);
   _SyncVec(GroupFileFormat, MPI_CHAR);
@@ -419,6 +420,7 @@ void Parameter_t::DumpParameters()
   DumpPar(SourceSubRelaxFactor);
   DumpPar(RefineMostboundParticle);
   DumpPar(MaxSampleSizeOfPotentialEstimate);
+  DumpPar(RandomSeedFactor);
 
   DumpHeader("Subhalo Tracking");
   DumpPar(MinNumPartOfSub);

src/config_parser.h

@@ -78,7 +78,8 @@ class Parameter_t
                                 // leads to more accuracy mostbound particle
 
   int TracerParticleBitMask; /* Bitmask used to identify which particle type can be used as tracer */
-
+  HBTInt RandomSeedFactor; // Modifies the random number seed used for sampling
+
   /*derived parameters; do not require user input*/
   HBTReal TreeNodeOpenAngleSquare;
   HBTReal TreeNodeResolution;
@@ -124,7 +125,8 @@ class Parameter_t
     RefineMostboundParticle = true;
     GroupLoadedFullParticle = false;
     MaxPhysicalSofteningHalo = -1; // Indicates no max. physical softening is used.
-
+    RandomSeedFactor = 0; // If zero, just use particle IDs to seed random number generator
+
     /* Tracer-related parameters. If unset, only use collisionless particles (DM
      * + Stars) as tracer. Here we assume they correspond to particle types 1
      * and 4, respectively. */

src/subhalo_tracking.cpp

@@ -735,15 +735,15 @@ void SubhaloSnapshot_t::FeedCentrals(HaloSnapshot_t &halo_snap)
     // Subhalos.reserve(Snapshot->size()*0.1);//reserve enough	branches.......
     Subhalos.resize(Npro + MemberTable.NBirth);
   }
-#pragma omp for
+#pragma omp for ordered
   for (HBTInt hostid = 0; hostid < halo_snap.Halos.size(); hostid++)
   {
     MemberShipTable_t::MemberList_t &Members = MemberTable.SubGroups[hostid];
     auto &Host = halo_snap.Halos[hostid];
     if (0 == Members.size()) // create a new sub
     {
       HBTInt subid;
-#pragma omp critical(AddNewSub) // maybe consider ordered for here..
+#pragma omp ordered
       {
         subid = Npro++;
       }

src/subhalo_unbind.cpp

@@ -3,6 +3,7 @@
 #include <iostream>
 #include <new>
 #include <omp.h>
+#include <random>
 
 #include "datatypes.h"
 #include "gravity_tree.h"
@@ -314,8 +315,17 @@ void Subhalo_t::Unbind(const Snapshot_t &epoch)
   GravityTree_t tree;
   tree.Reserve(Particles.size());
   Nbound = Particles.size(); // start from full set
-  if (MaxSampleSize > 0 && Nbound > MaxSampleSize)
-    random_shuffle(Particles.begin(), Particles.end()); // shuffle for easy resampling later.
+  if (MaxSampleSize > 0 && Nbound > MaxSampleSize) {
+    // Initialize random number generator with first few particle IDs for reproducibility
+    int nr_seed_vals = min(Nbound, (HBTInt) 20);
+    vector<HBTInt> ids(nr_seed_vals);
+    for(int i=0; i<nr_seed_vals; i+=1)
+      ids[i] = Particles[i].Id ^ HBTConfig.RandomSeedFactor;
+    seed_seq seed(ids.begin(), ids.end());
+    mt19937 rng(seed);    
+    // shuffle for easy resampling later.
+    shuffle(Particles.begin(), Particles.end(), rng);    
+  }
   HBTInt Nlast;
 
   vector<ParticleEnergy_t> Elist(Nbound);