coarse collision returns colliding bodies in deterministic order

src/pe/ccd/HashGrids.h

@@ -492,6 +492,10 @@ void HashGrids::HashGrid::processBodies( BodyID* bodies, size_t bodyCount, Conta
 template< typename Contacts >  // Contact container type
 void HashGrids::collide( BodyID a, BodyID b, Contacts& contacts )
 {
+   //make sure to always check in the correct order (a<b)
+   if (a->getSystemID() > b->getSystemID())
+      std::swap(a, b);
+
    if( ( !a->hasInfiniteMass() || !b->hasInfiniteMass() ) &&        // Ignoring contacts between two fixed bodies
        ( a->getAABB().intersects( b->getAABB() ) ) )  // Testing for overlapping bounding boxes
    {

src/pe/ccd/SimpleCCD.cpp

@@ -57,14 +57,26 @@ PossibleContacts& SimpleCCD::generatePossibleContacts( WcTimingTree* tt ){
 
    if (tt != NULL) tt->start("SimpleCCD");
    for (auto it1 = bodies_.begin(); it1 != bodies_.end(); ++it1){
-      for (auto it2 = it1 + 1; it2 !=bodies_.end(); ++it2){
+      for (auto it2 = it1 + 1; it2 !=bodies_.end(); ++it2)
+      {
          if (!((*it1)->hasInfiniteMass() && (*it2)->hasInfiniteMass()))
-            contacts_.push_back(std::make_pair(*it1, *it2));
+         {
+            if ( (*it1)->getSystemID() > (*it2)->getSystemID() )
+               contacts_.push_back(std::make_pair(*it2, *it1));
+            else
+               contacts_.push_back(std::make_pair(*it1, *it2));
+         }
       }
 
-      for (auto it2 = globalStorage_.begin(); it2 != globalStorage_.end(); ++it2){
+      for (auto it2 = globalStorage_.begin(); it2 != globalStorage_.end(); ++it2)
+      {
          if (!((*it1)->hasInfiniteMass() && (*it2)->hasInfiniteMass()))
-            contacts_.push_back(std::make_pair(*it1, *it2));
+         {
+            if ( (*it1)->getSystemID() > (*it2)->getSystemID() )
+               contacts_.push_back(std::make_pair(*it2, *it1));
+            else
+               contacts_.push_back(std::make_pair(*it1, *it2));
+         }
       }
    }
    if (tt != NULL) tt->stop("SimpleCCD");

tests/pe/HashGrids.cpp

@@ -115,7 +115,7 @@ int main( int argc, char** argv )
                      iron, true, false, true);
 
     syncShadowOwners<BodyTuple>( forest->getBlockForest(), storageID);
-    for (int i=0; i < 100; ++i){
+    for (int step=0; step < 100; ++step){
        cr( real_c(0.1) );
        syncShadowOwners<BodyTuple>( forest->getBlockForest(), storageID);
 
@@ -132,6 +132,13 @@ int main( int argc, char** argv )
           WALBERLA_LOG_DETAIL_ON_ROOT("tracked particles: " << sccd->getObservedBodyCount() << "/" << hccd->getObservedBodyCount());
           WALBERLA_CHECK_EQUAL(tmp1, tmp2);
           WALBERLA_LOG_DETAIL_ON_ROOT("contacts on root: " << cont1.size());
+
+          // check for correct ordering of bodies within contacts
+          for (size_t i = 0; i < cont1.size(); ++i)
+          {
+             WALBERLA_CHECK_LESS(cont1[i].getBody1()->getSystemID(), cont1[i].getBody2()->getSystemID());
+             WALBERLA_CHECK_LESS(cont2[i].getBody1()->getSystemID(), cont2[i].getBody2()->getSystemID());
+          }
        }
     }