Merge branch 'issue-Moore#538' into 'master'

More deterministic comparison for ProtoParticle Relations

Closes Moore#538

See merge request !3998

Event/RecEvent/include/Event/ProtoParticle.h

@@ -13,6 +13,7 @@
 // Include files
 #include "Event/CaloHypo.h"
 #include "Event/Track.h"
+#include "GaudiKernel/IRegistry.h"
 #include "GaudiKernel/KeyedContainer.h"
 #include "GaudiKernel/KeyedObject.h"
 #include "GaudiKernel/SmartRef.h"
@@ -749,3 +750,32 @@ inline int LHCb::ProtoParticle::charge() const { return ( track() != nullptr ? t
 namespace Gaudi::Parsers {
   StatusCode parse( std::vector<LHCb::ProtoParticle::additionalInfo>& v, const std::string& in );
 } // namespace Gaudi::Parsers
+
+namespace Relations {
+  template <typename>
+  struct ObjectTypeTraits;
+
+  // TODO: generalize Relations/Pointer.h to do this for all classes derived from KeyedObject<K>...
+  template <>
+  struct ObjectTypeTraits<LHCb::ProtoParticle> {
+    // ========================================================================
+    using Type   = LHCb::ProtoParticle;
+    using Input  = const LHCb::ProtoParticle*;
+    using Output = const LHCb::ProtoParticle*;
+    using Inner  = SmartRef<LHCb::ProtoParticle>;
+    struct Less {
+      bool operator()( const LHCb::ProtoParticle& lhs, const LHCb::ProtoParticle& rhs ) const {
+        // relations should only be between objects in the event store, hence there _should_ be a
+        // valid `parent()`, and that `parent()` should have a non-nullptr `registry()`.
+        return lhs.parent() == rhs.parent()
+                   ? ( lhs.key() < rhs.key() )
+                   : ( lhs.parent()->registry()->identifier() < rhs.parent()->registry()->identifier() );
+      }
+      bool operator()( const LHCb::ProtoParticle* lhs, const LHCb::ProtoParticle* rhs ) const {
+        // there should not be any nullptrs in the 'from' side of a relation table...
+        return ( *this )( *lhs, *rhs );
+      }
+    };
+    // ========================================================================
+  };
+} // namespace Relations