Modernize deterministic mixer code

• replace uint32_t state and free functions with a small struct
• define a single overload set for mixing
• be paranoid about implicit conversions of the 'extra' state
• use the above mentioned struct everywhere, and remove copies of the code
• add a unit test (which contains a copy of the original code to verify the results are the same)
• replace use of boost::integer_traits with std::numeric_limits

Kernel/LHCbAlgs/src/DeterministicPrescaler.cpp

@@ -8,19 +8,14 @@
 * granted to it by virtue of its status as an Intergovernmental Organization  *
 * or submit itself to any jurisdiction.                                       *
 \*****************************************************************************/
-
-// Gaudi
-#include "GaudiAlg/FilterPredicate.h"
-// from LHCb core
 #include "Event/ODIN.h"
-// STL
+#include "GaudiAlg/FilterPredicate.h"
+#include "LHCbMath/DeterministicMixer.h"
 #include <cmath>
 #include <cstdint>
 #include <limits>
 #include <string>
 
-#include "DeterministicPrescalerGenerator.h"
-
 /** @class DeterministicPrescaler DeterministicPrescaler.cpp
  *
  * @brief Return a positive decision for a configurable fraction of executions.
@@ -51,7 +46,7 @@ public:
 
   bool operator()( const LHCb::ODIN& odin ) const override {
     auto accept = [&]( const LHCb::ODIN& odin ) {
-      auto x = mix64( mix32( mix64( m_initial, odin.gpsTime() ), odin.runNumber() ), odin.eventNumber() );
+      auto x = LHCb::DeterministicMixer{m_initial}( odin.gpsTime(), odin.runNumber(), odin.eventNumber() ).state;
       if ( msgLevel( MSG::DEBUG ) )
         debug() << "  gpsTime: " << odin.gpsTime() << "  run#: " << odin.runNumber() << "  evt#: " << odin.eventNumber()
                 << "  -->  " << x << endmsg;
@@ -72,12 +67,12 @@ private:
                                                throw GaudiException( "Required property SeedName is empty", name(),
                                                                      StatusCode::FAILURE );
                                              }
-                                             m_initial = mixString( m_seed_name.size(), m_seed_name );
+                                             m_initial = LHCb::DeterministicMixer{m_seed_name};
                                            },
                                            "String used to seed pseudo-random number generation"};
 
   /// initial seed unique to this instance (computed from the name)
-  std::uint32_t m_initial{0};
+  LHCb::DeterministicMixer m_initial{0};
   /// integer representation of the above
   std::uint32_t m_acc{std::numeric_limits<uint32_t>::max()};