Add tests for empty RelationTable

Phys/FunctorCore/include/Functors/Functional.h

@@ -221,6 +221,8 @@ namespace Functors::Functional {
             using std::next;
             using std::make_pair;
 
+            if ( begin( range ) == end( range ) ) throw std::runtime_error( "Attempt to iterate over an empty range" );
+
             auto min_entry = std::accumulate(
                 next( begin( range ) ), end( range ),
                 make_pair( *begin( range ), functor( mask_arg, mask, *begin( range ), input... ) ),
@@ -264,6 +266,8 @@ namespace Functors::Functional {
             using std::next;
             using std::make_pair;
 
+            if ( begin( range ) == end( range ) ) throw std::runtime_error( "Attempt to iterate over an empty range" );
+
             auto max_entry = std::accumulate(
                 next( begin( range ) ), end( range ),
                 make_pair( *begin( range ), functor( mask_arg, mask, *begin( range ), input... ) ),

Phys/FunctorCore/tests/src/TestFunctors.cpp

@@ -1222,6 +1222,7 @@ BOOST_AUTO_TEST_CASE( test_relation_table_isolation ) {
   Functors::TopLevelInfo top_level;
 
   IsolationTable table = make_relations2D( parts );
+  IsolationTable empty_table;
 
   auto const front_weight =
       Functors::chain( Functors::Common::Weight{}, Functors::Functional::Front{}, Functors::Common::Relations{} );
@@ -1246,13 +1247,26 @@ BOOST_AUTO_TEST_CASE( test_relation_table_isolation ) {
       PT, Functors::Common::To{}, Functors::Functional::EntryWithMaxRelatedValueOf( Functors::Common::Weight{} ),
       Functors::Common::Relations{} );
   auto const pt_to_max_weight_entry_prepped = pt_to_max_weight_entry.prepare( evtCtx, top_level );
+  auto       empty_test                     = pt_to_min_weight_entry_prepped( true, empty_table, parts( 0 ) );
+  auto       empty_test2                    = pt_to_max_weight_entry_prepped( true, empty_table, parts( 1 ) );
   auto       pt_wmin                        = pt_to_min_weight_entry_prepped( true, table, parts( 2 ) );
   auto       pt_wmax                        = pt_to_max_weight_entry_prepped( true, table, parts( 2 ) );
+  BOOST_CHECK( !empty_test.has_value() );
+  BOOST_CHECK( !empty_test2.has_value() );
   BOOST_CHECK( pt_wmin.has_value() );
   BOOST_CHECK( pt_wmax.has_value() );
   BOOST_CHECK( LHCb::Utils::as_arithmetic( pt_wmin.value() ) < 100 );
   BOOST_CHECK( LHCb::Utils::as_arithmetic( pt_wmax.value() ) > 100 );
 
+  auto const value_or_pt_to_min_weight_entry = Functors::chain(
+      Functors::Functional::ValueOr{0}, PT, Functors::Common::To{},
+      Functors::Functional::EntryWithMinRelatedValueOf( Functors::Common::Weight{} ), Functors::Common::Relations{} );
+
+  auto const value_or_pt_to_min_weight_entry_prepped = value_or_pt_to_min_weight_entry.prepare( evtCtx, top_level );
+  auto       value_or_empty_test = value_or_pt_to_min_weight_entry_prepped( true, empty_table, parts( 0 ) );
+
+  BOOST_CHECK( LHCb::Utils::as_arithmetic( value_or_empty_test ) == 0 );
+
   auto const relation = Functors::Common::Relations{}( table, parts( 1 ) );
   BOOST_CHECK( relation.has_value() );
   auto       pt_to     = chain( PT, Functors::Common::To{} );