Monitor algorithm for event variables

Add algorithm which can histogram a single functor.

Needed for Moore!2798 (merged)

Phys/SelAlgorithms/src/VoidMonitor.cpp

+/*****************************************************************************\
+* (c) Copyright 2019 CERN for the benefit of the LHCb Collaboration           *
+*                                                                             *
+* This software is distributed under the terms of the GNU General Public      *
+* Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
+*                                                                             *
+* In applying this licence, CERN does not waive the privileges and immunities *
+* granted to it by virtue of its status as an Intergovernmental Organization  *
+* or submit itself to any jurisdiction.                                       *
+\*****************************************************************************/
+#include "Functors/with_functors.h"
+#include "Functors/with_output_tree.h"
+#include "Gaudi/Accumulators/Histogram.h"
+#include "LHCbAlgs/Consumer.h"
+
+namespace {
+  struct VoidFunctor {
+    using Signature                    = std::any();
+    static constexpr auto PropertyName = "Variable";
+  };
+  using base_type  = with_functors<LHCb::Algorithm::Consumer<void()>, VoidFunctor>;
+  using value_type = double;
+} // namespace
+
+/** @class EventVariableMonitor EventVariableMonitor.cpp
+ */
+struct EventVariableMonitor final : public base_type {
+  EventVariableMonitor( const std::string& name, ISvcLocator* pSvcLocator ) : base_type{name, pSvcLocator, {}} {}
+
+  void operator()() const override {
+    auto const&                                               functor = getFunctor<VoidFunctor>();
+    Functors::detail::with_output_tree::numeric_value_wrapper w( functor.rtype() );
+
+    std::visit(
+        [&]( auto& x ) {
+          auto unwrapped = std::any_cast<std::decay_t<decltype( x )>>( std::invoke( functor ) );
+          ++( *m_histogram )[LHCb::Utils::as_arithmetic( unwrapped )];
+        },
+        w.get() );
+  }
+
+  StatusCode initialize() override {
+
+    auto sc = base_type::initialize();
+    if ( sc.isFailure() ) return sc;
+    m_histogram.emplace( this, m_histogram_name.value(), m_histodef.value().title(),
+                         Gaudi::Accumulators::Axis<value_type>{static_cast<unsigned int>( m_histodef.value().bins() ),
+                                                               m_histodef.value().lowEdge(),
+                                                               m_histodef.value().highEdge()} );
+    return sc;
+  }
+
+private:
+  // Counter for recording cut retention statistics
+  mutable Gaudi::Accumulators::BinomialCounter<> m_cutEff{this, "Cut selection efficiency"};
+
+private:
+  // properties
+  Gaudi::Property<Gaudi::Histo1DDef> m_histodef{
+      this, "HistogramDef", {"DefaultTitle", 0., 1., 100}, "Histogram definition"};
+
+  Gaudi::Property<std::string> m_histogram_name{this, "HistogramName", "Histogram", "Histogram name"};
+  mutable std::optional<Gaudi::Accumulators::Histogram<1, Gaudi::Accumulators::atomicity::full, value_type>>
+      m_histogram;
+};
+
+DECLARE_COMPONENT_WITH_ID( EventVariableMonitor, "Monitor__EventVariable" )