Modernise CountIterator

Event/TrackEvent/src/Track_v1.cpp

@@ -10,6 +10,7 @@
 \*****************************************************************************/
 #include "Event/Track_v1.h"
 #include "Event/TrackFunctor.h"
+#include "Kernel/CountIterator.h"
 #include "Kernel/TaggedBool.h"
 #include "gsl/gsl_cdf.h"
 #include <algorithm>
@@ -244,25 +245,10 @@ namespace LHCb::Event {
     //=============================================================================
     // Count the number of LHCbIDs that two tracks have in common
     //=============================================================================
-    namespace {
-
-      // inserter which only counts how often something is inserted
-      struct counting_inserter {
-        size_t             count = 0;
-        counting_inserter& operator++() { return *this; } // nop
-        counting_inserter& operator*() { return *this; }  // redirect to self, so that our op= is called
-        counting_inserter& operator=( const LHCbID& ) {
-          ++count;
-          return *this;
-        } // raison d'etre
-      };
-
-    } // namespace
-
     size_t Track::nCommonLhcbIDs( const Track& rhs ) const {
       return std::set_intersection( begin( m_lhcbIDs ), end( m_lhcbIDs ), begin( rhs.m_lhcbIDs ), end( rhs.m_lhcbIDs ),
-                                    counting_inserter{} )
-          .count;
+                                    CountIterator{} )
+          .count();
     }
 
     //=============================================================================

Event/TrackEvent/src/Track_v2.cpp

@@ -21,6 +21,7 @@
 // local
 #include "Event/TrackFunctor.h"
 #include "Event/Track_v2.h"
+#include "Kernel/CountIterator.h"
 #include "Kernel/TaggedBool.h"
 
 // ============================================================================
@@ -40,18 +41,6 @@ namespace {
                                   TrackFunctor::increasingByZ(), std::forward<F>( f ) );
   }
 
-  // inserter which only counts how often something is inserted
-  struct counting_inserter {
-    size_t             count = 0;
-    counting_inserter& operator++() { return *this; } // nop
-    counting_inserter& operator*() { return *this; }  // redirect to self, so that our op= is called
-    template <typename T>
-    counting_inserter& operator=( T&& ) {
-      ++count; // raison d'etre
-      return *this;
-    }
-  };
-
   template <size_t N>
   std::array<double, N + 1> generate_chi2max( double limit ) {
     std::array<double, N + 1> c = {0};
@@ -219,8 +208,8 @@ namespace LHCb::Event::v2 {
   //=============================================================================
   size_t Track::nCommonLhcbIDs( const Track& rhs ) const {
     return std::set_intersection( begin( m_lhcbIDs ), end( m_lhcbIDs ), begin( rhs.m_lhcbIDs ), end( rhs.m_lhcbIDs ),
-                                  counting_inserter{} )
-        .count;
+                                  CountIterator{} )
+        .count();
   }
 
   //=============================================================================

Kernel/LHCbKernel/include/Kernel/CountIterator.h

 /*****************************************************************************\
-* (c) Copyright 2000-2018 CERN for the benefit of the LHCb Collaboration      *
+* (c) Copyright 2023 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".   *
@@ -8,48 +8,54 @@
 * granted to it by virtue of its status as an Intergovernmental Organization  *
 * or submit itself to any jurisdiction.                                       *
 \*****************************************************************************/
-// ============================================================================
-#ifndef KERNEL_COUNTITERATOR_H
-#define KERNEL_COUNTITERATOR_H 1
-// ============================================================================
-// STD & STL
-// ============================================================================
-#include <iterator>
-// ============================================================================
-/** @class CountIterator CountIterator.h Kernel/CountIterator.h
- *  Trivial "count"-iterator: output iterator that only counts
- *
- *  @author Vanya Belyaev
- *  @date   2015-03-17
- */
-template <typename TYPE>
-class count_iterator {
-public:
-  // ========================================================================
-  /// Constructor
-  count_iterator() = default;
-  //
-  count_iterator& operator=( const TYPE& /* value */ ) {
-    ++m_count;
-    return *this;
-  }
-  //
-  count_iterator& operator*() { return *this; }
-  count_iterator& operator++() { return *this; }
-  count_iterator& operator++( int ) { return *this; }
-  //
-  // ========================================================================
-public:
-  // ==========================================================================
-  /// the the counter
-  [[nodiscard]] unsigned long count() const { return m_count; }
-  // ==========================================================================
-private:
-  ///
-  unsigned long m_count{0};
-  // ==========================================================================
-};
-// ============================================================================
-// The END
-// ============================================================================
-#endif // KERNEL_COUNTITERATOR_H
+#pragma once
+
+#include <cstddef>
+#include <functional>
+#include <utility>
+
+namespace LHCb {
+
+  namespace details {
+    struct AlwaysTrue {
+      template <typename T>
+      constexpr bool operator()( const T& ) const noexcept {
+        return true;
+      }
+    };
+  } // namespace details
+
+  /**
+   * @brief OutputIterator struct that increments a count when using the assignment operator and Predicate is true.
+   *
+   * @tparam Predicate
+   *
+   * @details Fulfills https://en.cppreference.com/w/cpp/named_req/OutputIterator
+   * and can be used e.g. in a std::set_intersection counting only elements fulfilling the predicate.
+   */
+  template <typename Predicate>
+  class CountIfIterator {
+    std::size_t m_count{0};
+    Predicate   m_predicate;
+
+  public:
+    explicit CountIfIterator( Predicate pred = {} ) : m_predicate{std::move( pred )} {}
+    CountIfIterator& operator++() { return *this; }
+    CountIfIterator& operator*() { return *this; }
+    template <typename T>
+    CountIfIterator& operator=( const T& data ) {
+      if ( std::invoke( m_predicate, data ) ) ++m_count;
+      return *this;
+    }
+
+    auto count() { return m_count; }
+  };
+
+  /**
+   * @brief OutputIterator struct that increments a count when using the assignment operator.
+   * @details Fulfills https://en.cppreference.com/w/cpp/named_req/OutputIterator
+   * and can be used e.g. in a std::set_intersection.
+   */
+  using CountIterator = CountIfIterator<details::AlwaysTrue>;
+
+} // namespace LHCb

Kernel/LHCbKernel/tests/src/test_count_iterator.cpp

@@ -16,16 +16,24 @@
 #include <vector>
 
 BOOST_AUTO_TEST_CASE( test_count_iterator ) {
-  count_iterator<int> i;
+  using namespace LHCb;
+  auto i = CountIterator{};
   BOOST_CHECK( i.count() == 0 );
-
   std::vector<int> data = {0, 1, 2, 3, 4, 5, 6};
   {
-    auto result = std::copy( begin( data ), end( data ), count_iterator<int>() );
+    auto result = std::copy( begin( data ), end( data ), CountIterator{} );
     BOOST_CHECK( result.count() == 7 );
   }
+  auto isNotEven = []( auto i ) { return i % 2; };
+  {
+    auto result = std::copy_if( begin( data ), end( data ), CountIterator{}, isNotEven );
+    BOOST_CHECK( result.count() == 3 );
+  }
+
+  auto j = CountIfIterator{isNotEven};
+  BOOST_CHECK( j.count() == 0 );
   {
-    auto result = std::copy_if( begin( data ), end( data ), count_iterator<int>(), []( auto i ) { return i % 2; } );
+    auto result = std::copy( begin( data ), end( data ), j );
     BOOST_CHECK( result.count() == 3 );
   }
 }