MapKeyIterator.h

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#ifndef NURANDOM_RANDOMUTILS_PROVIDERS_MAPKEYITERATOR_H
#define NURANDOM_RANDOMUTILS_PROVIDERS_MAPKEYITERATOR_H 1

// C++ standard libraries
#include <iterator>
#include <tuple>
#include <type_traits>


namespace rndm {
  namespace NuRandomServiceHelper {
    
    
    template <typename Iter>
    class MapKeyIterator: public Iter {
        public:
      using map_iterator_t = Iter;
      using iterator_traits_t = std::iterator_traits<map_iterator_t>;
      
      // iterator traits
      using difference_type = typename iterator_traits_t::difference_type;
      using value_type
        = typename std::tuple_element<0, typename iterator_traits_t::value_type>::type;
      using pointer = typename std::add_pointer<value_type>::type;
      using reference = typename std::add_lvalue_reference<value_type>::type;
      using iterator_category = typename iterator_traits_t::iterator_category;
      
      MapKeyIterator() = default;
      
      // accept quietly all the other constructery C++ provides
      
      MapKeyIterator(map_iterator_t const& from):
        map_iterator_t(from) {}
      
      reference operator*() { return get_key(get_value()); }
      
      pointer operator->() { return &get_key(get_value()); }
      
        protected:
      using pair_reference = typename iterator_traits_t::reference;
      
      pair_reference get_value() const { return map_iterator_t::operator*(); }
      
      static reference get_key(pair_reference value) 
        { return std::get<0>(value); }
      
    }; // MapKeyIterator<>
    
    
    
    template <typename Map>
    class MapKeyConstIteratorBox {
        public:
      using map_t = Map;
      
      using const_iterator = MapKeyIterator<typename map_t::const_iterator>;
      
      MapKeyConstIteratorBox(map_t const& map):
        beginKey(map.cbegin()), endKey(map.cend())
        {}
      
      const_iterator begin()  { return beginKey; }
      const_iterator end()    { return endKey; }
      const_iterator cbegin() { return beginKey; }
      const_iterator cend()   { return endKey; }
      
        protected:
      const_iterator beginKey; 
      const_iterator endKey;   
      
    }; // class MapKeyConstIteratorBox<>
  
  } // namespace NuRandomServiceHelper
} // namespace rndm

#endif // NURANDOM_RANDOMUTILS_PROVIDERS_MAPKEYITERATOR_H