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 // Copyright 2019 Hans Dembinski
 //
 // Distributed under the Boost Software License, Version 1.0.
 // (See accompanying file LICENSE_1_0.txt
 // or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 // Uses code segments from boost/iterator/iterator_adaptor.hpp
 // and boost/iterator/iterator_fascade.hpp
 
 #ifndef BOOST_HISTOGRAM_DETAIL_ITERATOR_ADAPTOR_HPP
 #define BOOST_HISTOGRAM_DETAIL_ITERATOR_ADAPTOR_HPP
 
 #include <cstddef>
 #include <iterator>
 #include <memory>
 #include <type_traits>
 #include <utility>
 
 namespace boost {
 namespace histogram {
 namespace detail {
 
 // operator->() needs special support for input iterators to strictly meet the
 // standard's requirements. If *i is not a reference type, we must still
 // produce an lvalue to which a pointer can be formed.  We do that by
 // returning a proxy object containing an instance of the reference object.
 template <class Reference>
 struct operator_arrow_dispatch_t {
   struct pointer {
     explicit pointer(Reference const& x) noexcept : m_ref(x) {}
     Reference* operator->() noexcept { return std::addressof(m_ref); }
     Reference m_ref;
   };
 
   using result_type = pointer;
   static result_type apply(Reference const& x) noexcept { return pointer(x); }
 };
 
 // specialization for "real" references
 template <class T>
 struct operator_arrow_dispatch_t<T&> {
   using result_type = T*;
   static result_type apply(T& x) noexcept { return std::addressof(x); }
 };
 
 // it is ok if void_t is already defined in another header
 template <class...>
 using void_t = void;
 
 template <class T, class = void>
 struct get_difference_type_impl : std::make_signed<T> {};
 
 template <class T>
 struct get_difference_type_impl<
     T, void_t<typename std::iterator_traits<T>::difference_type>> {
   using type = typename std::iterator_traits<T>::difference_type;
 };
 
 template <class T>
 using get_difference_type = typename get_difference_type_impl<T>::type;
 
 // adaptor supports only random access Base
 // Base: underlying base type of the iterator; can be iterator, pointer, integer
 // Reference: type returned when pointer is dereferenced
 template <class Derived, class Base, class Reference = std::remove_pointer_t<Base>&,
           class Value = std::decay_t<Reference>>
 class iterator_adaptor {
   using operator_arrow_dispatch = operator_arrow_dispatch_t<Reference>;
 
 public:
   using base_type = Base;
 
   using reference = Reference;
   using value_type = Value;
   using pointer = typename operator_arrow_dispatch::result_type;
   using difference_type = get_difference_type<base_type>;
   using iterator_category = std::random_access_iterator_tag;
 
   iterator_adaptor() = default;
 
   explicit iterator_adaptor(base_type const& iter) : iter_(iter) {}
 
   // you can override this in derived
   decltype(auto) operator*() const noexcept { return *iter_; }
 
   // you can override this in derived
   Derived& operator+=(difference_type n) {
     iter_ += n;
     return this->derived();
   }
 
   // you should override this in derived if there is an override for operator+=
   template <class... Ts>
   difference_type operator-(const iterator_adaptor<Ts...>& x) const noexcept {
     return iter_ - x.iter_;
   }
 
   // you can override this in derived
   template <class... Ts>
   bool operator==(const iterator_adaptor<Ts...>& x) const noexcept {
     return iter_ == x.iter_;
   }
 
   reference operator[](difference_type n) const { return *(this->derived() + n); }
   pointer operator->() const noexcept {
     return operator_arrow_dispatch::apply(this->derived().operator*());
   }
 
   Derived& operator-=(difference_type n) { return this->derived().operator+=(-n); }
   Derived& operator++() { return this->derived().operator+=(1); }
   Derived& operator--() { return this->derived().operator+=(-1); }
 
   Derived operator++(int) {
     Derived tmp(this->derived());
     operator++();
     return tmp;
   }
 
   Derived operator--(int) {
     Derived tmp(this->derived());
     operator--();
     return tmp;
   }
 
   Derived operator+(difference_type n) const { return Derived(this->derived()) += n; }
   Derived operator-(difference_type n) const { return Derived(this->derived()) -= n; }
 
   template <class... Ts>
   bool operator!=(const iterator_adaptor<Ts...>& x) const noexcept {
     return !this->derived().operator==(x);
   }
 
   template <class... Ts>
   bool operator<(const iterator_adaptor<Ts...>& x) const noexcept {
     return iter_ < x.iter_;
   }
 
   template <class... Ts>
   bool operator>=(const iterator_adaptor<Ts...>& x) const noexcept {
     return iter_ >= x.iter_;
   }
 
   template <class... Ts>
   bool operator>(const iterator_adaptor<Ts...>& x) const noexcept {
     return iter_ > x.iter_;
   }
 
   template <class... Ts>
   bool operator<=(const iterator_adaptor<Ts...>& x) const noexcept {
     return iter_ <= x.iter_;
   }
 
   friend Derived operator+(difference_type n, const Derived& x) { return x + n; }
 
   Base const& base() const noexcept { return iter_; }
 
 protected:
   // for convenience: refer to base class in derived class
   using iterator_adaptor_ = iterator_adaptor;
 
 private:
   Derived& derived() noexcept { return *static_cast<Derived*>(this); }
   const Derived& derived() const noexcept { return *static_cast<Derived const*>(this); }
 
   base_type iter_;
 
   template <class, class, class, class>
   friend class iterator_adaptor;
 };
 
 } // namespace detail
 } // namespace histogram
 } // namespace boost
 
 #endif

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