enumerate-contrib-cry.h

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/* *****************************************************************************
 * Copyright (c) 2020 Krystian Stasiowski
 *
 * This code adapted from code written by Krystian Stasiowski
 * <sdkrystian@gmail.com>
 *
 * No specific license provisions were given, however permission was granted for
 * me to include it in kblib.
 *
 * His code is much faster and cleaner than my magic_enumerate is. HOWEVER, it
 * is fundamentally unable to detect the copying-nonconst-from-const case.
 *
 * My modifications are:
 * - Change from [val, idx] to [idx, val] to match Python's enumerate()
 * - Wrote documentation
 * - Change name from value_and_index() to cry_enumerate()
 * - Silenced Clang warnings about std::tuple_element specializations with
 *   mismatched tags.
 * - Formatting
 *
 * All credit for everything else goes to Krystian.
 *
 * ****************************************************************************/
 
#ifndef ENUMERATECONTRIB_H
#define ENUMERATECONTRIB_H
#include "tdecl.h"
 
#include <iterator>
#include <type_traits>
 
#if KBLIB_USE_CXX17
 
namespace KBLIB_NS {
namespace detail_cry {
   template <typename>
   struct value_index_pair;
 
   template <std::size_t N, typename T, std::enable_if_t<N == 0>* = nullptr>
   auto get(T&& t)
       -> std::conditional_t<std::is_reference_v<T>, const std::size_t&,
                             const std::size_t> {
      return t.index;
   }
 
   template <std::size_t N, typename T, std::enable_if_t<N == 1>* = nullptr>
   auto get(T&& t)
       -> std::conditional_t<std::is_reference_v<T>,
                             typename std::remove_reference_t<T>::value_type&,
                             typename std::remove_reference_t<T>::value_type> {
      // static_assert(std::is_reference_v<T>);
      // static_assert(
      //     std::is_const_v<std::remove_reference_t<decltype(*t.iter)>>);
      return *t.iter;
   }
 
   template <typename Iterator>
   struct value_index_pair {
      using value_type
          = std::remove_reference_t<decltype(*std::declval<Iterator&>())>;
 
      std::size_t index;
      Iterator iter;
   };
 
   template <typename Range, typename = void>
   struct value_and_index_base {
    public:
      using iterator_type = decltype(std::begin(std::declval<Range&>()));
 
      value_and_index_base(Range& range)
          : range_begin_(std::begin(range))
          , range_end_(std::end(range)) {}
 
      auto range_begin() -> iterator_type { return range_begin_; }
 
      auto range_end() -> iterator_type { return range_end_; }
 
      iterator_type range_begin_;
      iterator_type range_end_;
   };
 
   template <typename Range>
   struct value_and_index_base<
       Range, std::enable_if_t<not std::is_reference_v<Range>>> {
    public:
      using iterator_type = decltype(std::begin(std::declval<Range&>()));
 
      value_and_index_base(Range& range)
          : range_(std::move(range)) {}
 
      auto range_begin() -> iterator_type { return std::begin(range_); }
 
      auto range_end() -> iterator_type { return std::end(range_); }
 
      Range range_;
   };
 
   template <typename Range>
   struct value_and_index_impl : value_and_index_base<Range> {
      using iterator_type = typename value_and_index_base<Range>::iterator_type;
 
      value_and_index_impl(Range& range)
          : value_and_index_base<Range>(range)
          , begin_(this->range_begin(), 0)
          , end_(this->range_end(), 0) {}
 
      struct iterator {
       private:
         value_index_pair<iterator_type> pair_;
 
       public:
         iterator(iterator_type iter, std::size_t index = 0)
             : pair_{index, iter} {}
 
         auto operator*() -> value_index_pair<iterator_type>& { return pair_; }
 
         auto operator++(int) -> iterator {
            iterator copy(pair_.iter, pair_.index);
            ++pair_.iter, ++pair_.index;
            return copy;
         }
 
         auto operator++() -> iterator& {
            ++pair_.iter, ++pair_.index;
            return *this;
         }
 
         auto operator==(const iterator& other) const -> bool {
            return other.pair_.iter == pair_.iter;
         }
 
         auto operator!=(const iterator& other) const -> bool {
            return not (other == *this);
         }
      };
 
      auto begin() -> iterator { return begin_; }
 
      auto end() -> iterator { return end_; }
 
    private:
      iterator begin_;
      iterator end_;
   };
} // namespace detail_cry
} // namespace KBLIB_NS
namespace std {
#  if defined(__clang__)
// Fix from: https://github.com/nlohmann/json/issues/1401
#     pragma clang diagnostic push
#     pragma clang diagnostic ignored "-Wmismatched-tags"
#  endif
template <typename T>
struct tuple_size<kblib::detail_cry::value_index_pair<T>> {
   static constexpr std::size_t value = 2;
};
 
template <typename T>
struct tuple_element<0, kblib::detail_cry::value_index_pair<T>> {
   using type = const std::size_t;
};
 
template <typename T>
struct tuple_element<1, kblib::detail_cry::value_index_pair<T>> {
   using type = std::remove_reference_t<decltype(*std::declval<T&>())>;
};
 
#  if defined(__clang__)
#     pragma clang diagnostic pop
#  endif
} // namespace std
 
namespace KBLIB_NS {
 
template <typename Range>
auto cry_enumerate(Range&& range) -> auto {
   return detail_cry::value_and_index_impl<Range>(range);
}
 
} // namespace KBLIB_NS
 
#endif
 
#endif // ENUMERATECONTRIB_H