cpp-useful-iterators/include/iterator/facade.h

#pragma once

#include <iterator>
#include <type_traits>

#include "detail/arrow_proxy.h"

#define _val(type) std::declval<type>()
#define exists(expr) void_t<decltype(expr)>

namespace iterator::detail {
  template <typename> using void_t = void;

  template <typename, typename = void> struct has_equal_to : std::false_type {};
  template <typename T>
  struct has_equal_to<T, exists(_val(T).equal_to(_val(T)))> : std::true_type {};

  template <typename, typename = void>
  struct has_distance_to : std::false_type {};
  template <typename T>
  struct has_distance_to<T, exists(_val(T).distance_to(_val(T)))>
      : std::true_type {};

  template <typename, typename = void>
  struct is_advanceable_iterator : std::false_type {};
  template <typename T>
  struct is_advanceable_iterator<T, exists(_val(T).advance({}))>
      : std::true_type {};

  template <typename, typename = void>
  struct is_single_pass_iterator : std::false_type {};
  template <typename T>
  struct is_single_pass_iterator<T, void_t<typename T::single_pass_iterator>>
      : std::true_type {};

  template <typename, typename = void>
  struct has_increment : std::false_type {};
  template <typename T>
  struct has_increment<T, exists(_val(T).increment())> : std::true_type {};

  template <typename, typename = void>
  struct has_decrement : std::false_type {};
  template <typename T>
  struct has_decrement<T, exists(_val(T).decrement())> : std::true_type {};

  template <typename, typename = void> struct distance_to {
    using type = std::ptrdiff_t;
  };

  template <typename T>
  struct distance_to<T, std::enable_if_t<has_distance_to<T>{}>> {
    using type = decltype(_val(T).distance_to(_val(T)));
  };

  template <typename T> using distance_to_t = typename distance_to<T>::type;
  template <typename T> constexpr bool has_equal_to_v = has_equal_to<T>{};
  template <typename T> constexpr bool has_distance_to_v = has_distance_to<T>{};
  template <typename T> constexpr bool has_increment_v = has_increment<T>{};
  template <typename T> constexpr bool has_decrement_v = has_decrement<T>{};
  template <typename T>
  constexpr bool is_advanceable_iterator_v = is_advanceable_iterator<T>{};

  template <typename T>
  constexpr bool is_random_access_iterator_v =
      has_distance_to_v<T> && is_advanceable_iterator_v<T>;
  template <typename T>
  constexpr bool is_bidirectional_iterator_v =
      has_decrement_v<T> && has_increment_v<T> && has_equal_to_v<T>;
  template <typename T>
  constexpr bool is_single_pass_iterator_v = is_single_pass_iterator<T>{};

  template <typename T>
  using iterator_category_t = std::conditional_t<
      is_random_access_iterator_v<T>, std::random_access_iterator_tag,
      std::conditional_t<is_bidirectional_iterator_v<T>,
                         std::bidirectional_iterator_tag,
                         std::conditional_t<is_single_pass_iterator_v<T>,
                                            std::input_iterator_tag,
                                            std::forward_iterator_tag>>>;
}

#undef _val
#undef exists

namespace iterator {

  template <typename D, typename T>
  using difference_type_arg_t =
      std::enable_if_t<std::is_convertible_v<D, detail::distance_to_t<T>>>;

  template <typename self_type> class facade {
  public:
    decltype(auto) operator*() const { return self().dereference(); }

    decltype(auto) operator->() const {
      if constexpr (std::is_reference<decltype(**this)>{}) {
        return std::addressof(**this);
      } else {
        return detail::arrow_proxy{**this};
      }
    }

    template <typename D, typename = difference_type_arg_t<D, self_type>>
    decltype(auto) operator[](D off) const {
      return *(self() + off);
    }

    self_type & operator++() {
      if constexpr (detail::is_advanceable_iterator_v<self_type>) {
        *this += 1;
      } else {
        self().increment();
      }
      return self();
    }

    auto operator++(int) {
      if constexpr (detail::is_single_pass_iterator_v<self_type>) {
        ++*this;
      } else {
        auto tmp = self();
        ++*this;
        return tmp;
      }
    }

    self_type & operator--() {
      if constexpr (detail::is_advanceable_iterator_v<self_type>) {
        *this -= 1;
      } else {
        self().decrement();
      }
      return self();
    }

    self_type operator--(int) {
      auto tmp = self();
      --*this;
      return tmp;
    }

    template <typename D, typename = difference_type_arg_t<D, self_type>>
    friend self_type & operator+=(self_type & self, D off) {
      static_assert(detail::is_advanceable_iterator_v<self_type>,
                    "must be advancable");
      self.advance(off);
      return self;
    }

    template <typename D, typename = difference_type_arg_t<D, self_type>>
    friend self_type & operator-=(self_type & self, D off) {
      static_assert(detail::is_advanceable_iterator_v<self_type>,
                    "must be advancable");
      self.advance(-off);
      return self;
    }

    template <typename D, typename = difference_type_arg_t<D, self_type>>
    friend auto operator+(self_type self, D off) {
      static_assert(detail::is_advanceable_iterator_v<self_type>,
                    "must be advancable");
      return self += off;
    }

    template <typename D, typename = difference_type_arg_t<D, self_type>>
    friend auto operator+(D off, self_type self) {
      static_assert(detail::is_advanceable_iterator_v<self_type>,
                    "must be advancable");
      return self += off;
    }

    template <typename D, typename = difference_type_arg_t<D, self_type>>
    friend auto operator-(self_type self, D off) {
      static_assert(detail::is_advanceable_iterator_v<self_type>,
                    "must be advancable");
      return self -= off;
    }

    friend auto operator-(self_type const & left, self_type const & right) {
      return right.distance_to(left);
    }

    friend bool operator==(self_type const & left, self_type const & right) {
      if constexpr (detail::has_distance_to_v<self_type>) {
        return (left - right) == 0;
      } else {
        return left.equal_to(right);
      }
    }

    friend bool operator!=(self_type const & left, self_type const & right) {
      return !(left == right);
    }

    friend bool operator<(self_type const & left, self_type const & right) {
      return (left - right) < 0;
    }

    friend bool operator<=(self_type const & left, self_type const & right) {
      return (left - right) <= 0;
    }

    friend bool operator>(self_type const & left, self_type const & right) {
      return (left - right) > 0;
    }

    friend bool operator>=(self_type const & left, self_type const & right) {
      return (left - right) >= 0;
    }

  private:
    self_type & self() { return *static_cast<self_type *>(this); }
    self_type const & self() const {
      return *static_cast<self_type const *>(this);
    }
  };
}

namespace std {
  // In C++20, a concept/requires could be used to eschew the need for the below
  // macros.
  template <typename I> struct iterator_traits<::iterator::facade<I>> {
    using reference = decltype(*std::declval<I>());
    using value_type = std::remove_cv_t<std::remove_reference_t<reference>>;
    using pointer = decltype(std::declval<I>().operator->());
    using difference_type = ::iterator::detail::distance_to_t<I>;
    using iterator_category = ::iterator::detail::iterator_category_t<I>;
  };
}

#define MAKE_ITERATOR_FACADE_TYPEDEFS(type)                                    \
  namespace std {                                                              \
    template <>                                                                \
    struct iterator_traits<type>                                               \
        : std::iterator_traits<::iterator::facade<type>> {};                   \
  }

#define MAKE_ITERATOR_FACADE_TYPEDEFS_T(type)                                  \
  namespace std {                                                              \
    template <typename T>                                                      \
    struct iterator_traits<type<T>>                                            \
        : std::iterator_traits<::iterator::facade<type<T>>> {};                \
  }