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

#pragma once

#include <iterator>
#include <type_traits>

#include <iterator/detail/arrow_proxy.h>
#include <iterator/detail/traits.h>

#include <iterator/detail/macro.h>

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

template <typename T>
struct distance_to<T, EXISTS(VAL(T).distance_to(VAL(T)))> {
  using type = decltype(VAL(T).distance_to(VAL(T)));
};

template <typename T> using distance_to_t = typename distance_to<T>::type;
}

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 CRTP, category C> class facade {
private:
  using self_type = CRTP;

public:
  constexpr static auto category_enum = C;

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>,
            REQUIRES(DEFER(D) && C == category::random_access)>
  decltype(auto) operator[](D off) const {
    return *(self() + off);
  }

  self_type & operator++() {
    if constexpr (C == category::random_access) {
      self() += 1;
    } else {
      self().increment();
    }
    return self();
  }

  auto operator++(int) {
    if constexpr (C == category::single_pass) {
      ++*this;
    } else {
      auto tmp = self();
      ++*this;
      return tmp;
    }
  }

  SFINAE(C >= category::bidirectional)
  self_type & operator--() {
    if constexpr (C == category::random_access) {
      self() -= 1;
    } else {
      self().decrement();
    }
    return self();
  }

  SFINAE(C >= category::bidirectional)
  self_type operator--(int) {
    auto tmp = self();
    --*this;
    return tmp;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>,
            REQUIRES(DEFER(D) && C == category::random_access)>
  friend self_type & operator+=(self_type & self, D off) {
    self.advance(off);
    return self;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>,
            REQUIRES(DEFER(D) && C == category::random_access)>
  friend self_type & operator-=(self_type & self, D off) {
    self.advance(-off);
    return self;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>,
            REQUIRES(DEFER(D) && C == category::random_access)>
  friend auto operator+(self_type self, D off) {
    return self += off;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>,
            REQUIRES(DEFER(D) && C == category::random_access)>
  friend auto operator+(D off, self_type self) {
    return self += off;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>,
            REQUIRES(DEFER(D) && C == category::random_access)>
  friend auto operator-(self_type self, D off) {
    return self -= off;
  }

  SFINAE(C == category::random_access)
  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 (C == category::random_access) {
      return (left - right) == 0;
    } else {
      return left.equal_to(right);
    }
  }

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

  SFINAE(C == category::random_access)
  friend bool operator<(self_type const & left, self_type const & right) {
    return (left - right) < 0;
  }

  SFINAE(C == category::random_access)
  friend bool operator<=(self_type const & left, self_type const & right) {
    return (left - right) <= 0;
  }

  SFINAE(C == category::random_access)
  friend bool operator>(self_type const & left, self_type const & right) {
    return (left - right) > 0;
  }

  SFINAE(C == category::random_access)
  friend bool operator>=(self_type const & left, self_type const & right) {
    return (left - right) >= 0;
  }

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

// In C++20, a concept/requires could be used to eschew the need for the below
// macros.
#define MAKE_ITERATOR_FACADE_TYPEDEFS_T(Iter)                                  \
  template <typename... T> struct std::iterator_traits<Iter<T...>> {           \
    using type = Iter<T...>;                                                   \
    using reference = decltype(*std::declval<type>());                         \
    using value_type = std::decay_t<reference>;                                \
    using pointer = decltype(std::declval<type>().operator->());               \
    using difference_type = ::iterator::detail::distance_to_t<type>;           \
    using iterator_category = ::iterator::detail::tag_for_t<type>;             \
  }

#include <iterator/detail/undef.h>