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

#pragma once

#include <iterator>
#include <type_traits>

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

namespace iterator {
template <typename CRTP> class Facade {
private:
  using self_type = CRTP;

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::ArrowProxy{**this};
    }
  }

  template <typename D>
    requires(difference_type_arg<D, self_type> && random_access<self_type>)
  decltype(auto) operator[](D off) const {
    return *(self() + off);
  }

  self_type & operator++() {
    if constexpr (forward<self_type>) {
      self().increment();
    } else {
      static_assert(random_access<self_type>,
                    "requires .increment() or .advance()");
      self() += 1;
    }
    return self();
  }

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

  self_type & operator--() {
    if constexpr (bidirectional<self_type>) {
      self().decrement();
    } else {
      static_assert(random_access<self_type>,
                    "requires .decrement() or .advance()");
      self() -= 1;
    }
    return self();
  }

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

  template <typename D>
    requires(difference_type_arg<D, self_type> && random_access<self_type>)
  friend self_type & operator+=(self_type & self, D off) {
    self.advance(off);
    return self;
  }

  template <typename D>
    requires(difference_type_arg<D, self_type> && random_access<self_type>)
  friend self_type & operator-=(self_type & self, D off) {
    self.advance(-off);
    return self;
  }

  template <typename D>
    requires(difference_type_arg<D, self_type> && random_access<self_type>)
  friend auto operator+(self_type self, D off) {
    return self += off;
  }

  template <typename D>
    requires(difference_type_arg<D, self_type> && random_access<self_type>)
  friend auto operator+(D off, self_type self) {
    return self += off;
  }

  template <typename D>
    requires(difference_type_arg<D, self_type> && random_access<self_type>)
  friend auto operator-(self_type self, D off) {
    return self -= off;
  }

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

  friend bool operator==(self_type const & left, self_type const & right) {
    if constexpr (has_sentinel<self_type>) {
      return (left.at_end() && right.at_end()) || left.equal_to(right);
    } else {
      return left.equal_to(right);
    }
  }

  friend auto operator<=>(self_type const & left, self_type const & right)
    requires(random_access<self_type>)
  {
    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);
  }
};
}

template <typename It>
  requires std::is_base_of_v<iterator::Facade<It>, It>
struct std::iterator_traits<It> {
  static const It & _it;
  using reference = decltype(*_it);
  using pointer = decltype(_it.operator->());
  using value_type = ::iterator::infer_value_type_t<It>;
  using difference_type = ::iterator::infer_difference_type_t<It>;
  using iterator_category = std::conditional_t<
      ::iterator::random_access<It>, random_access_iterator_tag,
      std::conditional_t<
          ::iterator::bidirectional<It>, bidirectional_iterator_tag,
          std::conditional_t<::iterator::single_pass<It>, input_iterator_tag,
                             forward_iterator_tag>>>;
};