cpp-useful-iterators/include/iterator/detail/facade_traits.h

//
//  facade_traits.h
//  iterator
//
//  Created by Sam Jaffe on 4/1/23.
//  Copyright © 2023 Sam Jaffe. All rights reserved.
//

#pragma once

#include <iterator>
#include <type_traits>

#include <iterator/detail/traits.h>

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

namespace iterator::detail {
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, std::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 It>
constexpr bool is_single_pass_iterator_v = is_single_pass_iterator<It>{};

template <typename It> struct facade_category {
  constexpr static bool has_random_access =
      has_distance_to_v<It> && is_advanceable_iterator_v<It>;
  constexpr static bool has_bidirectional =
      has_decrement_v<It> && has_increment_v<It> && has_equal_to_v<It>;

  using type = std::conditional_t<
      has_random_access, std::random_access_iterator_tag,
      std::conditional_t<has_bidirectional, std::bidirectional_iterator_tag,
                         std::conditional_t<is_single_pass_iterator_v<It>,
                                            std::input_iterator_tag,
                                            std::forward_iterator_tag>>>;
};

template <typename It>
using facade_category_t = typename facade_category<It>::type;
}