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

//
//  concepts.h
//  iterator
//
//  Created by Sam Jaffe on 9/20/25.
//  Copyright © 2025 Sam Jaffe. All rights reserved.
//

#pragma once

#include <concepts>
#include <ranges>
#include <string_view>

namespace iterator {
template <typename It>
concept implements_distance_to = requires(It const & it) {
  { it.distance_to(it) } -> std::integral;
};

template <typename> struct infer_difference_type {
  using type = std::ptrdiff_t;
};

template <implements_distance_to It> struct infer_difference_type<It> {
  static const It & _it;
  using type = decltype(_it.distance_to(_it));
};

template <typename It>
using infer_difference_type_t = typename infer_difference_type<It>::type;

template <typename D, typename It>
concept difference_type_arg =
    std::convertible_to<D, infer_difference_type_t<It>>;

template <typename It> struct infer_value_type {
  static const It & _it;
  using type = std::remove_cvref_t<decltype(*_it)>;
};

template <typename It>
  requires requires { typename It::value_type; }
struct infer_value_type<It> {
  using type = typename It::value_type;
};

template <typename It> using infer_value_type_t = infer_value_type<It>::type;

template <typename T, typename U>
concept not_same_as = not std::same_as<T, U>;

template <typename It>
concept has_sentinel = requires(It const & it) {
  typename It::sentinel_type;
  { it.at_end() } -> std::same_as<bool>;
};

template <typename It, typename S>
concept sentinel_for = has_sentinel<It> && requires(It const & it, S s) {
  { s - it } -> std::integral;
};

template <typename It>
concept single_pass = bool(It::single_pass_iterator);

template <typename It>
concept forward = requires(It & it) {
  { it.dereference() } -> not_same_as<void>;
  { it.equal_to(it) } -> std::same_as<bool>;
  { it.increment() } -> std::same_as<void>;
};

template <typename It>
concept bidirectional = forward<It> && requires(It & it) {
  { it.decrement() } -> std::same_as<void>;
};

template <typename It>
concept random_access = requires(It & it, infer_difference_type_t<It> offset) {
  { it.dereference() } -> not_same_as<void>;
  { it.equal_to(it) } -> std::same_as<bool>;
  { it.advance(offset) } -> std::same_as<void>;
};

template <typename C>
concept Char = std::same_as<C, char> || std::same_as<C, wchar_t> ||
               std::same_as<C, char16_t> || std::same_as<C, char32_t> ||
               std::same_as<C, char8_t>;

template <typename S>
concept LegacyString = requires(S const & t) {
  { t.c_str() };
};

template <typename S>
concept StringViewCompat =
    Char<typename S::value_type> && requires(S const & t) {
      { std::basic_string_view{t} };
    };

template <typename S>
concept String = LegacyString<S> || StringViewCompat<S>;

template <typename C>
concept Range = (not String<C>) && std::ranges::range<C>;

template <typename V>
concept Assoc = std::is_const_v<typename V::first_type>;

template <typename V>
concept AssocRange = Assoc<V> && Range<typename V::second_type>;

template <typename C>
using iterator_t = decltype(std::begin(std::declval<C &>()));
}