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

#pragma once

#include <iterator>

#include "traits.h"

namespace iterator::recursive {
// Type deduction guides for constructing recursive iterators.
enum class recursion_type { END, THRU, ASSOC };

// Helpers for condensing type deductions
template <typename It> using value = decltype(std::begin(*std::declval<It>()));

template <typename It>
using mapped = decltype(std::begin(std::declval<It>()->second));

template <typename T> using val_key_t = typename T::value_type::first_type;

// Type trait to identify value_type ~~ std::pair<K const, V>, which is
// a safe bet towards 'this is an associative container type'
template <typename T, typename = void>
struct is_associative : std::false_type {};
template <typename T>
struct is_associative<T, std::enable_if_t<std::is_const_v<val_key_t<T>>>>
    : std::true_type {};

template <typename T, typename = void> struct typeclass_t {
  static constexpr recursion_type const value{recursion_type::END};
};

template <typename T> struct is_string_iter : std::false_type {};
template <> struct is_string_iter<std::string::iterator> : std::true_type {};
template <>
struct is_string_iter<std::string::const_iterator> : std::true_type {};

template <typename T> constexpr bool is_string_iter_v = is_string_iter<T>{};

template <typename T>
struct typeclass_t<T, std::enable_if_t<!is_string_iter_v<value<T>>>> {
  constexpr static recursion_type value{recursion_type::THRU};
};

template <typename T>
struct typeclass_t<T, std::enable_if_t<is_string_iter_v<value<T>>>> {
  constexpr static recursion_type value{recursion_type::END};
};

template <typename T> struct typeclass_t<T, detail::void_t<mapped<T>>> {
  constexpr static recursion_type value{recursion_type::ASSOC};
};
}