<xstd/bits/bit_set/bit_key_mapping.hpp>

bit_key_mapping, bit_range_mapping, bit_find_mapping, enum_traits

template<class Key, Key First, size_t N>
  requires see below
struct bit_range_mapping {
  static constexpr size_t size = N;
  static constexpr auto is_key(Key key) noexcept -> bool;
  static constexpr auto to_index(Key key) noexcept -> size_t;
  static constexpr auto from_index(size_t index) noexcept -> Key;
};

template<class Key, const auto& Keys>
  requires see below
struct bit_find_mapping {
  static constexpr size_t size = ranges::size(Keys);
  static constexpr auto is_key(Key key) noexcept -> bool;
  static constexpr auto to_index(Key key) noexcept -> size_t;
  static constexpr auto from_index(size_t index) noexcept -> Key;
};

A key's mapping is an order-preserving bijection onto the positions 0 to size - 1: a < b exactly where to_index(a) < to_index(b), and from_index inverts to_index.

• bit_range_mapping<Key, First, N> maps the N consecutive keys from First, of an integer or an enumeration, onto 0 to N - 1. is_key(k) holds for First <= k < First + N, compared as an unsigned distance from First, so no bound overflows Key. A key outside the range ranks at N or above.
• bit_find_mapping<Key, Keys> maps each key of the list Keys to its rank there, found by binary search, so a gap costs no position. Mandates: Keys ascends strictly. is_key(k) holds for a listed key, and any other ranks at size.
• bit_key_mapping<Key> is the mapping a set takes by default. Its primary template is declared and not defined, and it is specialized as char_traits is:
  • for every unsigned integer Key, the identity, with no size, leaving the universe open; is_key(k) holds where k survives the round trip through size_t;
  • for an enumeration whose enum_traits<Key>::values is declared, bit_range_mapping where the values run without a gap and bit_find_mapping over the list otherwise;
  • for an enumeration that declares no list and has an enumerator count, bit_range_mapping from Key{} to count, the sentinel itself no key. Mandates: count is not negative.
  • A program may specialize it for a key type of its own, such as a strong index.
• enum_traits<Enum> is empty. A program specializes it beside an enumeration with a static constexpr std::array named values, listing the enumerators ascending, each once.