Function enum_prev_value

include/magic_enum/magic_enum_utility.hpp:109–120

template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
[[nodiscard]] constexpr auto enum_prev_value(E value, std::ptrdiff_t n = 1) noexcept -> detail::enable_if_t<E, optional<std::decay_t<E>>> {
  using D = std::decay_t<E>;
  constexpr std::ptrdiff_t count = detail::count_v<D, S>;

  if (const auto i = enum_index<D, S>(value)) {
    const std::ptrdiff_t index = (static_cast<std::ptrdiff_t>(*i) - n);
    if (index >= 0 && index < count) {
      return enum_value<D, S>(static_cast<std::size_t>(index));
    }
  }
  return {};
}

template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
[[nodiscard]] constexpr auto enum_prev_value_circular(E value, std::ptrdiff_t n = 1) noexcept -> detail::enable_if_t<E, std::decay_t<E>> {