single.hpp

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/* This file is part of OpenKalman, a header-only C++ library for
 * Kalman filters and other recursive filters.
 *
 * Copyright (c) 2025 Christopher Lee Ogden <ogden@gatech.edu>
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */

#ifndef OPENKALMAN_COMPATIBILITY_VIEWS_SINGLE_HPP
#define OPENKALMAN_COMPATIBILITY_VIEWS_SINGLE_HPP

#ifndef __cpp_lib_ranges

#include "view-concepts.hpp"
#include "view_interface.hpp"

namespace OpenKalman::ranges
{
#ifdef __cpp_concepts
  template<std::move_constructible T> requires std::is_object_v<T>
#else
  template<typename T>
#endif
  struct single_view : view_interface<single_view<T>>
  {
#ifdef __cpp_concepts
    constexpr
    single_view() requires std::default_initializable<T> = default;
#else
    template<typename aT = T, std::enable_if_t<std::is_default_constructible_v<aT>, int> = 0>
    constexpr
    single_view() {}
#endif


#ifdef __cpp_concepts
    explicit constexpr
    single_view(const T& t) requires std::copy_constructible<T> : t_ {t} {}
    requires std::copy_constructible<T>
#else
    template<typename aT = T, std::enable_if_t<std::is_copy_constructible_v<aT>, int> = 0>
    explicit constexpr
    single_view(const T& t) : t_ {t} {}
#endif


    explicit constexpr
    single_view(T&& t) : t_ {std::move(t)} {}


    constexpr T*
    begin() noexcept { return data(); }

    constexpr const T*
    begin() const noexcept { return data(); }


    constexpr T*
    end() noexcept { return data() + 1_uz; }

    constexpr const T*
    end() const noexcept { return data() + 1_uz; }


    static constexpr auto
    empty() noexcept { return false; }


    static constexpr std::size_t
    size() noexcept { return 1_uz; }


    constexpr T*
    data() noexcept { return std::addressof(std::get<0>(t_)); }


    constexpr const T*
    data() const noexcept { return std::addressof(std::get<0>(t_)); }

  private:

    std::tuple<T> t_;

  };


  template<typename Arg>
  single_view(Arg) -> single_view<Arg>;


#ifdef __cpp_impl_three_way_comparison
  template<typename T>
  constexpr std::partial_ordering
  operator<=>(const single_view<T>& lhs, const T& rhs) noexcept
  {
    return get(lhs, std::integral_constant<std::size_t, 0>{}) <=> rhs;
  }

  template<typename T>
  constexpr bool
  operator==(const single_view<T>& lhs, const T& rhs) noexcept
  {
    return std::is_eq(operator<=>(lhs, rhs));
  }
#else
  template<typename T>
  constexpr bool operator==(const single_view<T>& lhs, const T& rhs) noexcept { return get(lhs, std::integral_constant<std::size_t, 0>{}) == rhs; }

  template<typename T>
  constexpr bool operator!=(const single_view<T>& lhs, const T& rhs) noexcept { return get(lhs, std::integral_constant<std::size_t, 0>{}) != rhs; }

  template<typename T>
  constexpr bool operator<(const single_view<T>& lhs, const T& rhs) noexcept { return get(lhs, std::integral_constant<std::size_t, 0>{}) < rhs; }

  template<typename T>
  constexpr bool operator>(const single_view<T>& lhs, const T& rhs) noexcept { return get(lhs, std::integral_constant<std::size_t, 0>{}) > rhs; }

  template<typename T>
  constexpr bool operator<=(const single_view<T>& lhs, const T& rhs) noexcept { return get(lhs, std::integral_constant<std::size_t, 0>{}) <= rhs; }

  template<typename T>
  constexpr bool operator>=(const single_view<T>& lhs, const T& rhs) noexcept { return get(lhs, std::integral_constant<std::size_t, 0>{}) >= rhs; }


  template<typename T>
  constexpr bool operator==(const T& lhs, const single_view<T>& rhs) noexcept { return lhs == get(rhs, std::integral_constant<std::size_t, 0>{}); }

  template<typename T>
  constexpr bool operator!=(const T& lhs, const single_view<T>& rhs) noexcept { return lhs != get(rhs, std::integral_constant<std::size_t, 0>{}); }

  template<typename T>
  constexpr bool operator<(const T& lhs, const single_view<T>& rhs) noexcept { return lhs < get(rhs, std::integral_constant<std::size_t, 0>{}); }

  template<typename T>
  constexpr bool operator>(const T& lhs, const single_view<T>& rhs) noexcept { return lhs > get(rhs, std::integral_constant<std::size_t, 0>{}); }

  template<typename T>
  constexpr bool operator<=(const T& lhs, const single_view<T>& rhs) noexcept { return lhs <= get(rhs, std::integral_constant<std::size_t, 0>{}); }

  template<typename T>
  constexpr bool operator>=(const T& lhs, const single_view<T>& rhs) noexcept { return lhs >= get(rhs, std::integral_constant<std::size_t, 0>{}); }

#endif

} // namespace OpenKalman


namespace OpenKalman::ranges::views
{
  namespace detail
  {
    struct single_impl
    {
      template<typename R>
      constexpr auto
      operator() [[nodiscard]] (R&& r) const { return single_view<R> {std::forward<R>(r)}; }
    };
  }


  inline constexpr detail::single_impl single;

}

#endif

#endif //OPENKALMAN_COMPATIBILITY_VIEWS_SINGLE_HPP