empty.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_EMPTY_HPP
#define OPENKALMAN_COMPATIBILITY_VIEWS_EMPTY_HPP

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

namespace OpenKalman::stdex::ranges
{
#ifdef __cpp_lib_ranges
  using std::ranges::empty_view;
  namespace views
  {
    using std::ranges::views::empty;
  }
#else

  template<typename T>
  struct empty_view : stdex::ranges::view_interface<empty_view<T>>
  {
    static constexpr T* begin() noexcept { return nullptr; }

    static constexpr T* end() noexcept { return nullptr; }

    static constexpr T* data() noexcept { return nullptr; }

    static constexpr std::size_t size() noexcept { return 0; }

    static constexpr bool empty() noexcept { return true; }
  };


  template<typename T>
  constexpr bool enable_borrowed_range<empty_view<T>> = true;


  namespace views
  {
    template<class T>
    constexpr empty_view<T> empty{};
  }

#endif
}

#endif