get.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_COLLECTIONS_GET_HPP
#define OPENKALMAN_COLLECTIONS_GET_HPP

#include <tuple>
#ifdef __cpp_lib_ranges
#include <ranges>
#endif
#include "basics/compatibility/ranges.hpp"
#include "basics/global-definitions.hpp"
#include "values/concepts/fixed.hpp"
#include "values/concepts/index.hpp"
#include "values/traits/fixed_number_of.hpp"
#include "collections/concepts/collection.hpp"
#include "collections/traits/size_of.hpp"

namespace OpenKalman::collections
{
#ifndef __cpp_lib_ranges
  namespace detail
  {
    template<typename T, typename I>
    inline constexpr bool gettable_with_i = []
    {
      if constexpr (values::fixed<I>) { return gettable<values::fixed_number_of_v<I>, T>; }
      else { return false; }
    }();
  }
#endif


#ifdef __cpp_lib_ranges
  template<collection Arg, values::index I> requires std::ranges::random_access_range<Arg> or
    (values::fixed<I> and gettable<values::fixed_number_of_v<I>, Arg>)
#else
  template<typename Arg, typename I, std::enable_if_t<collection<Arg> and values::index<I> and
    (ranges::random_access_range<Arg> or detail::gettable_with_i<Arg, I>), int> = 0>
#endif
  constexpr decltype(auto)
  get(Arg&& arg, I i)
  {
    if constexpr (sized<Arg> and values::fixed<I>)
    { static_assert(size_of_v<Arg> == dynamic_size or values::fixed_number_of_v<I> < size_of_v<Arg>, "Index out of range"); }

#ifdef __cpp_lib_ranges
    if constexpr (values::fixed<I> and requires { requires gettable<values::fixed_number_of<I>::value, Arg>; })
#else
    if constexpr (detail::gettable_with_i<Arg, I>)
#endif
    {
      return OpenKalman::internal::generalized_std_get<values::fixed_number_of_v<I>>(std::forward<Arg>(arg));
    }
    else
    {
#ifdef __cpp_lib_remove_cvref
      using std::remove_cvref_t;
#endif
#ifdef __cpp_lib_ranges
      namespace ranges = std::ranges;
#endif

      std::size_t n = values::to_number(std::move(i));
      if constexpr (std::is_array_v<remove_cvref_t<Arg>>)
        return std::forward<Arg>(arg)[n];
      else if constexpr (ranges::borrowed_range<Arg>)
        return ranges::begin(std::forward<Arg>(arg))[n];
      else
        return begin(std::forward<Arg>(arg))[n];
    }
  }


} // namespace OpenKalman::collections

#endif //OPENKALMAN_COLLECTIONS_GET_HPP