get_element.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_ELEMENT_HPP
#define OPENKALMAN_COLLECTIONS_GET_ELEMENT_HPP

#include "values/values.hpp"
#include "collections/traits/size_of.hpp"

namespace OpenKalman::collections
{
  namespace detail_get
  {
#ifdef __cpp_concepts
    template<typename T, typename I>
#else
    template<typename T, typename I, typename = void>
#endif
    struct has_member_get_function : std::false_type {};

#ifdef __cpp_concepts
    template<typename T, typename I> requires requires { std::declval<T&&>().template get<values::fixed_value_of<I>::value>(); }
    struct has_member_get_function<T, I>
#else
    template<typename T, typename I>
    struct has_member_get_function<T, I, std::void_t<decltype(std::declval<T>().template get<values::fixed_value_of<I>::value>())>>
#endif
      : std::true_type {};


    using std::get;

#ifdef __cpp_concepts
    template<typename T, typename I>
#else
    template<typename T, typename I, typename = void>
#endif
    struct has_adl_get_function : std::false_type {};

#ifdef __cpp_concepts
    template<typename T, typename I> requires requires { get<values::fixed_value_of<I>::value>(std::declval<T&&>()); }
    struct has_adl_get_function<T, I>
#else
    template<typename T, typename I>
    struct has_adl_get_function<T, I, std::void_t<decltype(get<values::fixed_value_of<I>::value>(std::declval<T>()))>>
#endif
      : std::true_type {};


    template<typename Arg, typename I>
    constexpr decltype(auto)
    get_element_impl(Arg&& arg, I ix)
    {
      if constexpr (has_member_get_function<Arg, I>::value)
      {
        return std::forward<Arg>(arg).template get<values::fixed_value_of_v<I>>();
      }
      else if constexpr (has_adl_get_function<Arg, I>::value)
      {
        return get<values::fixed_value_of_v<I>>(std::forward<Arg>(arg));
      }
      else
      {
        std::size_t n = values::to_value_type(std::move(ix));
        if constexpr (std::is_array_v<stdex::remove_cvref_t<Arg>>)
        {
          return std::forward<Arg>(arg)[n];
        }
        else if constexpr (stdex::ranges::borrowed_range<Arg>)
        {
          return stdex::ranges::begin(std::forward<Arg>(arg))[n];
        }
        else
        {
          using namespace std;
          return begin(std::forward<Arg>(arg))[n];
        }
      }

    };

  }


#ifdef __cpp_concepts
  template<typename Arg, values::index I> requires
    (not values::size_compares_with<I, size_of<Arg>, &stdex::is_gteq>) and
    (stdex::ranges::random_access_range<Arg> or
      detail_get::has_member_get_function<Arg, I>::value or
      detail_get::has_adl_get_function<Arg, I>::value)
#else
  template<typename Arg, typename I, std::enable_if_t<values::index<I> and
    (not values::size_compares_with<I, size_of<Arg>, &stdex::is_gteq>) and
    (stdex::ranges::random_access_range<Arg> or
      detail_get::has_member_get_function<Arg, I>::value or
      detail_get::has_adl_get_function<Arg, I>::value), int> = 0>
#endif
  constexpr decltype(auto)
  get_element(Arg&& arg, I i)
  {
    return detail_get::get_element_impl(std::forward<Arg>(arg), i);
  }


}

#endif