algorithm.hh

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// SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
// vi: set et ts=4 sw=2 sts=2:
#ifndef DUNE_COMMON_STD_ALGORITHM_HH
#define DUNE_COMMON_STD_ALGORITHM_HH

#include <version>
#if !(__cpp_impl_three_way_comparison >= 201907L && __cpp_lib_concepts && __has_include(<compare>))
  #error "Three-way comparison requires language support!"
#endif

#include <algorithm>
#include <compare>
#include <type_traits>

#include <dune/common/std/compare.hh>

namespace Dune::Std {

#if __cpp_lib_three_way_comparison >= 201907L

using std::lexicographical_compare_three_way;

#else // __cpp_lib_three_way_comparison

template <class I1, class I2, class Cmp = Std::compare_three_way>
constexpr auto lexicographical_compare_three_way(I1 f1, I1 l1, I2 f2, I2 l2, Cmp comp = {})
    -> decltype(comp(*f1, *f2))
{
  using ret_t = decltype(comp(*f1, *f2));
  static_assert(std::disjunction_v<
                    std::is_same<ret_t, std::strong_ordering>,
                    std::is_same<ret_t, std::weak_ordering>,
                    std::is_same<ret_t, std::partial_ordering>>,
                "The return type must be a comparison category type.");

  bool exhaust1 = (f1 == l1);
  bool exhaust2 = (f2 == l2);
  for (; !exhaust1 && !exhaust2; exhaust1 = (++f1 == l1), exhaust2 = (++f2 == l2))
    if (auto c = comp(*f1, *f2); c != 0)
      return c;

  return !exhaust1 ? std::strong_ordering::greater:
         !exhaust2 ? std::strong_ordering::less:
                     std::strong_ordering::equal;
}

#endif // __cpp_lib_three_way_comparison

} // end namespace Dune::Std

#endif // DUNE_COMMON_STD_ALGORITHM_HH