__cxx03/__iterator/advance.h

0001 // -*- C++ -*-
0002 //===----------------------------------------------------------------------===//
0003 //
0004 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
0005 // See https://llvm.org/LICENSE.txt for license information.
0006 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
0007 //
0008 //===----------------------------------------------------------------------===//
0009
0010 #ifndef _LIBCPP___CXX03___ITERATOR_ADVANCE_H
0011 #define _LIBCPP___CXX03___ITERATOR_ADVANCE_H
0012
0013 #include <__cxx03/__assert>
0014 #include <__cxx03/__concepts/assignable.h>
0015 #include <__cxx03/__concepts/same_as.h>
0016 #include <__cxx03/__config>
0017 #include <__cxx03/__iterator/concepts.h>
0018 #include <__cxx03/__iterator/incrementable_traits.h>
0019 #include <__cxx03/__iterator/iterator_traits.h>
0020 #include <__cxx03/__type_traits/enable_if.h>
0021 #include <__cxx03/__type_traits/is_integral.h>
0022 #include <__cxx03/__utility/convert_to_integral.h>
0023 #include <__cxx03/__utility/declval.h>
0024 #include <__cxx03/__utility/move.h>
0025 #include <__cxx03/__utility/unreachable.h>
0026 #include <__cxx03/limits>
0027
0028 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0029 #  pragma GCC system_header
0030 #endif
0031
0032 _LIBCPP_PUSH_MACROS
0033 #include <__cxx03/__undef_macros>
0034
0035 _LIBCPP_BEGIN_NAMESPACE_STD
0036
0037 template <class _InputIter>
0038 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void
0039 __advance(_InputIter& __i, typename iterator_traits<_InputIter>::difference_type __n, input_iterator_tag) {
0040   for (; __n > 0; --__n)
0041     ++__i;
0042 }
0043
0044 template <class _BiDirIter>
0045 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void
0046 __advance(_BiDirIter& __i, typename iterator_traits<_BiDirIter>::difference_type __n, bidirectional_iterator_tag) {
0047   if (__n >= 0)
0048     for (; __n > 0; --__n)
0049       ++__i;
0050   else
0051     for (; __n < 0; ++__n)
0052       --__i;
0053 }
0054
0055 template <class _RandIter>
0056 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void
0057 __advance(_RandIter& __i, typename iterator_traits<_RandIter>::difference_type __n, random_access_iterator_tag) {
0058   __i += __n;
0059 }
0060
0061 template < class _InputIter,
0062            class _Distance,
0063            class _IntegralDistance = decltype(std::__convert_to_integral(std::declval<_Distance>())),
0064            __enable_if_t<is_integral<_IntegralDistance>::value, int> = 0>
0065 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void advance(_InputIter& __i, _Distance __orig_n) {
0066   typedef typename iterator_traits<_InputIter>::difference_type _Difference;
0067   _Difference __n = static_cast<_Difference>(std::__convert_to_integral(__orig_n));
0068   // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
0069   _LIBCPP_ASSERT_PEDANTIC(__n >= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
0070                           "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
0071   std::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
0072 }
0073
0074 #if _LIBCPP_STD_VER >= 20
0075
0076 // [range.iter.op.advance]
0077
0078 namespace ranges {
0079 namespace __advance {
0080
0081 struct __fn {
0082 private:
0083   template <class _Ip>
0084   _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_forward(_Ip& __i, iter_difference_t<_Ip> __n) {
0085     while (__n > 0) {
0086       --__n;
0087       ++__i;
0088     }
0089   }
0090
0091   template <class _Ip>
0092   _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_backward(_Ip& __i, iter_difference_t<_Ip> __n) {
0093     while (__n < 0) {
0094       ++__n;
0095       --__i;
0096     }
0097   }
0098
0099 public:
0100   // Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative.
0101   template <input_or_output_iterator _Ip>
0102   _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const {
0103     // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
0104     _LIBCPP_ASSERT_PEDANTIC(
0105         __n >= 0 || bidirectional_iterator<_Ip>, "If `n < 0`, then `bidirectional_iterator<I>` must be true.");
0106
0107     // If `I` models `random_access_iterator`, equivalent to `i += n`.
0108     if constexpr (random_access_iterator<_Ip>) {
0109       __i += __n;
0110       return;
0111     } else if constexpr (bidirectional_iterator<_Ip>) {
0112       // Otherwise, if `n` is non-negative, increments `i` by `n`.
0113       __advance_forward(__i, __n);
0114       // Otherwise, decrements `i` by `-n`.
0115       __advance_backward(__i, __n);
0116       return;
0117     } else {
0118       // Otherwise, if `n` is non-negative, increments `i` by `n`.
0119       __advance_forward(__i, __n);
0120       return;
0121     }
0122   }
0123
0124   // Preconditions: Either `assignable_from<I&, S> || sized_sentinel_for<S, I>` is modeled, or [i, bound_sentinel)
0125   // denotes a range.
0126   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
0127   _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, _Sp __bound_sentinel) const {
0128     // If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound_sentinel)`.
0129     if constexpr (assignable_from<_Ip&, _Sp>) {
0130       __i = std::move(__bound_sentinel);
0131     }
0132     // Otherwise, if `S` and `I` model `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound_sentinel -
0133     // i)`.
0134     else if constexpr (sized_sentinel_for<_Sp, _Ip>) {
0135       (*this)(__i, __bound_sentinel - __i);
0136     }
0137     // Otherwise, while `bool(i != bound_sentinel)` is true, increments `i`.
0138     else {
0139       while (__i != __bound_sentinel) {
0140         ++__i;
0141       }
0142     }
0143   }
0144
0145   // Preconditions:
0146   //   * If `n > 0`, [i, bound_sentinel) denotes a range.
0147   //   * If `n == 0`, [i, bound_sentinel) or [bound_sentinel, i) denotes a range.
0148   //   * If `n < 0`, [bound_sentinel, i) denotes a range, `I` models `bidirectional_iterator`, and `I` and `S` model
0149   //   `same_as<I, S>`.
0150   // Returns: `n - M`, where `M` is the difference between the ending and starting position.
0151   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
0152   _LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip>
0153   operator()(_Ip& __i, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
0154     // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
0155     _LIBCPP_ASSERT_PEDANTIC((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>),
0156                             "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true.");
0157     // If `S` and `I` model `sized_sentinel_for<S, I>`:
0158     if constexpr (sized_sentinel_for<_Sp, _Ip>) {
0159       // If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`.
0160       // __magnitude_geq(a, b) returns |a| >= |b|, assuming they have the same sign.
0161       auto __magnitude_geq = [](auto __a, auto __b) { return __a == 0 ? __b == 0 : __a > 0 ? __a >= __b : __a <= __b; };
0162       if (const auto __m = __bound_sentinel - __i; __magnitude_geq(__n, __m)) {
0163         (*this)(__i, __bound_sentinel);
0164         return __n - __m;
0165       }
0166
0167       // Otherwise, equivalent to `ranges::advance(i, n)`.
0168       (*this)(__i, __n);
0169       return 0;
0170     } else {
0171       // Otherwise, if `n` is non-negative, while `bool(i != bound_sentinel)` is true, increments `i` but at
0172       // most `n` times.
0173       while (__n > 0 && __i != __bound_sentinel) {
0174         ++__i;
0175         --__n;
0176       }
0177
0178       // Otherwise, while `bool(i != bound_sentinel)` is true, decrements `i` but at most `-n` times.
0179       if constexpr (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) {
0180         while (__n < 0 && __i != __bound_sentinel) {
0181           --__i;
0182           ++__n;
0183         }
0184       }
0185       return __n;
0186     }
0187
0188     __libcpp_unreachable();
0189   }
0190 };
0191
0192 } // namespace __advance
0193
0194 inline namespace __cpo {
0195 inline constexpr auto advance = __advance::__fn{};
0196 } // namespace __cpo
0197 } // namespace ranges
0198
0199 #endif // _LIBCPP_STD_VER >= 20
0200
0201 _LIBCPP_END_NAMESPACE_STD
0202
0203 _LIBCPP_POP_MACROS
0204
0205 #endif // _LIBCPP___CXX03___ITERATOR_ADVANCE_H