v1/__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___ITERATOR_ADVANCE_H
0011 #define _LIBCPP___ITERATOR_ADVANCE_H
0012
0013 #include <__assert>
0014 #include <__concepts/assignable.h>
0015 #include <__concepts/same_as.h>
0016 #include <__config>
0017 #include <__iterator/concepts.h>
0018 #include <__iterator/incrementable_traits.h>
0019 #include <__iterator/iterator_traits.h>
0020 #include <__type_traits/enable_if.h>
0021 #include <__type_traits/is_integral.h>
0022 #include <__utility/convert_to_integral.h>
0023 #include <__utility/declval.h>
0024 #include <__utility/move.h>
0025 #include <__utility/unreachable.h>
0026 #include <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 <__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 struct __advance {
0080 private:
0081   template <class _Ip>
0082   _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_forward(_Ip& __i, iter_difference_t<_Ip> __n) {
0083     while (__n > 0) {
0084       --__n;
0085       ++__i;
0086     }
0087   }
0088
0089   template <class _Ip>
0090   _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_backward(_Ip& __i, iter_difference_t<_Ip> __n) {
0091     while (__n < 0) {
0092       ++__n;
0093       --__i;
0094     }
0095   }
0096
0097 public:
0098   // Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative.
0099   template <input_or_output_iterator _Ip>
0100   _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const {
0101     // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
0102     _LIBCPP_ASSERT_PEDANTIC(
0103         __n >= 0 || bidirectional_iterator<_Ip>, "If `n < 0`, then `bidirectional_iterator<I>` must be true.");
0104
0105     // If `I` models `random_access_iterator`, equivalent to `i += n`.
0106     if constexpr (random_access_iterator<_Ip>) {
0107       __i += __n;
0108       return;
0109     } else if constexpr (bidirectional_iterator<_Ip>) {
0110       // Otherwise, if `n` is non-negative, increments `i` by `n`.
0111       __advance_forward(__i, __n);
0112       // Otherwise, decrements `i` by `-n`.
0113       __advance_backward(__i, __n);
0114       return;
0115     } else {
0116       // Otherwise, if `n` is non-negative, increments `i` by `n`.
0117       __advance_forward(__i, __n);
0118       return;
0119     }
0120   }
0121
0122   // Preconditions: Either `assignable_from<I&, S> || sized_sentinel_for<S, I>` is modeled, or [i, bound_sentinel)
0123   // denotes a range.
0124   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
0125   _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, _Sp __bound_sentinel) const {
0126     // If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound_sentinel)`.
0127     if constexpr (assignable_from<_Ip&, _Sp>) {
0128       __i = std::move(__bound_sentinel);
0129     }
0130     // Otherwise, if `S` and `I` model `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound_sentinel -
0131     // i)`.
0132     else if constexpr (sized_sentinel_for<_Sp, _Ip>) {
0133       (*this)(__i, __bound_sentinel - __i);
0134     }
0135     // Otherwise, while `bool(i != bound_sentinel)` is true, increments `i`.
0136     else {
0137       while (__i != __bound_sentinel) {
0138         ++__i;
0139       }
0140     }
0141   }
0142
0143   // Preconditions:
0144   //   * If `n > 0`, [i, bound_sentinel) denotes a range.
0145   //   * If `n == 0`, [i, bound_sentinel) or [bound_sentinel, i) denotes a range.
0146   //   * If `n < 0`, [bound_sentinel, i) denotes a range, `I` models `bidirectional_iterator`, and `I` and `S` model
0147   //   `same_as<I, S>`.
0148   // Returns: `n - M`, where `M` is the difference between the ending and starting position.
0149   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
0150   _LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip>
0151   operator()(_Ip& __i, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
0152     // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
0153     _LIBCPP_ASSERT_PEDANTIC((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>),
0154                             "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true.");
0155     // If `S` and `I` model `sized_sentinel_for<S, I>`:
0156     if constexpr (sized_sentinel_for<_Sp, _Ip>) {
0157       // If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`.
0158       // __magnitude_geq(a, b) returns |a| >= |b|, assuming they have the same sign.
0159       auto __magnitude_geq = [](auto __a, auto __b) { return __a == 0 ? __b == 0 : __a > 0 ? __a >= __b : __a <= __b; };
0160       if (const auto __m = __bound_sentinel - __i; __magnitude_geq(__n, __m)) {
0161         (*this)(__i, __bound_sentinel);
0162         return __n - __m;
0163       }
0164
0165       // Otherwise, equivalent to `ranges::advance(i, n)`.
0166       (*this)(__i, __n);
0167       return 0;
0168     } else {
0169       // Otherwise, if `n` is non-negative, while `bool(i != bound_sentinel)` is true, increments `i` but at
0170       // most `n` times.
0171       while (__n > 0 && __i != __bound_sentinel) {
0172         ++__i;
0173         --__n;
0174       }
0175
0176       // Otherwise, while `bool(i != bound_sentinel)` is true, decrements `i` but at most `-n` times.
0177       if constexpr (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) {
0178         while (__n < 0 && __i != __bound_sentinel) {
0179           --__i;
0180           ++__n;
0181         }
0182       }
0183       return __n;
0184     }
0185
0186     __libcpp_unreachable();
0187   }
0188 };
0189
0190 inline namespace __cpo {
0191 inline constexpr auto advance = __advance{};
0192 } // namespace __cpo
0193 } // namespace ranges
0194
0195 #endif // _LIBCPP_STD_VER >= 20
0196
0197 _LIBCPP_END_NAMESPACE_STD
0198
0199 _LIBCPP_POP_MACROS
0200
0201 #endif // _LIBCPP___ITERATOR_ADVANCE_H