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1//===----------------------------------------------------------------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8 9#ifndef _LIBCPP___ALGORITHM_LOWER_BOUND_H10#define _LIBCPP___ALGORITHM_LOWER_BOUND_H11 12#include <__algorithm/comp.h>13#include <__algorithm/half_positive.h>14#include <__algorithm/iterator_operations.h>15#include <__config>16#include <__functional/identity.h>17#include <__iterator/advance.h>18#include <__iterator/distance.h>19#include <__iterator/iterator_traits.h>20#include <__type_traits/invoke.h>21#include <__type_traits/is_callable.h>22 23#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)24# pragma GCC system_header25#endif26 27_LIBCPP_BEGIN_NAMESPACE_STD28 29template <class _AlgPolicy, class _Iter, class _Type, class _Proj, class _Comp>30[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Iter __lower_bound_bisecting(31 _Iter __first,32 const _Type& __value,33 typename iterator_traits<_Iter>::difference_type __len,34 _Comp& __comp,35 _Proj& __proj) {36 while (__len != 0) {37 auto __l2 = std::__half_positive(__len);38 _Iter __m = __first;39 _IterOps<_AlgPolicy>::advance(__m, __l2);40 if (std::__invoke(__comp, std::__invoke(__proj, *__m), __value)) {41 __first = ++__m;42 __len -= __l2 + 1;43 } else {44 __len = __l2;45 }46 }47 return __first;48}49 50// One-sided binary search, aka meta binary search, has been in the public domain for decades, and has the general51// advantage of being \Omega(1) rather than the classic algorithm's \Omega(log(n)), with the downside of executing at52// most 2*log(n) comparisons vs the classic algorithm's exact log(n). There are two scenarios in which it really shines:53// the first one is when operating over non-random-access iterators, because the classic algorithm requires knowing the54// container's size upfront, which adds \Omega(n) iterator increments to the complexity. The second one is when you're55// traversing the container in order, trying to fast-forward to the next value: in that case, the classic algorithm56// would yield \Omega(n*log(n)) comparisons and, for non-random-access iterators, \Omega(n^2) iterator increments,57// whereas the one-sided version will yield O(n) operations on both counts, with a \Omega(log(n)) bound on the number of58// comparisons.59template <class _AlgPolicy, class _ForwardIterator, class _Sent, class _Type, class _Proj, class _Comp>60[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator61__lower_bound_onesided(_ForwardIterator __first, _Sent __last, const _Type& __value, _Comp& __comp, _Proj& __proj) {62 // step = 0, ensuring we can always short-circuit when distance is 1 later on63 if (__first == __last || !std::__invoke(__comp, std::__invoke(__proj, *__first), __value))64 return __first;65 66 using _Distance = typename iterator_traits<_ForwardIterator>::difference_type;67 for (_Distance __step = 1; __first != __last; __step <<= 1) {68 auto __it = __first;69 auto __dist = __step - _IterOps<_AlgPolicy>::__advance_to(__it, __step, __last);70 // once we reach the last range where needle can be we must start71 // looking inwards, bisecting that range72 if (__it == __last || !std::__invoke(__comp, std::__invoke(__proj, *__it), __value)) {73 // we've already checked the previous value and it was less, we can save74 // one comparison by skipping bisection75 if (__dist == 1)76 return __it;77 return std::__lower_bound_bisecting<_AlgPolicy>(__first, __value, __dist, __comp, __proj);78 }79 // range not found, move forward!80 __first = __it;81 }82 return __first;83}84 85template <class _AlgPolicy, class _ForwardIterator, class _Sent, class _Type, class _Proj, class _Comp>86[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator87__lower_bound(_ForwardIterator __first, _Sent __last, const _Type& __value, _Comp& __comp, _Proj& __proj) {88 const auto __dist = _IterOps<_AlgPolicy>::distance(__first, __last);89 return std::__lower_bound_bisecting<_AlgPolicy>(__first, __value, __dist, __comp, __proj);90}91 92template <class _ForwardIterator, class _Tp, class _Compare>93[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator94lower_bound(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value, _Compare __comp) {95 static_assert(__is_callable<_Compare&, decltype(*__first), const _Tp&>::value, "The comparator has to be callable");96 auto __proj = std::__identity();97 return std::__lower_bound<_ClassicAlgPolicy>(__first, __last, __value, __comp, __proj);98}99 100template <class _ForwardIterator, class _Tp>101[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator102lower_bound(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) {103 return std::lower_bound(__first, __last, __value, __less<>());104}105 106_LIBCPP_END_NAMESPACE_STD107 108#endif // _LIBCPP___ALGORITHM_LOWER_BOUND_H109