chromium/third_party/libc++/src/include/__algorithm/set_intersection.h

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___ALGORITHM_SET_INTERSECTION_H
#define _LIBCPP___ALGORITHM_SET_INTERSECTION_H

#include <__algorithm/comp.h>
#include <__algorithm/comp_ref_type.h>
#include <__algorithm/iterator_operations.h>
#include <__algorithm/lower_bound.h>
#include <__config>
#include <__functional/identity.h>
#include <__iterator/iterator_traits.h>
#include <__iterator/next.h>
#include <__type_traits/is_same.h>
#include <__utility/exchange.h>
#include <__utility/move.h>
#include <__utility/swap.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _InIter1, class _InIter2, class _OutIter>
struct __set_intersection_result { … };

// Helper for __set_intersection() with one-sided binary search: populate result and advance input iterators if they
// are found to potentially contain the same value in two consecutive calls. This function is very intimately related to
// the way it is used and doesn't attempt to abstract that, it's not appropriate for general usage outside of its
// context.
template <class _InForwardIter1, class _InForwardIter2, class _OutIter>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __set_intersection_add_output_if_equal(
    bool __may_be_equal,
    _InForwardIter1& __first1,
    _InForwardIter2& __first2,
    _OutIter& __result,
    bool& __prev_may_be_equal) { … }

// With forward iterators we can make multiple passes over the data, allowing the use of one-sided binary search to
// reduce best-case complexity to log(N). Understanding how we can use binary search and still respect complexity
// guarantees is _not_ straightforward: the guarantee is "at most 2*(N+M)-1 comparisons", and one-sided binary search
// will necessarily overshoot depending on the position of the needle in the haystack -- for instance, if we're
// searching for 3 in (1, 2, 3, 4), we'll check if 3<1, then 3<2, then 3<4, and, finally, 3<3, for a total of 4
// comparisons, when linear search would have yielded 3. However, because we won't need to perform the intervening
// reciprocal comparisons (ie 1<3, 2<3, 4<3), that extra comparison doesn't run afoul of the guarantee. Additionally,
// this type of scenario can only happen for match distances of up to 5 elements, because 2*log2(8) is 6, and we'll
// still be worse-off at position 5 of an 8-element set. From then onwards these scenarios can't happen. TL;DR: we'll be
// 1 comparison worse-off compared to the classic linear-searching algorithm if matching position 3 of a set with 4
// elements, or position 5 if the set has 7 or 8 elements, but we'll never exceed the complexity guarantees from the
// standard.
template <class _AlgPolicy,
          class _Compare,
          class _InForwardIter1,
          class _Sent1,
          class _InForwardIter2,
          class _Sent2,
          class _OutIter>
_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter>
__set_intersection(
    _InForwardIter1 __first1,
    _Sent1 __last1,
    _InForwardIter2 __first2,
    _Sent2 __last2,
    _OutIter __result,
    _Compare&& __comp,
    std::forward_iterator_tag,
    std::forward_iterator_tag) { … }

// input iterators are not suitable for multipass algorithms, so we stick to the classic single-pass version
template <class _AlgPolicy,
          class _Compare,
          class _InInputIter1,
          class _Sent1,
          class _InInputIter2,
          class _Sent2,
          class _OutIter>
_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter>
__set_intersection(
    _InInputIter1 __first1,
    _Sent1 __last1,
    _InInputIter2 __first2,
    _Sent2 __last2,
    _OutIter __result,
    _Compare&& __comp,
    std::input_iterator_tag,
    std::input_iterator_tag) { … }

template <class _AlgPolicy, class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter>
_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InIter1, _InIter2, _OutIter>
__set_intersection(
    _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) { … }

template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection(
    _InputIterator1 __first1,
    _InputIterator1 __last1,
    _InputIterator2 __first2,
    _InputIterator2 __last2,
    _OutputIterator __result,
    _Compare __comp) { … }

template <class _InputIterator1, class _InputIterator2, class _OutputIterator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection(
    _InputIterator1 __first1,
    _InputIterator1 __last1,
    _InputIterator2 __first2,
    _InputIterator2 __last2,
    _OutputIterator __result) { … }

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP___ALGORITHM_SET_INTERSECTION_H