xref: /llvm-project/libcxx/include/__algorithm/nth_element.h (revision 9783f28cbb155e4a8d49c12e1c60ce14dcfaf0c7)

//===----------------------------------------------------------------------===//
//
// 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_NTH_ELEMENT_H
#define _LIBCPP___ALGORITHM_NTH_ELEMENT_H

#include <__algorithm/comp.h>
#include <__algorithm/comp_ref_type.h>
#include <__algorithm/iterator_operations.h>
#include <__algorithm/sort.h>
#include <__assert>
#include <__config>
#include <__debug_utils/randomize_range.h>
#include <__iterator/iterator_traits.h>
#include <__utility/move.h>

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

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool __nth_element_find_guard(
    _RandomAccessIterator& __i, _RandomAccessIterator& __j, _RandomAccessIterator __m, _Compare __comp) {
  // manually guard downward moving __j against __i
  while (true) {
    if (__i == --__j) {
      return false;
    }
    if (__comp(*__j, *__m)) {
      return true; // found guard for downward moving __j, now use unguarded partition
    }
  }
}

template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
// NOLINTNEXTLINE(readability-function-cognitive-complexity)
__nth_element(
    _RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) {
  using _Ops = _IterOps<_AlgPolicy>;

  // _Compare is known to be a reference type
  typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
  const difference_type __limit = 7;
  while (true) {
    if (__nth == __last)
      return;
    difference_type __len = __last - __first;
    switch (__len) {
    case 0:
    case 1:
      return;
    case 2:
      if (__comp(*--__last, *__first))
        _Ops::iter_swap(__first, __last);
      return;
    case 3: {
      _RandomAccessIterator __m = __first;
      std::__sort3<_AlgPolicy, _Compare>(__first, ++__m, --__last, __comp);
      return;
    }
    }
    if (__len <= __limit) {
      std::__selection_sort<_AlgPolicy, _Compare>(__first, __last, __comp);
      return;
    }
    // __len > __limit >= 3
    _RandomAccessIterator __m   = __first + __len / 2;
    _RandomAccessIterator __lm1 = __last;
    unsigned __n_swaps          = std::__sort3<_AlgPolicy, _Compare>(__first, __m, --__lm1, __comp);
    // *__m is median
    // partition [__first, __m) < *__m and *__m <= [__m, __last)
    // (this inhibits tossing elements equivalent to __m around unnecessarily)
    _RandomAccessIterator __i = __first;
    _RandomAccessIterator __j = __lm1;
    // j points beyond range to be tested, *__lm1 is known to be <= *__m
    // The search going up is known to be guarded but the search coming down isn't.
    // Prime the downward search with a guard.
    if (!__comp(*__i, *__m)) // if *__first == *__m
    {
      // *__first == *__m, *__first doesn't go in first part
      if (std::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
        _Ops::iter_swap(__i, __j);
        ++__n_swaps;
      } else {
        // *__first == *__m, *__m <= all other elements
        // Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
        ++__i; // __first + 1
        __j = __last;
        if (!__comp(*__first, *--__j)) { // we need a guard if *__first == *(__last-1)
          while (true) {
            if (__i == __j) {
              return; // [__first, __last) all equivalent elements
            } else if (__comp(*__first, *__i)) {
              _Ops::iter_swap(__i, __j);
              ++__n_swaps;
              ++__i;
              break;
            }
            ++__i;
          }
        }
        // [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
        if (__i == __j) {
          return;
        }
        while (true) {
          while (!__comp(*__first, *__i)) {
            ++__i;
            _LIBCPP_ASSERT_UNCATEGORIZED(
                __i != __last,
                "Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
          }
          do {
            _LIBCPP_ASSERT_UNCATEGORIZED(
                __j != __first,
                "Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
            --__j;
          } while (__comp(*__first, *__j));
          if (__i >= __j)
            break;
          _Ops::iter_swap(__i, __j);
          ++__n_swaps;
          ++__i;
        }
        // [__first, __i) == *__first and *__first < [__i, __last)
        // The first part is sorted,
        if (__nth < __i) {
          return;
        }
        // __nth_element the second part
        // std::__nth_element<_Compare>(__i, __nth, __last, __comp);
        __first = __i;
        continue;
      }
    }
    ++__i;
    // j points beyond range to be tested, *__lm1 is known to be <= *__m
    // if not yet partitioned...
    if (__i < __j) {
      // known that *(__i - 1) < *__m
      while (true) {
        // __m still guards upward moving __i
        while (__comp(*__i, *__m)) {
          ++__i;
          _LIBCPP_ASSERT_UNCATEGORIZED(
              __i != __last,
              "Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
        }
        // It is now known that a guard exists for downward moving __j
        do {
          _LIBCPP_ASSERT_UNCATEGORIZED(
              __j != __first,
              "Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
          --__j;
        } while (!__comp(*__j, *__m));
        if (__i >= __j)
          break;
        _Ops::iter_swap(__i, __j);
        ++__n_swaps;
        // It is known that __m != __j
        // If __m just moved, follow it
        if (__m == __i)
          __m = __j;
        ++__i;
      }
    }
    // [__first, __i) < *__m and *__m <= [__i, __last)
    if (__i != __m && __comp(*__m, *__i)) {
      _Ops::iter_swap(__i, __m);
      ++__n_swaps;
    }
    // [__first, __i) < *__i and *__i <= [__i+1, __last)
    if (__nth == __i)
      return;
    if (__n_swaps == 0) {
      // We were given a perfectly partitioned sequence.  Coincidence?
      if (__nth < __i) {
        // Check for [__first, __i) already sorted
        __j = __m = __first;
        while (true) {
          if (++__j == __i) {
            // [__first, __i) sorted
            return;
          }
          if (__comp(*__j, *__m)) {
            // not yet sorted, so sort
            break;
          }
          __m = __j;
        }
      } else {
        // Check for [__i, __last) already sorted
        __j = __m = __i;
        while (true) {
          if (++__j == __last) {
            // [__i, __last) sorted
            return;
          }
          if (__comp(*__j, *__m)) {
            // not yet sorted, so sort
            break;
          }
          __m = __j;
        }
      }
    }
    // __nth_element on range containing __nth
    if (__nth < __i) {
      // std::__nth_element<_Compare>(__first, __nth, __i, __comp);
      __last = __i;
    } else {
      // std::__nth_element<_Compare>(__i+1, __nth, __last, __comp);
      __first = ++__i;
    }
  }
}

template <class _AlgPolicy, class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __nth_element_impl(
    _RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare& __comp) {
  if (__nth == __last)
    return;

  std::__debug_randomize_range<_AlgPolicy>(__first, __last);

  std::__nth_element<_AlgPolicy, __comp_ref_type<_Compare> >(__first, __nth, __last, __comp);

  std::__debug_randomize_range<_AlgPolicy>(__first, __nth);
  if (__nth != __last) {
    std::__debug_randomize_range<_AlgPolicy>(++__nth, __last);
  }
}

template <class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) {
  std::__nth_element_impl<_ClassicAlgPolicy>(std::move(__first), std::move(__nth), std::move(__last), __comp);
}

template <class _RandomAccessIterator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) {
  std::nth_element(std::move(__first), std::move(__nth), std::move(__last), __less<>());
}

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H