std::ranges::partial_sort(3)

NAME
std::ranges::partial_sort - std::ranges::partial_sort

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::random_access_iterator I, std::sentinel_for<I> S,
             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<I, Comp, Proj> (1) (since C++20)
   constexpr I
       partial_sort( I first, I middle, S last, Comp comp = {}, Proj
   proj = {} );
   template< ranges::random_access_range R,
             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
   constexpr ranges::borrowed_iterator_t<R> (2) (since C++20)
       partial_sort( R&& r, ranges::iterator_t<R> middle, Comp comp =
   {},
                     Proj proj = {} );
   1) Rearranges elements such that the range [first, middle) contains the sorted
   middle - first smallest elements in the range [first, last).
   The order of equal elements is not guaranteed to be preserved. The order of the
   remaining elements in the range [middle, last) is unspecified.
   The elements are compared using the given binary comparison function comp and
   projected using proj function object.
   2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - iterator-sentinel defining the range to sort
   r - the range to sort
   middle - the iterator defining the last element to be sorted
   comp - comparator to apply to the projected elements
   proj - projection to apply to the elements

Return value
   An iterator equal to last.

Complexity
   \(\scriptsize \mathcal{O}(N\cdot\log{(M)})\)𝓞(N·log(M)) comparisons and twice as
   many projections, where \(\scriptsize N\)N is ranges::distance(first, last),
   \(\scriptsize M\)M is ranges::distance(first, middle).

Possible implementation
struct partial_sort_fn {
    template<std::random_access_iterator I, std::sentinel_for<I> S,
             class Comp = ranges::less, class Proj = std::identity>
    requires std::sortable<I, Comp, Proj>
    constexpr I
        operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == middle)
            return ranges::next(first, last);
        ranges::make_heap(first, middle, comp, proj);
        auto it {middle};
        for (; it != last; ++it)
        {
            if (std::invoke(comp, std::invoke(proj, *it), std::invoke(proj, *first)))
            {
                ranges::pop_heap(first, middle, comp, proj);
                ranges::iter_swap(middle - 1, it);
                ranges::push_heap(first, middle, comp, proj);
            }
        }
        ranges::sort_heap(first, middle, comp, proj);
        return it;
    }
    template<ranges::random_access_range R, class Comp = ranges::less,
             class Proj = std::identity>
    requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
                       std::move(comp), std::move(proj));
    } }; inline constexpr partial_sort_fn partial_sort {};

Example
// Run this code
 #include <algorithm>
 #include <functional>
 #include <iostream>
 #include <string>
 #include <vector>
 void print(const auto& v)
 {
     for (const char e : v)
         std::cout << e << ' ';
     std::cout << '\n';
 }
 void underscore(int n)
 {
     while (n-- > 0)
         std::cout << "^ ";
     std::cout << '\n';
 }
 int main()
 {
     static_assert('A' < 'a');
     std::vector<char> v {'x', 'P', 'y', 'C', 'z', 'w', 'P', 'o'};
     print(v);
     const int m {3};
     std::ranges::partial_sort(v, v.begin() + m);
     print(v), underscore(m);
     static_assert('1' < 'a');
     std::string s {"3a1b41c5"};
     print(s);
     std::ranges::partial_sort(s.begin(), s.begin() + m, s.end(), std::greater {});
     print(s), underscore(m);
 }

Output:
 x P y C z w P o
 C P P y z x w o
 ^ ^ ^
 3 a 1 b 4 1 c 5
 c b a 1 3 1 4 5
 ^ ^ ^

See also
   ranges::partial_sort_copy copies and partially sorts a range of elements
   (C++20) (niebloid)
   ranges::sort sorts a range into ascending order
   (C++20) (niebloid)
   ranges::stable_sort sorts a range of elements while preserving order between
   (C++20) equal elements
                        (niebloid)
   ranges::nth_element partially sorts the given range making sure that it is
   (C++20) partitioned by the given element
                        (niebloid)
   ranges::make_heap creates a max heap out of a range of elements
   (C++20) (niebloid)
   ranges::pop_heap removes the largest element from a max heap
   (C++20) (niebloid)
   ranges::push_heap adds an element to a max heap
   (C++20) (niebloid)
   ranges::sort_heap turns a max heap into a range of elements sorted in
   (C++20) ascending order
                        (niebloid)
   partial_sort sorts the first N elements of a range
                        (function template)