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std::ranges::stable_partition() algorithm

// (1)
ranges::subrange<I>
    stable_partition( I first, S last, Pred pred, Proj proj = {} );

// (2)
ranges::borrowed_subrange_t<R>
    stable_partition( R&& r, Pred pred, Proj proj = {} );

The type of arguments are generic and have the following constraints:

  • I - std::bidirectional_iterator
  • S - std::sentinel_for<I>
  • R - std::ranges::bidirectional_range
  • Pred:
    • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
    • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
  • Proj - (none)

The Proj template argument has the following default type std::identity for all overloads.

Additionally, each overload has the following constraints:

  • (1) - std::permutable<I>
  • (2) - std::permutable<ranges::iterator_t<R>>

  • (1) Reorders the elements in the range [first; last) in such a way that the projection proj of all elements for which the predicate pred returns true precede the projection proj of elements for which predicate pred returns false.

    The algorithms is stable, the relative order of elements is preserved.

  • (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.

Parameters​

first
last

The range of elements to reorder.

r

The range of elements to reorder.

pred

The predicate to apply to the projected elements.

proj

The projection to apply to the elements.

Return value​

An object equal to

{
  pivot,
  last
}

Where pivot is an iterator to the first element of the second group.

Complexity​

Given N ranges::distance(first, last):

The complexity is at worst N * log(N) swaps, and only O(N) swaps in case an extra memory buffer is used.

Exactly N applications of the predicate pred and projection proj.

Exceptions​

(none)

Possible implementation​

stable_partition(1) and stable_partition(2)
struct stable_partition_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    requires std::permutable<I>
    constexpr ranges::subrange<I>
        operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        first = ranges::find_if_not(first, last, pred, proj);
        I mid = first;
        while (mid != last)
        {
            mid = ranges::find_if(mid, last, pred, proj);
            if (mid == last)
                break;
            I last2 = ranges::find_if_not(mid, last, pred, proj);
            ranges::rotate(first, mid, last2);
            first = ranges::next(first, ranges::distance(mid, last2));
            mid = last2;
        }
        return {std::move(first), std::move(mid)};
    }

    template<ranges::bidirectional_range R, class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>> Pred>
    requires std::permutable<ranges::iterator_t<R>>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(pred), std::move(proj));
    }
};

inline constexpr stable_partition_fn stable_partition {};

Notes​

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

Examples​

Main.cpp
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

namespace rng = std::ranges;

template<std::permutable I, std::sentinel_for<I> S>
constexpr void stable_sort(I first, S last)
{
    if (first == last)
        return;

    auto pivot = *rng::next(first, rng::distance(first, last) / 2, last);
    auto left = [pivot](const auto& em) { return em < pivot; };
    auto tail1 = rng::stable_partition(first, last, left);
    auto right = [pivot](const auto& em) { return !(pivot < em); };
    auto tail2 = rng::stable_partition(tail1, right);

    stable_sort(first, tail1.begin());
    stable_sort(tail2.begin(), tail2.end());
}

void print(const auto rem, auto first, auto last, bool end = true)
{
    std::cout << rem;
    for (; first != last; ++first)
        std::cout << *first << ' ';
    std::cout << (end ? "\n" : "");
}

int main()
{
    const auto original = {9, 6, 5, 2, 3, 1, 7, 8};

    std::vector<int> vi {};
    auto even = [](int x) { return 0 == (x % 2); };

    print("Original vector:\t", original.begin(), original.end(), "\n");

    vi = original;
    const auto ret1 = rng::stable_partition(vi, even);
    print("Stable partitioned:\t", vi.begin(), ret1.begin(), 0);
    print("│ ", ret1.begin(), ret1.end());

    vi = original;
    const auto ret2 = rng::partition(vi, even);
    print("Partitioned:\t\t", vi.begin(), ret2.begin(), 0);
    print("│ ", ret2.begin(), ret2.end());


    vi = {16, 30, 44, 30, 15, 24, 10, 18, 12, 35};
    print("Unsorted vector: ", vi.begin(), vi.end());

    stable_sort(rng::begin(vi), rng::end(vi));
    print("Sorted vector:   ", vi.begin(), vi.end());
}
Possible Output
Original vector:        9 6 5 2 3 1 7 8
Stable partitioned:     6 2 8 │ 9 5 3 1 7
Partitioned:            8 6 2 │ 5 3 1 7 9
Unsorted vector: 16 30 44 30 15 24 10 18 12 35 
Sorted vector:   10 12 15 16 18 24 30 30 35 44
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