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[parallel] Fix hpx::nth_element to correctly support C++20 projections and sentinels #7191

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[parallel] Fix hpx::nth_element to correctly support C++20 projections and sentinels #7191

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5fd7338
[parallel] Fix hpx::nth_element to correctly support projections and …
dasaneek007-cpu Apr 16, 2026
181ec0a
Apply clang-format fixes
dasaneek007-cpu Apr 16, 2026
f895d19
Merge branch 'master' into fix/nth-element-projections-sentinels
aneek22112007-tech Apr 16, 2026
b0ebd62
Remove <iostream> inclusion to fix hpxinspect failure
dasaneek007-cpu Apr 16, 2026
9740280
Revert hpx::nth_element to standard compliance, move projections/sent…
dasaneek007-cpu Apr 16, 2026
e58f6aa
Address maintainer feedback: consolidate wrapped_comp_type and use st…
dasaneek007-cpu Apr 16, 2026
8199058
Apply clang-format fixes
dasaneek007-cpu Apr 16, 2026
5590bc1
Fix macOS CI: avoid detail::min_element type-matching issue with proj…
dasaneek007-cpu Apr 17, 2026
891e829
Merge branch 'master' into fix/nth-element-projections-sentinels
aneek22112007-tech Apr 17, 2026
5f9a80b
Fix type deduction in minmax.hpp and remove workaround in nth_element…
dasaneek007-cpu Apr 17, 2026
e61e974
Apply clang-format corrections to minmax.hpp
dasaneek007-cpu Apr 17, 2026
3ad02be
Fix clang-format in minmax.hpp after CI failure
dasaneek007-cpu Apr 17, 2026
c860663
Fix clang-format in minmax.hpp (final attempt)
dasaneek007-cpu Apr 17, 2026
e993817
Automated clang-format fix using opt/homebrew/bin/clang-format
dasaneek007-cpu Apr 17, 2026
30cf465
Refine element_type deduction to handle HPX proxies using proxy_value_t
dasaneek007-cpu Apr 18, 2026
0e2e53e
Remove redundant typename keywords as requested by maintainer
dasaneek007-cpu Apr 19, 2026
865d418
Restore typename keywords (strictly required for CI builds)
dasaneek007-cpu Apr 19, 2026
a4b4a86
Separate core fix into independent PR and restore temporary workaround
dasaneek007-cpu Apr 19, 2026
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33 changes: 20 additions & 13 deletions libs/core/algorithms/include/hpx/parallel/algorithms/minmax.hpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -404,7 +404,8 @@ namespace hpx::parallel {
return it;

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<FwdIter>::reference>>>;
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auto smallest = it;

Expand Down Expand Up @@ -442,9 +443,9 @@ namespace hpx::parallel {

auto smallest = *it;

using element_type =
hpx::traits::proxy_value_t<typename std::iterator_traits<
decltype(smallest)>::value_type>;
using element_type = hpx::traits::proxy_value_t<
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<Iter>::reference>>>;

element_type value = HPX_INVOKE(proj, *smallest);
util::loop_n<std::decay_t<ExPolicy>>(
Expand Down Expand Up @@ -474,7 +475,8 @@ namespace hpx::parallel {
return first;

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<FwdIter>::reference>>>;

auto smallest = first;

Expand Down Expand Up @@ -557,7 +559,8 @@ namespace hpx::parallel {
return it;

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<FwdIter>::reference>>>;

auto largest = it;

Expand Down Expand Up @@ -594,9 +597,9 @@ namespace hpx::parallel {

auto largest = *it;

using element_type =
hpx::traits::proxy_value_t<typename std::iterator_traits<
decltype(largest)>::value_type>;
using element_type = hpx::traits::proxy_value_t<
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<Iter>::reference>>>;

element_type value = HPX_INVOKE(proj, *largest);
util::loop_n<std::decay_t<ExPolicy>>(
Expand Down Expand Up @@ -626,7 +629,8 @@ namespace hpx::parallel {
return first;

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<FwdIter>::reference>>>;

auto largest = first;

Expand Down Expand Up @@ -711,7 +715,8 @@ namespace hpx::parallel {
return result;

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<FwdIter>::reference>>>;

element_type min_value = HPX_INVOKE(proj, *it);
element_type max_value = min_value;
Expand Down Expand Up @@ -752,7 +757,8 @@ namespace hpx::parallel {
return *it;

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<Iter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<Iter>::reference>>>;

auto result = *it;

Expand Down Expand Up @@ -800,7 +806,8 @@ namespace hpx::parallel {
}

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;
std::decay_t<hpx::util::invoke_result_t<Proj,
typename std::iterator_traits<FwdIter>::reference>>>;

element_type min_value = HPX_INVOKE(proj, *min);
element_type max_value = HPX_INVOKE(proj, *max);
Expand Down
30 changes: 19 additions & 11 deletions libs/core/algorithms/include/hpx/parallel/algorithms/nth_element.hpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -139,11 +139,14 @@ namespace hpx {
#include <hpx/modules/executors.hpp>
#include <hpx/modules/functional.hpp>
#include <hpx/modules/iterator_support.hpp>
#include <hpx/parallel/algorithms/detail/advance_to_sentinel.hpp>
#include <hpx/parallel/algorithms/detail/dispatch.hpp>
#include <hpx/parallel/algorithms/detail/pivot.hpp>
#include <hpx/parallel/algorithms/make_heap.hpp>
#include <hpx/parallel/algorithms/minmax.hpp>
#include <hpx/parallel/algorithms/partial_sort.hpp>
#include <hpx/parallel/algorithms/partition.hpp>
#include <hpx/parallel/util/compare_projected.hpp>
#include <hpx/parallel/util/detail/algorithm_result.hpp>
#include <hpx/parallel/util/detail/sender_util.hpp>

Expand Down Expand Up @@ -180,6 +183,10 @@ namespace hpx::parallel {
constexpr void nth_element_seq(RandomIt first, RandomIt nth,
RandomIt end, std::uint32_t level, Compare&& comp, Proj&& proj)
{
using wrapped_comp_type =
hpx::parallel::util::compare_projected<std::decay_t<Compare>,
std::decay_t<Proj>>;

constexpr std::uint32_t nmin_sort = 24;
auto nelem = end - first;

Expand All @@ -206,13 +213,13 @@ namespace hpx::parallel {
}
if (level == 0)
{
std::make_heap(first, end, comp);
std::sort_heap(first, nth, comp);
std::make_heap(first, end, wrapped_comp_type(comp, proj));
std::sort_heap(first, nth, wrapped_comp_type(comp, proj));
return;
}

// Filter the range and check which part contains the nth element
RandomIt c_last = filter(first, end, comp);
RandomIt c_last = filter(first, end, wrapped_comp_type(comp, proj));

if (c_last == nth)
return;
Expand Down Expand Up @@ -263,9 +270,6 @@ namespace hpx::parallel {
parallel(ExPolicy&& policy, RandomIt first, RandomIt nth, Sent last,
Pred&& pred, Proj&& proj)
{
using value_type =
typename std::iterator_traits<RandomIt>::value_type;

RandomIt partition_iter, return_last;

if (first == last)
Expand All @@ -288,17 +292,21 @@ namespace hpx::parallel {

while (first != last_iter)
{
detail::pivot9(first, last_iter, pred);
detail::pivot9(first, last_iter,
hpx::parallel::util::compare_projected<
std::decay_t<Pred>, std::decay_t<Proj>>(
pred, proj));

partition_iter =
hpx::parallel::detail::partition<RandomIt>().call(
policy(hpx::execution::non_task), first + 1,
last_iter,
[val = HPX_INVOKE(proj, *first), &pred](
value_type const& elem) {
return HPX_INVOKE(pred, elem, val);
[val = HPX_INVOKE(proj, *first), &pred, &proj](
auto const& elem) {
return HPX_INVOKE(
pred, HPX_INVOKE(proj, elem), val);
},
proj);
hpx::identity_v);

--partition_iter;

Expand Down
72 changes: 72 additions & 0 deletions libs/core/algorithms/tests/unit/container_algorithms/nth_element_range.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,11 @@

#include "test_utils.hpp"

struct S
{
std::size_t val;
};

////////////////////////////////////////////////////////////////////////////
#define SIZE 10007

Expand Down Expand Up @@ -198,6 +203,65 @@ void test_nth_element_async(ExPolicy policy, IteratorTag)
}
}

template <typename ExPolicy>
void test_nth_element_projection(ExPolicy policy)
{
static_assert(hpx::is_execution_policy<ExPolicy>::value,
"hpx::is_execution_policy<ExPolicy>::value");

std::vector<S> c(SIZE);
for (std::size_t i = 0; i < SIZE; ++i)
c[i].val = SIZE - i;

auto rand_index = std::rand() % SIZE;
auto nth = std::begin(c) + rand_index;

hpx::ranges::nth_element(policy, c, nth, std::less<std::size_t>{}, &S::val);

for (int k = 0; k < rand_index; k++)
{
HPX_TEST(c[k].val <= c[rand_index].val);
}

for (int k = rand_index + 1; k < SIZE; k++)
{
HPX_TEST(c[k].val >= c[rand_index].val);
}
}

template <typename ExPolicy, typename IteratorTag>
void test_nth_element_sent_projection(ExPolicy policy, IteratorTag)
{
static_assert(hpx::is_execution_policy<ExPolicy>::value,
"hpx::is_execution_policy<ExPolicy>::value");

using base_iterator = std::vector<S>::iterator;
using iterator = test::test_iterator<base_iterator, IteratorTag>;
using sentinel = test::sentinel_from_iterator<iterator>;

std::vector<S> c(SIZE);
for (std::size_t i = 0; i < SIZE; ++i)
c[i].val = SIZE - i;

auto rand_index = std::rand() % SIZE;

auto result = hpx::ranges::nth_element(policy, iterator(std::begin(c)),
iterator(std::begin(c) + rand_index),
sentinel(iterator(std::end(c) - 1)), std::less<std::size_t>{}, &S::val);

HPX_TEST(result == iterator(std::end(c) - 1));

for (int k = 0; k < rand_index; k++)
{
HPX_TEST(c[k].val <= c[rand_index].val);
}

for (int k = rand_index + 1; k < SIZE - 1; k++)
{
HPX_TEST(c[k].val >= c[rand_index].val);
}
}

template <typename IteratorTag>
void test_nth_element()
{
Expand All @@ -215,6 +279,14 @@ void test_nth_element()
test_nth_element_sent(seq, IteratorTag());
test_nth_element_sent(par, IteratorTag());
test_nth_element_sent(par_unseq, IteratorTag());

test_nth_element_projection(seq);
test_nth_element_projection(par);
test_nth_element_projection(par_unseq);

test_nth_element_sent_projection(seq, IteratorTag());
test_nth_element_sent_projection(par, IteratorTag());
test_nth_element_sent_projection(par_unseq, IteratorTag());
}

void nth_element_test()
Expand Down
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