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Add semantic tests for extrema algorithms with repeated values #6684

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ac38469
Fix extrema algorithm semantics and add semantic tests for min_elemen…
Apr 27, 2025
9755e1a
Fix extrema algorithm semantics and add semantic tests for min_elemen…
Apr 27, 2025
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52 changes: 30 additions & 22 deletions libs/core/algorithms/include/hpx/parallel/algorithms/minmax.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -706,33 +706,41 @@ namespace hpx::parallel {
minmax_element_result<FwdIter> sequential_minmax_element(ExPolicy&&,
FwdIter it, std::size_t count, F const& f, Proj const& proj)
{
minmax_element_result<FwdIter> result = {it, it};
HPX_ASSERT(count != 0);

if (count == 0 || count == 1)
return result;
if (count == 1)
return minmax_element_result<FwdIter>{it, it};

using element_type = hpx::traits::proxy_value_t<
typename std::iterator_traits<FwdIter>::value_type>;

element_type min_value = HPX_INVOKE(proj, *it);
element_type max_value = min_value;
util::loop_n<std::decay_t<ExPolicy>>(
++it, count - 1, [&](FwdIter const& curr) -> void {
element_type curr_value = HPX_INVOKE(proj, *curr);
if (HPX_INVOKE(f, curr_value, min_value))
{
result.min = curr;
min_value = curr_value;
}
auto min = it;
auto max = it;
++it;

if (!HPX_INVOKE(f, curr_value, max_value))
{
result.max = curr;
max_value = HPX_MOVE(curr_value);
}
});
element_type min_value = HPX_INVOKE(proj, *min);
element_type max_value = HPX_INVOKE(proj, *max);

for (std::size_t i = 1; i < count; ++i, ++it)
{
element_type curr_value = HPX_INVOKE(proj, *it);

// Update min if curr_value < min_value
if (HPX_INVOKE(f, curr_value, min_value))
{
min = it;
min_value = curr_value;
}

// Update max if min_value <= curr_value (not curr_value < min_value)
if (!HPX_INVOKE(f, curr_value, max_value))
{
max = it;
max_value = curr_value;
}
}

return result;
return minmax_element_result<FwdIter>{min, max};
}

template <typename Iter>
Expand Down Expand Up @@ -771,7 +779,7 @@ namespace hpx::parallel {

element_type curr_max_value =
HPX_INVOKE(proj, *curr->max);
if (!HPX_INVOKE(f, curr_max_value, max_value))
if (HPX_INVOKE(f, curr_max_value, max_value))
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@hkaiser hkaiser Apr 28, 2025

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This is now inconsistent with your sequential implementation above (same for the change below). Which version is correct?

{
result.max = curr->max;
max_value = HPX_MOVE(curr_max_value);
Expand Down Expand Up @@ -813,7 +821,7 @@ namespace hpx::parallel {
min_value = curr_value;
}

if (!HPX_INVOKE(f, curr_value, max_value))
if (HPX_INVOKE(f, curr_value, max_value))
{
max = curr;
max_value = HPX_MOVE(curr_value);
Expand Down
3 changes: 3 additions & 0 deletions libs/core/algorithms/tests/unit/algorithms/max_element.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@
#include <hpx/algorithm.hpp>
#include <hpx/init.hpp>
#include <hpx/modules/testing.hpp>
#include <hpx/parallel/algorithms/minmax.hpp>

#include <cstddef>
#include <ctime>
Expand Down Expand Up @@ -125,6 +126,8 @@ void max_element_test()
{
test_max_element<std::random_access_iterator_tag>();
test_max_element<std::forward_iterator_tag>();
test_max_element_semantics<std::random_access_iterator_tag>();
test_max_element_semantics<std::forward_iterator_tag>();
}

///////////////////////////////////////////////////////////////////////////////
Expand Down
3 changes: 3 additions & 0 deletions libs/core/algorithms/tests/unit/algorithms/min_element.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@
#include <hpx/algorithm.hpp>
#include <hpx/init.hpp>
#include <hpx/modules/testing.hpp>
#include <hpx/parallel/algorithms/minmax.hpp>

#include <cstddef>
#include <ctime>
Expand Down Expand Up @@ -126,6 +127,8 @@ void min_element_test()
{
test_min_element<std::random_access_iterator_tag>();
test_min_element<std::forward_iterator_tag>();
test_min_element_semantics<std::random_access_iterator_tag>();
test_min_element_semantics<std::forward_iterator_tag>();
}

///////////////////////////////////////////////////////////////////////////////
Expand Down
3 changes: 3 additions & 0 deletions libs/core/algorithms/tests/unit/algorithms/minmax_element.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@
#include <hpx/algorithm.hpp>
#include <hpx/init.hpp>
#include <hpx/modules/testing.hpp>
#include <hpx/parallel/algorithms/minmax.hpp>

#include <cstddef>
#include <ctime>
Expand Down Expand Up @@ -139,6 +140,8 @@ void minmax_element_test()
{
test_minmax_element<std::random_access_iterator_tag>();
test_minmax_element<std::forward_iterator_tag>();
test_minmax_element_semantics<std::random_access_iterator_tag>();
test_minmax_element_semantics<std::forward_iterator_tag>();
}

///////////////////////////////////////////////////////////////////////////////
Expand Down
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