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P3709R2 Reconsider parallel ranges::rotate_copy and ranges::reverse_copy #8048

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Jul 15, 2025
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P3709R2 Reconsider parallel ranges::rotate_copy and ranges::reverse_copy #8048

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51 changes: 23 additions & 28 deletions source/algorithms.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -2599,7 +2599,9 @@

namespace ranges {
template<class I, class O>
using reverse_copy_result = in_out_result<I, O>;
using @\libglobal{reverse_copy_result}@ = in_out_result<I, O>;
template<class I, class O>
using @\libglobal{reverse_copy_truncated_result}@ = in_in_out_result<I, I, O>;

template<@\libconcept{bidirectional_iterator}@ I, @\libconcept{sentinel_for}@<I> S, @\libconcept{weakly_incrementable}@ O>
requires @\libconcept{indirectly_copyable}@<I, O>
Expand All @@ -2613,12 +2615,12 @@
template<@\exposconcept{execution-policy}@ Ep, @\libconcept{random_access_iterator}@ I, @\libconcept{sized_sentinel_for}@<I> S,
@\libconcept{random_access_iterator}@ O, @\libconcept{sized_sentinel_for}@<O> OutS>
requires @\libconcept{indirectly_copyable}@<I, O>
reverse_copy_result<I, O>
reverse_copy_truncated_result<I, O>
reverse_copy(Ep&& exec, I first, S last, O result,
OutS result_last); // freestanding-deleted
template<@\exposconcept{execution-policy}@ Ep, @\exposconcept{sized-random-access-range}@ R, @\exposconcept{sized-random-access-range}@ OutR>
requires @\libconcept{indirectly_copyable}@<iterator_t<R>, iterator_t<OutR>>
reverse_copy_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
reverse_copy_truncated_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
reverse_copy(Ep&& exec, R&& r, OutR&& result_r); // freestanding-deleted
}

Expand Down Expand Up @@ -2662,7 +2664,9 @@

namespace ranges {
template<class I, class O>
using rotate_copy_result = in_out_result<I, O>;
using @\libglobal{rotate_copy_result}@ = in_out_result<I, O>;
template<class I, class O>
using @\libglobal{rotate_copy_truncated_result}@ = in_in_out_result<I, I, O>;

template<@\libconcept{forward_iterator}@ I, @\libconcept{sentinel_for}@<I> S, @\libconcept{weakly_incrementable}@ O>
requires @\libconcept{indirectly_copyable}@<I, O>
Expand All @@ -2676,12 +2680,12 @@
template<@\exposconcept{execution-policy}@ Ep, @\libconcept{random_access_iterator}@ I, @\libconcept{sized_sentinel_for}@<I> S,
@\libconcept{random_access_iterator}@ O, @\libconcept{sized_sentinel_for}@<O> OutS>
requires @\libconcept{indirectly_copyable}@<I, O>
rotate_copy_result<I, O>
rotate_copy_truncated_result<I, O>
rotate_copy(Ep&& exec, I first, I middle, S last, O result, // freestanding-deleted
OutS result_last);
template<@\exposconcept{execution-policy}@ Ep, @\exposconcept{sized-random-access-range}@ R, @\exposconcept{sized-random-access-range}@ OutR>
requires @\libconcept{indirectly_copyable}@<iterator_t<R>, iterator_t<OutR>>
rotate_copy_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
rotate_copy_truncated_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
rotate_copy(Ep&& exec, R&& r, iterator_t<R> middle, // freestanding-deleted
OutR&& result_r);
}
Expand Down Expand Up @@ -8140,12 +8144,12 @@
template<@\exposconcept{execution-policy}@ Ep, @\libconcept{random_access_iterator}@ I, @\libconcept{sized_sentinel_for}@<I> S,
@\libconcept{random_access_iterator}@ O, @\libconcept{sized_sentinel_for}@<O> OutS>
requires @\libconcept{indirectly_copyable}@<I, O>
ranges::reverse_copy_result<I, O>
ranges::reverse_copy_truncated_result<I, O>
ranges::reverse_copy(Ep&& exec, I first, S last, O result,
OutS result_last);
template<@\exposconcept{execution-policy}@ Ep, @\exposconcept{sized-random-access-range}@ R, @\exposconcept{sized-random-access-range}@ OutR>
requires @\libconcept{indirectly_copyable}@<iterator_t<R>, iterator_t<OutR>>
ranges::reverse_copy_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
ranges::reverse_copy_truncated_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
ranges::reverse_copy(Ep&& exec, R&& r, OutR&& result_r);
\end{itemdecl}

Expand All @@ -8169,15 +8173,7 @@

\pnum
\returns
\tcode{\{\exposid{NEW_FIRST}, result + $N$\}}.
\begin{note}
While the return type for the parallel and non-parallel algorithm overloads
in the namespace \tcode{ranges} is the same,
the semantics is different
because for the parallel range algorithm overloads
\tcode{result_last - result} can be insufficient
to copy all data from the input.
\end{note}
\tcode{\{last, \exposid{NEW_FIRST}, result + $N$\}}.

\pnum
\complexity
Expand Down Expand Up @@ -8320,7 +8316,7 @@
template<@\exposconcept{execution-policy}@ Ep, @\libconcept{random_access_iterator}@ I, @\libconcept{sized_sentinel_for}@<I> S,
@\libconcept{random_access_iterator}@ O, @\libconcept{sized_sentinel_for}@<O> OutS>
requires @\libconcept{indirectly_copyable}@<I, O>
ranges::rotate_copy_result<I, O>
ranges::rotate_copy_truncated_result<I, O>
ranges::rotate_copy(Ep&& exec, I first, I middle, S last, O result, OutS result_last);
\end{itemdecl}

Expand All @@ -8346,15 +8342,14 @@

\pnum
\returns
\tcode{\{first + ($N$ + (middle - first)) \% $M$, result + $N$\}}.
\begin{note}
While the return type for the parallel and non-parallel algorithm overloads
in the namespace \tcode{ranges} is the same,
the semantics is different
because for the parallel range algorithm overloads
\tcode{result_last - result} can be insufficient
to copy all data from the input.
\end{note}
\begin{itemize}
\item
\tcode{\{middle + $N$, first, result + $N$\}}
if $N$ is less than \tcode{last - middle}.
\item
Otherwise,
\tcode{\{last, first + ($N$ + (middle - first)) \% $M$, result + $N$\}}.
\end{itemize}

\pnum
\complexity
Expand All @@ -8380,7 +8375,7 @@
\begin{itemdecl}
template<@\exposconcept{execution-policy}@ Ep, @\exposconcept{sized-random-access-range}@ R, @\exposconcept{sized-random-access-range}@ OutR>
requires @\libconcept{indirectly_copyable}@<iterator_t<R>, iterator_t<OutR>>
ranges::rotate_copy_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
ranges::rotate_copy_truncated_result<borrowed_iterator_t<R>, borrowed_iterator_t<OutR>>
ranges::rotate_copy(Ep&& exec, R&& r, iterator_t<R> middle, OutR&& result_r);
\end{itemdecl}

Expand Down