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Use fold for tuple accumulation #2873

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e546e19
Updated for_each to use a fold over recurssion
spectre-ns Nov 19, 2025
1fb92a7
Updated accumulate to use C++17 folds rather than recursion
spectre-ns Nov 19, 2025
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78 changes: 31 additions & 47 deletions include/xtensor/utils/xutils.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -163,52 +163,37 @@ namespace xt

namespace detail
{
template <std::size_t I, class F, class... T>
inline typename std::enable_if<I == sizeof...(T), void>::type
for_each_impl(F&& /*f*/, std::tuple<T...>& /*t*/) noexcept
template <class F, size_t... I, class... Ts>
void for_each(F&& f, std::tuple<Ts...>& t, std::index_sequence<I...>) noexcept(
(noexcept(f(std::get<I>(t))) && ...)
)
{
(f(std::get<I>(t)), ...);
}

template <std::size_t I, class F, class... T>
inline typename std::enable_if < I<sizeof...(T), void>::type
for_each_impl(F&& f, std::tuple<T...>& t) noexcept(noexcept(f(std::get<I>(t))))
template <class F, size_t... I, class... Ts>
void for_each(F&& f, const std::tuple<Ts...>& t, std::index_sequence<I...>) noexcept(
(noexcept(f(std::get<I>(t))) && ...)
)
{
f(std::get<I>(t));
for_each_impl<I + 1, F, T...>(std::forward<F>(f), t);
(f(std::get<I>(t)), ...);
}
}

template <class F, class... T>
inline void for_each(F&& f, std::tuple<T...>& t) noexcept(
noexcept(detail::for_each_impl<0, F, T...>(std::forward<F>(f), t))
template <class F, class... Ts>
inline void for_each(F&& f, std::tuple<Ts...>& t) noexcept(
noexcept(detail::for_each(std::forward<F>(f), t, std::make_index_sequence<sizeof...(Ts)>{}))
)
{
detail::for_each_impl<0, F, T...>(std::forward<F>(f), t);
detail::for_each(std::forward<F>(f), t, std::make_index_sequence<sizeof...(Ts)>{});
}

namespace detail
{
template <std::size_t I, class F, class... T>
inline typename std::enable_if<I == sizeof...(T), void>::type
for_each_impl(F&& /*f*/, const std::tuple<T...>& /*t*/) noexcept
{
}

template <std::size_t I, class F, class... T>
inline typename std::enable_if < I<sizeof...(T), void>::type
for_each_impl(F&& f, const std::tuple<T...>& t) noexcept(noexcept(f(std::get<I>(t))))
{
f(std::get<I>(t));
for_each_impl<I + 1, F, T...>(std::forward<F>(f), t);
}
}

template <class F, class... T>
inline void for_each(F&& f, const std::tuple<T...>& t) noexcept(
noexcept(detail::for_each_impl<0, F, T...>(std::forward<F>(f), t))
template <class F, class... Ts>
inline void for_each(F&& f, const std::tuple<Ts...>& t) noexcept(
noexcept(detail::for_each(std::forward<F>(f), t, std::make_index_sequence<sizeof...(Ts)>{}))
)
{
detail::for_each_impl<0, F, T...>(std::forward<F>(f), t);
detail::for_each(std::forward<F>(f), t, std::make_index_sequence<sizeof...(Ts)>{});
}

/*****************************
Expand All @@ -219,28 +204,27 @@ namespace xt

namespace detail
{
template <std::size_t I, class F, class R, class... T>
inline std::enable_if_t<I == sizeof...(T), R>
accumulate_impl(F&& /*f*/, R init, const std::tuple<T...>& /*t*/) noexcept
{
return init;
}

template <std::size_t I, class F, class R, class... T>
inline std::enable_if_t < I<sizeof...(T), R>
accumulate_impl(F&& f, R init, const std::tuple<T...>& t) noexcept(noexcept(f(init, std::get<I>(t))))
template <class F, class R, class... T, size_t... I>
R accumulate_impl(F&& f, R init, const std::tuple<T...>& t, std::index_sequence<I...> /*I*/) noexcept(
(noexcept(f(init, std::get<I>(t))) && ...)
)
{
R res = f(init, std::get<I>(t));
return accumulate_impl<I + 1, F, R, T...>(std::forward<F>(f), res, t);
R res = init;
auto wrapper = [&](const auto& i, const auto& j)
{
res = f(i, j);
};
(wrapper(res, std::get<I>(t)), ...);
return res;
}
}

template <class F, class R, class... T>
inline R accumulate(F&& f, R init, const std::tuple<T...>& t) noexcept(
noexcept(detail::accumulate_impl<0, F, R, T...>(std::forward<F>(f), init, t))
noexcept(detail::accumulate_impl(std::forward<F>(f), init, t, std::make_index_sequence<sizeof...(T)>{}))
)
{
return detail::accumulate_impl<0, F, R, T...>(std::forward<F>(f), init, t);
return detail::accumulate_impl(std::forward<F>(f), init, t, std::make_index_sequence<sizeof...(T)>{});
}

/// @endcond
Expand Down
3 changes: 3 additions & 0 deletions test/test_xutils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,9 @@ namespace xt
};
EXPECT_EQ(8, accumulate(func_ne, 0, t));
EXPECT_TRUE(noexcept(accumulate(func_ne, 0, t)));

const std::tuple<> t_empty{};
EXPECT_EQ(8, accumulate(func_ne, 8, t_empty));
}

template <class... T>
Expand Down
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