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Less clever, much faster, mp_take

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This commit is contained in:
Peter Dimov
2017-03-22 02:03:39 +02:00
parent 321a54728f
commit 32938df318
1 changed files with 34 additions and 10 deletions
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44
include/boost/mp11/algorithm.hpp
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@@ -290,23 +290,47 @@ template<class L, class I> using mp_at = typename detail::mp_at_impl<L, I>::type
namespace detail
{
template<class L, std::size_t N, class E = void> struct mp_take_c_impl;
template<template<class...> class L, class... T> struct mp_take_c_impl<L<T...>, 0>
{
using type = L<>;
};
template<template<class...> class L, class T1, class... T> struct mp_take_c_impl<L<T1, T...>, 1>
{
using type = L<T1>;
};
template<template<class...> class L, class T1, class T2, class... T> struct mp_take_c_impl<L<T1, T2, T...>, 2>
{
using type = L<T1, T2>;
};
template<template<class...> class L, class T1, class T2, class T3, class... T> struct mp_take_c_impl<L<T1, T2, T3, T...>, 3>
{
using type = L<T1, T2, T3>;
};
template<template<class...> class L, class T1, class T2, class T3, class T4, class... T> struct mp_take_c_impl<L<T1, T2, T3, T4, T...>, 4>
{
using type = L<T1, T2, T3, T4>;
};
template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class... T, std::size_t N> struct mp_take_c_impl<L<T1, T2, T3, T4, T5, T...>, N, typename std::enable_if<N >= 5>::type>
{
using type = mp_append<L<T1, T2, T3, T4, T5>, typename mp_take_c_impl<L<T...>, N-5>::type>;
};
template<class L, class N> struct mp_take_impl
{
static_assert( N::value >= 0, "mp_take<L, N>: N must not be negative" );
using _map = mp_transform<mp_list, mp_iota<mp_size<L>>, L>;
template<class I> using _f = mp_second<mp_map_find<_map, I>>;
using _ind = mp_iota_c<N::value>;
using type = mp_assign<L, mp_transform<_f, _ind>>;
using type = typename mp_take_c_impl<L, N::value>::type;
};
} // namespace detail
template<class L, std::size_t N> using mp_take_c = typename detail::mp_take_impl<L, mp_size_t<N>>::type;
template<class L, std::size_t N> using mp_take_c = typename detail::mp_take_c_impl<L, N>::type;
template<class L, class N> using mp_take = typename detail::mp_take_impl<L, N>::type;
// mp_replace<L, V, W>