598bfd3f
 Hu Chunming
提交_GLIBCXX_USE_CX...
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/*!
@file
Defines `boost::hana::cycle`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_CYCLE_HPP
#define BOOST_HANA_CYCLE_HPP
#include <boost/hana/fwd/cycle.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/concat.hpp>
#include <boost/hana/concept/integral_constant.hpp>
#include <boost/hana/concept/monad_plus.hpp>
#include <boost/hana/concept/sequence.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/core/make.hpp>
#include <boost/hana/detail/array.hpp>
#include <boost/hana/empty.hpp>
#include <boost/hana/length.hpp>
#include <cstddef>
#include <utility>
namespace boost { namespace hana {
//! @cond
template <typename Xs, typename N>
constexpr auto cycle_t::operator()(Xs&& xs, N const& n) const {
using M = typename hana::tag_of<Xs>::type;
using Cycle = BOOST_HANA_DISPATCH_IF(cycle_impl<M>,
hana::MonadPlus<M>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::MonadPlus<M>::value,
"hana::cycle(xs, n) requires 'xs' to be a MonadPlus");
static_assert(hana::IntegralConstant<N>::value,
"hana::cycle(xs, n) requires 'n' to be an IntegralConstant");
#endif
static_assert(N::value >= 0,
"hana::cycle(xs, n) requires 'n' to be non-negative");
return Cycle::apply(static_cast<Xs&&>(xs), n);
}
//! @endcond
namespace detail {
template <typename M, std::size_t n, bool = n % 2 == 0>
struct cycle_helper;
template <typename M>
struct cycle_helper<M, 0, true> {
template <typename Xs>
static constexpr auto apply(Xs const&)
{ return hana::empty<M>(); }
};
template <typename M, std::size_t n>
struct cycle_helper<M, n, true> {
template <typename Xs>
static constexpr auto apply(Xs const& xs)
{ return cycle_helper<M, n/2>::apply(hana::concat(xs, xs)); }
};
template <typename M, std::size_t n>
struct cycle_helper<M, n, false> {
template <typename Xs>
static constexpr auto apply(Xs const& xs)
{ return hana::concat(xs, cycle_helper<M, n-1>::apply(xs)); }
};
}
template <typename M, bool condition>
struct cycle_impl<M, when<condition>> : default_ {
template <typename Xs, typename N>
static constexpr auto apply(Xs const& xs, N const&) {
constexpr std::size_t n = N::value;
return detail::cycle_helper<M, n>::apply(xs);
}
};
namespace detail {
template <std::size_t N, std::size_t Len>
struct cycle_indices {
static constexpr auto compute_value() {
detail::array<std::size_t, N * Len> indices{};
// Avoid (incorrect) Clang warning about remainder by zero
// in the loop below.
std::size_t len = Len;
for (std::size_t i = 0; i < N * Len; ++i)
indices[i] = i % len;
return indices;
}
static constexpr auto value = compute_value();
};
}
template <typename S>
struct cycle_impl<S, when<Sequence<S>::value>> {
template <typename Indices, typename Xs, std::size_t ...i>
static constexpr auto cycle_helper(Xs&& xs, std::index_sequence<i...>) {
constexpr auto indices = Indices::value;
(void)indices; // workaround GCC warning when sizeof...(i) == 0
return hana::make<S>(hana::at_c<indices[i]>(xs)...);
}
template <typename Xs, typename N>
static constexpr auto apply(Xs&& xs, N const&) {
constexpr std::size_t n = N::value;
constexpr std::size_t len = decltype(hana::length(xs))::value;
using Indices = detail::cycle_indices<n, len>;
return cycle_helper<Indices>(static_cast<Xs&&>(xs),
std::make_index_sequence<n * len>{});
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_CYCLE_HPP
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