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/*!
@file
Defines `boost::hana::take_back`.
@copyright Louis Dionne 2013-2017
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_TAKE_BACK_HPP
#define BOOST_HANA_TAKE_BACK_HPP
#include <boost/hana/fwd/take_back.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/concept/integral_constant.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/integral_constant.hpp>
#include <boost/hana/length.hpp>
#include <cstddef>
#include <utility>
BOOST_HANA_NAMESPACE_BEGIN
//! @cond
template <typename Xs, typename N>
constexpr auto take_back_t::operator()(Xs&& xs, N const& n) const {
using S = typename hana::tag_of<Xs>::type;
using TakeBack = BOOST_HANA_DISPATCH_IF(take_back_impl<S>,
hana::Sequence<S>::value &&
hana::IntegralConstant<N>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Sequence<S>::value,
"hana::take_back(xs, n) requires 'xs' to be a Sequence");
static_assert(hana::IntegralConstant<N>::value,
"hana::take_back(xs, n) requires 'n' to be an IntegralConstant");
#endif
return TakeBack::apply(static_cast<Xs&&>(xs), n);
}
//! @endcond
template <typename S, bool condition>
struct take_back_impl<S, when<condition>> : default_ {
template <std::size_t start, typename Xs, std::size_t ...n>
static constexpr auto take_back_helper(Xs&& xs, std::index_sequence<n...>) {
return hana::make<S>(hana::at_c<start + n>(static_cast<Xs&&>(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;
constexpr std::size_t start = n < len ? len - n : 0;
return take_back_helper<start>(static_cast<Xs&&>(xs),
std::make_index_sequence<(n < len ? n : len)>{});
}
};
template <std::size_t n>
struct take_back_c_t {
template <typename Xs>
constexpr auto operator()(Xs&& xs) const {
return hana::take_back(static_cast<Xs&&>(xs), hana::size_c<n>);
}
};
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_TAKE_BACK_HPP
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