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Diffstat (limited to 'boost/hana/take_back.hpp')
-rw-r--r-- | boost/hana/take_back.hpp | 68 |
1 files changed, 68 insertions, 0 deletions
diff --git a/boost/hana/take_back.hpp b/boost/hana/take_back.hpp new file mode 100644 index 0000000000..9a12d93b23 --- /dev/null +++ b/boost/hana/take_back.hpp @@ -0,0 +1,68 @@ +/*! +@file +Defines `boost::hana::take_back`. + +@copyright Louis Dionne 2013-2016 +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 size = decltype(hana::length(xs))::value; + constexpr std::size_t start = n < size ? size - n : 0; + return take_back_helper<start>(static_cast<Xs&&>(xs), + std::make_index_sequence<(n < size ? n : size)>{}); + } + }; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_TAKE_BACK_HPP |