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+/*!
+@file
+Defines `boost::hana::slice` and `boost::hana::slice_c`.
+
+@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_SLICE_HPP
+#define BOOST_HANA_SLICE_HPP
+
+#include <boost/hana/fwd/slice.hpp>
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/concept/foldable.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/range.hpp>
+#include <boost/hana/unpack.hpp>
+
+#include <cstddef>
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //! @cond
+ template <typename Xs, typename Indices>
+ constexpr auto slice_t::operator()(Xs&& xs, Indices&& indices) const {
+ using S = typename hana::tag_of<Xs>::type;
+ using Slice = BOOST_HANA_DISPATCH_IF(slice_impl<S>,
+ hana::Sequence<S>::value &&
+ hana::Foldable<Indices>::value
+ );
+
+ #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
+ static_assert(hana::Sequence<S>::value,
+ "hana::slice(xs, indices) requires 'xs' to be a Sequence");
+
+ static_assert(hana::Foldable<Indices>::value,
+ "hana::slice(xs, indices) requires 'indices' to be Foldable");
+ #endif
+
+ return Slice::apply(static_cast<Xs&&>(xs), static_cast<Indices&&>(indices));
+ }
+ //! @endcond
+
+ namespace detail {
+ template <typename Xs>
+ struct take_arbitrary {
+ Xs& xs;
+ using S = typename hana::tag_of<Xs>::type;
+
+ template <typename ...N>
+ constexpr auto operator()(N const& ...) const {
+ return hana::make<S>(hana::at_c<N::value>(xs)...);
+ }
+ };
+ }
+
+ template <typename S, bool condition>
+ struct slice_impl<S, when<condition>> : default_ {
+ template <std::size_t from, typename Xs, std::size_t ...i>
+ static constexpr auto from_offset(Xs&& xs, std::index_sequence<i...>) {
+ return hana::make<S>(hana::at_c<from + i>(static_cast<Xs&&>(xs))...);
+ }
+
+ template <typename Xs, typename T, T from, T to>
+ static constexpr auto apply(Xs&& xs, hana::range<T, from, to> const&) {
+ return slice_impl::from_offset<from>(
+ static_cast<Xs&&>(xs), std::make_index_sequence<to - from>{}
+ );
+ }
+
+ //! @todo
+ //! Since we have the right to specify the same index more than once,
+ //! we can't move from the elements of the source sequence even if it
+ //! is a temporary object: we could end up double-moving. Perhaps it
+ //! would be possible to determine the indices from which we can move
+ //! without incurring a too large compile-time penalty?
+ template <typename Xs, typename Indices>
+ static constexpr auto apply(Xs const& xs, Indices const& indices) {
+ return hana::unpack(indices, detail::take_arbitrary<Xs const>{xs});
+ }
+ };
+
+ template <std::size_t from, std::size_t to>
+ struct slice_c_t {
+ template <typename Xs>
+ constexpr auto operator()(Xs&& xs) const {
+ return hana::slice(static_cast<Xs&&>(xs),
+ hana::range_c<std::size_t, from, to>);
+ }
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_SLICE_HPP