100 lines
3.4 KiB
C++
100 lines
3.4 KiB
C++
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/*!
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@file
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Defines `boost::hana::slice` and `boost::hana::slice_c`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_SLICE_HPP
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#define BOOST_HANA_SLICE_HPP
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#include <boost/hana/fwd/slice.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/concept/foldable.hpp>
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#include <boost/hana/concept/sequence.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/core/make.hpp>
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#include <boost/hana/range.hpp>
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#include <boost/hana/unpack.hpp>
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#include <cstddef>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs, typename Indices>
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constexpr auto slice_t::operator()(Xs&& xs, Indices&& indices) const {
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using S = typename hana::tag_of<Xs>::type;
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using Slice = BOOST_HANA_DISPATCH_IF(slice_impl<S>,
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hana::Sequence<S>::value &&
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hana::Foldable<Indices>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Sequence<S>::value,
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"hana::slice(xs, indices) requires 'xs' to be a Sequence");
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static_assert(hana::Foldable<Indices>::value,
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"hana::slice(xs, indices) requires 'indices' to be Foldable");
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#endif
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return Slice::apply(static_cast<Xs&&>(xs), static_cast<Indices&&>(indices));
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}
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//! @endcond
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namespace detail {
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template <typename Xs>
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struct take_arbitrary {
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Xs& xs;
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using S = typename hana::tag_of<Xs>::type;
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template <typename ...N>
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constexpr auto operator()(N const& ...) const {
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return hana::make<S>(hana::at_c<N::value>(xs)...);
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}
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};
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}
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template <typename S, bool condition>
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struct slice_impl<S, when<condition>> : default_ {
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template <std::size_t from, typename Xs, std::size_t ...i>
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static constexpr auto from_offset(Xs&& xs, std::index_sequence<i...>) {
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return hana::make<S>(hana::at_c<from + i>(static_cast<Xs&&>(xs))...);
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}
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template <typename Xs, typename T, T from, T to>
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static constexpr auto apply(Xs&& xs, hana::range<T, from, to> const&) {
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return slice_impl::from_offset<from>(
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static_cast<Xs&&>(xs), std::make_index_sequence<to - from>{}
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);
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}
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//! @todo
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//! Since we have the right to specify the same index more than once,
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//! we can't move from the elements of the source sequence even if it
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//! is a temporary object: we could end up double-moving. Perhaps it
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//! would be possible to determine the indices from which we can move
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//! without incurring a too large compile-time penalty?
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template <typename Xs, typename Indices>
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static constexpr auto apply(Xs const& xs, Indices const& indices) {
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return hana::unpack(indices, detail::take_arbitrary<Xs const>{xs});
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}
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};
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template <std::size_t from, std::size_t to>
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struct slice_c_t {
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template <typename Xs>
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constexpr auto operator()(Xs&& xs) const {
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return hana::slice(static_cast<Xs&&>(xs),
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hana::range_c<std::size_t, from, to>);
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_SLICE_HPP
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