boost/hana/span.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::span`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_SPAN_HPP
0011 #define BOOST_HANA_SPAN_HPP
0012
0013 #include <boost/hana/fwd/span.hpp>
0014
0015 #include <boost/hana/at.hpp>
0016 #include <boost/hana/concept/sequence.hpp>
0017 #include <boost/hana/config.hpp>
0018 #include <boost/hana/core/dispatch.hpp>
0019 #include <boost/hana/core/make.hpp>
0020 #include <boost/hana/detail/nested_by.hpp> // required by fwd decl
0021 #include <boost/hana/detail/first_unsatisfied_index.hpp>
0022 #include <boost/hana/length.hpp>
0023 #include <boost/hana/pair.hpp>
0024 #include <boost/hana/unpack.hpp>
0025
0026 #include <cstddef>
0027 #include <utility>
0028
0029
0030 namespace boost { namespace hana {
0031     //! @cond
0032     template <typename Xs, typename Pred>
0033     constexpr auto span_t::operator()(Xs&& xs, Pred&& pred) const {
0034         using S = typename hana::tag_of<Xs>::type;
0035         using Span = BOOST_HANA_DISPATCH_IF(span_impl<S>,
0036             hana::Sequence<S>::value
0037         );
0038
0039     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0040         static_assert(hana::Sequence<S>::value,
0041         "hana::span(xs, pred) requires 'xs' to be a Sequence");
0042     #endif
0043
0044         return Span::apply(static_cast<Xs&&>(xs), static_cast<Pred&&>(pred));
0045     }
0046     //! @endcond
0047
0048     template <typename S, bool condition>
0049     struct span_impl<S, when<condition>> : default_ {
0050         template <typename Xs, std::size_t ...before, std::size_t ...after>
0051         static constexpr auto span_helper(Xs&& xs, std::index_sequence<before...>,
0052                                                    std::index_sequence<after...>)
0053         {
0054             return hana::make_pair(
0055                 hana::make<S>(hana::at_c<before>(static_cast<Xs&&>(xs))...),
0056                 hana::make<S>(hana::at_c<sizeof...(before) + after>(static_cast<Xs&&>(xs))...)
0057             );
0058         }
0059
0060         template <typename Xs, typename Pred>
0061         static constexpr auto apply(Xs&& xs, Pred&&) {
0062             using FirstUnsatisfied = decltype(
0063                 hana::unpack(static_cast<Xs&&>(xs),
0064                              detail::first_unsatisfied_index<Pred&&>{})
0065             );
0066             constexpr std::size_t breakpoint = FirstUnsatisfied::value;
0067             constexpr std::size_t N = decltype(hana::length(xs))::value;
0068             return span_helper(static_cast<Xs&&>(xs),
0069                                std::make_index_sequence<breakpoint>{},
0070                                std::make_index_sequence<N - breakpoint>{});
0071         }
0072     };
0073 }} // end namespace boost::hana
0074
0075 #endif // !BOOST_HANA_SPAN_HPP