1 /*!
2 @file
3 Defines `boost::hana::take_front` and `boost::hana::take_front_c`.
4 
5 @copyright Louis Dionne 2013-2017
6 Distributed under the Boost Software License, Version 1.0.
7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
8  */
9 
10 #ifndef BOOST_HANA_TAKE_FRONT_HPP
11 #define BOOST_HANA_TAKE_FRONT_HPP
12 
13 #include <boost/hana/fwd/take_front.hpp>
14 
15 #include <boost/hana/at.hpp>
16 #include <boost/hana/concept/integral_constant.hpp>
17 #include <boost/hana/concept/sequence.hpp>
18 #include <boost/hana/config.hpp>
19 #include <boost/hana/core/dispatch.hpp>
20 #include <boost/hana/core/make.hpp>
21 #include <boost/hana/integral_constant.hpp>
22 #include <boost/hana/length.hpp>
23 
24 #include <cstddef>
25 #include <utility>
26 
27 
28 BOOST_HANA_NAMESPACE_BEGIN
29     //! @cond
30     template <typename Xs, typename N>
operator ()(Xs && xs,N const & n) const31     constexpr auto take_front_t::operator()(Xs&& xs, N const& n) const {
32         using S = typename hana::tag_of<Xs>::type;
33         using TakeFront = BOOST_HANA_DISPATCH_IF(take_front_impl<S>,
34             hana::Sequence<S>::value &&
35             hana::IntegralConstant<N>::value
36         );
37 
38 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
39         static_assert(hana::Sequence<S>::value,
40         "hana::take_front(xs, n) requires 'xs' to be a Sequence");
41 
42         static_assert(hana::IntegralConstant<N>::value,
43         "hana::take_front(xs, n) requires 'n' to be an IntegralConstant");
44 #endif
45 
46         return TakeFront::apply(static_cast<Xs&&>(xs), n);
47     }
48     //! @endcond
49 
50     template <typename S, bool condition>
51     struct take_front_impl<S, when<condition>> : default_ {
52         template <typename Xs, std::size_t ...n>
take_front_helpertake_front_impl53         static constexpr auto take_front_helper(Xs&& xs, std::index_sequence<n...>) {
54             return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...);
55         }
56 
57         template <typename Xs, typename N>
applytake_front_impl58         static constexpr auto apply(Xs&& xs, N const&) {
59             constexpr std::size_t n = N::value;
60             constexpr std::size_t size = decltype(hana::length(xs))::value;
61             return take_front_helper(static_cast<Xs&&>(xs),
62                         std::make_index_sequence<(n < size ? n : size)>{});
63         }
64     };
65 
66     template <std::size_t n>
67     struct take_front_c_t {
68         template <typename Xs>
operator ()take_front_c_t69         constexpr auto operator()(Xs&& xs) const {
70             return hana::take_front(static_cast<Xs&&>(xs), hana::size_c<n>);
71         }
72     };
73 BOOST_HANA_NAMESPACE_END
74 
75 #endif // !BOOST_HANA_TAKE_FRONT_HPP
76