1 /*!
2 @file
3 Defines `boost::hana::intersperse`.
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_INTERSPERSE_HPP
11 #define BOOST_HANA_INTERSPERSE_HPP
12 
13 #include <boost/hana/fwd/intersperse.hpp>
14 
15 #include <boost/hana/at.hpp>
16 #include <boost/hana/bool.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/length.hpp>
22 
23 #include <cstddef>
24 #include <utility>
25 
26 
27 BOOST_HANA_NAMESPACE_BEGIN
28     //! @cond
29     template <typename Xs, typename Z>
operator ()(Xs && xs,Z && z) const30     constexpr auto intersperse_t::operator()(Xs&& xs, Z&& z) const {
31         using S = typename hana::tag_of<Xs>::type;
32         using Intersperse = BOOST_HANA_DISPATCH_IF(intersperse_impl<S>,
33             hana::Sequence<S>::value
34         );
35 
36     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
37         static_assert(hana::Sequence<S>::value,
38         "hana::intersperse(xs, z) requires 'xs' to be a Sequence");
39     #endif
40 
41         return Intersperse::apply(static_cast<Xs&&>(xs), static_cast<Z&&>(z));
42     }
43     //! @endcond
44 
45     template <typename S, bool condition>
46     struct intersperse_impl<S, when<condition>> : default_ {
47         template <std::size_t i, typename Xs, typename Z>
48         static constexpr decltype(auto)
pickintersperse_impl49         pick(Xs&&, Z&& z, hana::false_ /* odd index */)
50         { return static_cast<Z&&>(z); }
51 
52         template <std::size_t i, typename Xs, typename Z>
53         static constexpr decltype(auto)
pickintersperse_impl54         pick(Xs&& xs, Z&&, hana::true_ /* even index */)
55         { return hana::at_c<(i + 1) / 2>(static_cast<Xs&&>(xs)); }
56 
57         template <typename Xs, typename Z, std::size_t ...i>
58         static constexpr auto
intersperse_helperintersperse_impl59         intersperse_helper(Xs&& xs, Z&& z, std::index_sequence<i...>) {
60             return hana::make<S>(
61                 pick<i>(static_cast<Xs&&>(xs), static_cast<Z&&>(z),
62                         hana::bool_c<(i % 2 == 0)>)...
63             );
64         }
65 
66         template <typename Xs, typename Z>
applyintersperse_impl67         static constexpr auto apply(Xs&& xs, Z&& z) {
68             constexpr std::size_t size = decltype(hana::length(xs))::value;
69             constexpr std::size_t new_size = size == 0 ? 0 : (size * 2) - 1;
70             return intersperse_helper(static_cast<Xs&&>(xs), static_cast<Z&&>(z),
71                                       std::make_index_sequence<new_size>{});
72         }
73     };
74 BOOST_HANA_NAMESPACE_END
75 
76 #endif // !BOOST_HANA_INTERSPERSE_HPP
77