boost/hana/filter.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::filter`.
   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_FILTER_HPP
  11 #define BOOST_HANA_FILTER_HPP
  12
  13 #include <boost/hana/fwd/filter.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/bool.hpp>
  17 #include <boost/hana/chain.hpp>
  18 #include <boost/hana/concept/monad_plus.hpp>
  19 #include <boost/hana/concept/sequence.hpp>
  20 #include <boost/hana/config.hpp>
  21 #include <boost/hana/core/dispatch.hpp>
  22 #include <boost/hana/core/make.hpp>
  23 #include <boost/hana/detail/algorithm.hpp>
  24 #include <boost/hana/detail/array.hpp>
  25 #include <boost/hana/detail/decay.hpp>
  26 #include <boost/hana/empty.hpp>
  27 #include <boost/hana/lift.hpp>
  28 #include <boost/hana/unpack.hpp>
  29
  30 #include <cstddef>
  31 #include <utility>
  32
  33
  34 BOOST_HANA_NAMESPACE_BEGIN
  35     //! @cond
  36     template <typename Xs, typename Pred>
  37     constexpr auto filter_t::operator()(Xs&& xs, Pred&& pred) const {
  38         using M = typename hana::tag_of<Xs>::type;
  39         using Filter = BOOST_HANA_DISPATCH_IF(filter_impl<M>,
  40             hana::MonadPlus<M>::value
  41         );
  42
  43     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  44         static_assert(hana::MonadPlus<M>::value,
  45         "hana::filter(xs, pred) requires 'xs' to be a MonadPlus");
  46     #endif
  47
  48         return Filter::apply(static_cast<Xs&&>(xs),
  49                              static_cast<Pred&&>(pred));
  50     }
  51     //! @endcond
  52
  53     namespace detail {
  54         template <typename Pred, typename M>
  55         struct lift_or_empty {
  56             template <typename X>
  57             static constexpr auto helper(X&& x, hana::true_)
  58             { return hana::lift<M>(static_cast<X&&>(x)); }
  59
  60             template <typename X>
  61             static constexpr auto helper(X&&, hana::false_)
  62             { return hana::empty<M>(); }
  63
  64             template <typename X>
  65             constexpr auto operator()(X&& x) const {
  66                 constexpr bool cond = decltype(std::declval<Pred>()(x))::value;
  67                 return helper(static_cast<X&&>(x), hana::bool_c<cond>);
  68             }
  69         };
  70     }
  71
  72     template <typename M, bool condition>
  73     struct filter_impl<M, when<condition>> : default_ {
  74         template <typename Xs, typename Pred>
  75         static constexpr decltype(auto) apply(Xs&& xs, Pred const&) {
  76             return hana::chain(static_cast<Xs&&>(xs),
  77                 detail::lift_or_empty<Pred, M>{}
  78             );
  79         }
  80     };
  81
  82     namespace detail {
  83         template <bool ...b>
  84         struct filter_indices {
  85             static constexpr auto compute_indices() {
  86                 constexpr bool bs[] = {b..., false}; // avoid empty array
  87                 constexpr std::size_t N = detail::count(bs, bs + sizeof(bs), true);
  88                 detail::array<std::size_t, N> indices{};
  89                 std::size_t* keep = &indices[0];
  90                 for (std::size_t i = 0; i < sizeof...(b); ++i)
  91                     if (bs[i])
  92                         *keep++ = i;
  93                 return indices;
  94             }
  95
  96             static constexpr auto cached_indices = compute_indices();
  97         };
  98
  99         template <typename Pred>
 100         struct make_filter_indices {
 101             Pred const& pred;
 102             template <typename ...X>
 103             auto operator()(X&& ...x) const -> filter_indices<
 104                 static_cast<bool>(detail::decay<
 105                     decltype(pred(static_cast<X&&>(x)))
 106                 >::type::value)...
 107             > { return {}; }
 108         };
 109     }
 110
 111     template <typename S>
 112     struct filter_impl<S, when<Sequence<S>::value>> {
 113         template <typename Indices, typename Xs, std::size_t ...i>
 114         static constexpr auto filter_helper(Xs&& xs, std::index_sequence<i...>) {
 115             return hana::make<S>(
 116                 hana::at_c<Indices::cached_indices[i]>(static_cast<Xs&&>(xs))...
 117             );
 118         }
 119
 120         template <typename Xs, typename Pred>
 121         static constexpr auto apply(Xs&& xs, Pred const& pred) {
 122             using Indices = decltype(
 123                 hana::unpack(static_cast<Xs&&>(xs),
 124                              detail::make_filter_indices<Pred>{pred})
 125             );
 126
 127             return filter_impl::filter_helper<Indices>(
 128                 static_cast<Xs&&>(xs),
 129                 std::make_index_sequence<Indices::cached_indices.size()>{}
 130             );
 131         }
 132     };
 133 BOOST_HANA_NAMESPACE_END
 134
 135 #endif // !BOOST_HANA_FILTER_HPP