text: /*!
  @file
  Defines `boost::hana::filter`.
  
  @copyright Louis Dionne 2013-2016
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   */
  
  #ifndef BOOST_HANA_FILTER_HPP
  #define BOOST_HANA_FILTER_HPP
  
  #include <boost/hana/fwd/filter.hpp>
  
  #include <boost/hana/at.hpp>
  #include <boost/hana/bool.hpp>
  #include <boost/hana/chain.hpp>
  #include <boost/hana/concept/monad_plus.hpp>
  #include <boost/hana/concept/sequence.hpp>
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/dispatch.hpp>
  #include <boost/hana/core/make.hpp>
  #include <boost/hana/detail/algorithm.hpp>
  #include <boost/hana/detail/array.hpp>
  #include <boost/hana/detail/decay.hpp>
  #include <boost/hana/empty.hpp>
  #include <boost/hana/lift.hpp>
  #include <boost/hana/unpack.hpp>
  
  #include <cstddef>
  #include <utility>
  
  
  BOOST_HANA_NAMESPACE_BEGIN
      //! @cond
      template <typename Xs, typename Pred>
      constexpr auto filter_t::operator()(Xs&& xs, Pred&& pred) const {
          using M = typename hana::tag_of<Xs>::type;
          using Filter = BOOST_HANA_DISPATCH_IF(filter_impl<M>,
              hana::MonadPlus<M>::value
          );
  
      #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
          static_assert(hana::MonadPlus<M>::value,
          "hana::filter(xs, pred) requires 'xs' to be a MonadPlus");
      #endif
  
          return Filter::apply(static_cast<Xs&&>(xs),
                               static_cast<Pred&&>(pred));
      }
      //! @endcond
  
      namespace detail {
          template <typename Pred, typename M>
          struct lift_or_empty {
              template <typename X>
              static constexpr auto helper(X&& x, hana::true_)
              { return hana::lift<M>(static_cast<X&&>(x)); }
  
              template <typename X>
              static constexpr auto helper(X&&, hana::false_)
              { return hana::empty<M>(); }
  
              template <typename X>
              constexpr auto operator()(X&& x) const {
                  constexpr bool cond = decltype(std::declval<Pred>()(x))::value;
                  return helper(static_cast<X&&>(x), hana::bool_c<cond>);
              }
          };
      }
  
      template <typename M, bool condition>
      struct filter_impl<M, when<condition>> : default_ {
          template <typename Xs, typename Pred>
          static constexpr decltype(auto) apply(Xs&& xs, Pred const&) {
              return hana::chain(static_cast<Xs&&>(xs),
                  detail::lift_or_empty<Pred, M>{}
              );
          }
      };
  
      namespace detail {
          template <bool ...b>
          struct filter_indices {
              static constexpr auto compute_indices() {
                  constexpr bool bs[] = {b..., false}; // avoid empty array
                  constexpr std::size_t N = detail::count(bs, bs + sizeof(bs), true);
                  detail::array<std::size_t, N> indices{};
                  std::size_t* keep = &indices[0];
                  for (std::size_t i = 0; i < sizeof...(b); ++i)
                      if (bs[i])
                          *keep++ = i;
                  return indices;
              }
  
              static constexpr auto indices = compute_indices();
          };
  
          template <typename Pred>
          struct make_filter_indices {
              Pred const& pred;
              template <typename ...X>
              auto operator()(X&& ...x) const -> filter_indices<
                  static_cast<bool>(detail::decay<
                      decltype(pred(static_cast<X&&>(x)))
                  >::type::value)...
              > { return {}; }
          };
      }
  
      template <typename S>
      struct filter_impl<S, when<Sequence<S>::value>> {
          template <typename Indices, typename Xs, std::size_t ...i>
          static constexpr auto filter_helper(Xs&& xs, std::index_sequence<i...>) {
              return hana::make<S>(
                  hana::at_c<Indices::indices[i]>(static_cast<Xs&&>(xs))...
              );
          }
  
          template <typename Xs, typename Pred>
          static constexpr auto apply(Xs&& xs, Pred const& pred) {
              using Indices = decltype(
                  hana::unpack(static_cast<Xs&&>(xs),
                               detail::make_filter_indices<Pred>{pred})
              );
  
              return filter_impl::filter_helper<Indices>(
                  static_cast<Xs&&>(xs),
                  std::make_index_sequence<Indices::indices.size()>{}
              );
          }
      };
  BOOST_HANA_NAMESPACE_END
  
  #endif // !BOOST_HANA_FILTER_HPP
  