/*!
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@file
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Defines `boost::hana::filter`.
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@copyright Louis Dionne 2013-2017
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FILTER_HPP
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#define BOOST_HANA_FILTER_HPP
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#include <boost/hana/fwd/filter.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/bool.hpp>
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#include <boost/hana/chain.hpp>
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#include <boost/hana/concept/monad_plus.hpp>
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#include <boost/hana/concept/sequence.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/core/make.hpp>
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#include <boost/hana/detail/algorithm.hpp>
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#include <boost/hana/detail/array.hpp>
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#include <boost/hana/detail/decay.hpp>
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#include <boost/hana/empty.hpp>
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#include <boost/hana/lift.hpp>
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#include <boost/hana/unpack.hpp>
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#include <cstddef>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs, typename Pred>
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constexpr auto filter_t::operator()(Xs&& xs, Pred&& pred) const {
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using M = typename hana::tag_of<Xs>::type;
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using Filter = BOOST_HANA_DISPATCH_IF(filter_impl<M>,
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hana::MonadPlus<M>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::MonadPlus<M>::value,
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"hana::filter(xs, pred) requires 'xs' to be a MonadPlus");
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#endif
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return Filter::apply(static_cast<Xs&&>(xs),
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static_cast<Pred&&>(pred));
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}
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//! @endcond
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namespace detail {
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template <typename Pred, typename M>
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struct lift_or_empty {
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template <typename X>
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static constexpr auto helper(X&& x, hana::true_)
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{ return hana::lift<M>(static_cast<X&&>(x)); }
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template <typename X>
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static constexpr auto helper(X&&, hana::false_)
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{ return hana::empty<M>(); }
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template <typename X>
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constexpr auto operator()(X&& x) const {
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constexpr bool cond = decltype(std::declval<Pred>()(x))::value;
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return helper(static_cast<X&&>(x), hana::bool_c<cond>);
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}
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};
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}
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template <typename M, bool condition>
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struct filter_impl<M, when<condition>> : default_ {
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template <typename Xs, typename Pred>
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static constexpr decltype(auto) apply(Xs&& xs, Pred const&) {
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return hana::chain(static_cast<Xs&&>(xs),
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detail::lift_or_empty<Pred, M>{}
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);
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}
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};
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namespace detail {
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template <bool ...b>
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struct filter_indices {
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static constexpr auto compute_indices() {
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constexpr bool bs[] = {b..., false}; // avoid empty array
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constexpr std::size_t N = detail::count(bs, bs + sizeof(bs), true);
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detail::array<std::size_t, N> indices{};
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std::size_t* keep = &indices[0];
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for (std::size_t i = 0; i < sizeof...(b); ++i)
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if (bs[i])
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*keep++ = i;
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return indices;
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}
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static constexpr auto cached_indices = compute_indices();
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};
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template <typename Pred>
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struct make_filter_indices {
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Pred const& pred;
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template <typename ...X>
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auto operator()(X&& ...x) const -> filter_indices<
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static_cast<bool>(detail::decay<
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decltype(pred(static_cast<X&&>(x)))
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>::type::value)...
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> { return {}; }
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};
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}
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template <typename S>
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struct filter_impl<S, when<Sequence<S>::value>> {
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template <typename Indices, typename Xs, std::size_t ...i>
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static constexpr auto filter_helper(Xs&& xs, std::index_sequence<i...>) {
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return hana::make<S>(
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hana::at_c<Indices::cached_indices[i]>(static_cast<Xs&&>(xs))...
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);
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}
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template <typename Xs, typename Pred>
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static constexpr auto apply(Xs&& xs, Pred const& pred) {
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using Indices = decltype(
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hana::unpack(static_cast<Xs&&>(xs),
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detail::make_filter_indices<Pred>{pred})
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);
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return filter_impl::filter_helper<Indices>(
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static_cast<Xs&&>(xs),
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std::make_index_sequence<Indices::cached_indices.size()>{}
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);
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FILTER_HPP
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