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 Hu Chunming
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/*!
@file
Defines `boost::hana::drop_while`.
Copyright Louis Dionne 2013-2022
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_DROP_WHILE_HPP
#define BOOST_HANA_DROP_WHILE_HPP
#include <boost/hana/fwd/drop_while.hpp>
#include <boost/hana/concept/foldable.hpp>
#include <boost/hana/concept/iterable.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/detail/first_unsatisfied_index.hpp>
#include <boost/hana/drop_front.hpp>
#include <boost/hana/eval_if.hpp>
#include <boost/hana/front.hpp>
#include <boost/hana/is_empty.hpp>
#include <boost/hana/lazy.hpp>
namespace boost { namespace hana {
//! @cond
template <typename Xs, typename Pred>
constexpr auto drop_while_t::operator()(Xs&& xs, Pred&& pred) const {
using It = typename hana::tag_of<Xs>::type;
using DropWhile = BOOST_HANA_DISPATCH_IF(drop_while_impl<It>,
hana::Iterable<It>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Iterable<It>::value,
"hana::drop_while(xs, pred) requires 'xs' to be an Iterable");
#endif
return DropWhile::apply(static_cast<Xs&&>(xs), static_cast<Pred&&>(pred));
}
//! @endcond
namespace iterable_detail {
struct drop_while_helper {
struct next {
template <typename Xs, typename Pred>
constexpr decltype(auto) operator()(Xs&& xs, Pred&& pred) const {
return hana::drop_while(
hana::drop_front(static_cast<Xs&&>(xs)),
static_cast<Pred&&>(pred)
);
}
};
template <typename Xs, typename Pred>
constexpr decltype(auto) operator()(Xs&& xs, Pred&& pred) const {
return hana::eval_if(pred(hana::front(xs)),
hana::make_lazy(next{})(xs, pred),
hana::make_lazy(xs)
);
}
};
}
template <typename It, bool condition>
struct drop_while_impl<It, when<condition>> : default_ {
template <typename Xs, typename Pred>
static constexpr auto apply(Xs&& xs, Pred&& pred) {
return hana::eval_if(hana::is_empty(xs),
hana::make_lazy(xs),
hana::make_lazy(iterable_detail::drop_while_helper{})(
xs, static_cast<Pred&&>(pred))
);
}
};
template <typename S>
struct drop_while_impl<S, when<hana::Foldable<S>::value>> {
template <typename Xs, typename Pred>
static constexpr auto apply(Xs&& xs, Pred&&) {
using FirstUnsatisfied = decltype(
hana::unpack(static_cast<Xs&&>(xs),
detail::first_unsatisfied_index<Pred&&>{})
);
return hana::drop_front(static_cast<Xs&&>(xs),
FirstUnsatisfied{});
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_DROP_WHILE_HPP
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