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 Hu Chunming
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/*!
@file
Defines `boost::hana::take_front` and `boost::hana::take_front_c`.
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_TAKE_FRONT_HPP
#define BOOST_HANA_TAKE_FRONT_HPP
#include <boost/hana/fwd/take_front.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/concept/integral_constant.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/integral_constant.hpp>
#include <boost/hana/length.hpp>
#include <cstddef>
#include <utility>
namespace boost { namespace hana {
//! @cond
template <typename Xs, typename N>
constexpr auto take_front_t::operator()(Xs&& xs, N const& n) const {
using S = typename hana::tag_of<Xs>::type;
using TakeFront = BOOST_HANA_DISPATCH_IF(take_front_impl<S>,
hana::Sequence<S>::value &&
hana::IntegralConstant<N>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Sequence<S>::value,
"hana::take_front(xs, n) requires 'xs' to be a Sequence");
static_assert(hana::IntegralConstant<N>::value,
"hana::take_front(xs, n) requires 'n' to be an IntegralConstant");
#endif
return TakeFront::apply(static_cast<Xs&&>(xs), n);
}
//! @endcond
template <typename S, bool condition>
struct take_front_impl<S, when<condition>> : default_ {
template <typename Xs, std::size_t ...n>
static constexpr auto take_front_helper(Xs&& xs, std::index_sequence<n...>) {
return hana::make<S>(hana::at_c<n>(static_cast<Xs&&>(xs))...);
}
template <typename Xs, typename N>
static constexpr auto apply(Xs&& xs, N const&) {
constexpr std::size_t n = N::value;
constexpr std::size_t size = decltype(hana::length(xs))::value;
return take_front_helper(static_cast<Xs&&>(xs),
std::make_index_sequence<(n < size ? n : size)>{});
}
};
template <std::size_t n>
struct take_front_c_t {
template <typename Xs>
constexpr auto operator()(Xs&& xs) const {
return hana::take_front(static_cast<Xs&&>(xs), hana::size_c<n>);
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_TAKE_FRONT_HPP
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