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 Hu Chunming
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/*!
@file
Defines `boost::hana::intersperse`.
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_INTERSPERSE_HPP
#define BOOST_HANA_INTERSPERSE_HPP
#include <boost/hana/fwd/intersperse.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/bool.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/length.hpp>
#include <cstddef>
#include <utility>
namespace boost { namespace hana {
//! @cond
template <typename Xs, typename Z>
constexpr auto intersperse_t::operator()(Xs&& xs, Z&& z) const {
using S = typename hana::tag_of<Xs>::type;
using Intersperse = BOOST_HANA_DISPATCH_IF(intersperse_impl<S>,
hana::Sequence<S>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Sequence<S>::value,
"hana::intersperse(xs, z) requires 'xs' to be a Sequence");
#endif
return Intersperse::apply(static_cast<Xs&&>(xs), static_cast<Z&&>(z));
}
//! @endcond
template <typename S, bool condition>
struct intersperse_impl<S, when<condition>> : default_ {
template <std::size_t i, typename Xs, typename Z>
static constexpr decltype(auto)
pick(Xs&&, Z&& z, hana::false_ /* odd index */)
{ return static_cast<Z&&>(z); }
template <std::size_t i, typename Xs, typename Z>
static constexpr decltype(auto)
pick(Xs&& xs, Z&&, hana::true_ /* even index */)
{ return hana::at_c<(i + 1) / 2>(static_cast<Xs&&>(xs)); }
template <typename Xs, typename Z, std::size_t ...i>
static constexpr auto
intersperse_helper(Xs&& xs, Z&& z, std::index_sequence<i...>) {
return hana::make<S>(
pick<i>(static_cast<Xs&&>(xs), static_cast<Z&&>(z),
hana::bool_c<(i % 2 == 0)>)...
);
}
template <typename Xs, typename Z>
static constexpr auto apply(Xs&& xs, Z&& z) {
constexpr std::size_t size = decltype(hana::length(xs))::value;
constexpr std::size_t new_size = size == 0 ? 0 : (size * 2) - 1;
return intersperse_helper(static_cast<Xs&&>(xs), static_cast<Z&&>(z),
std::make_index_sequence<new_size>{});
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_INTERSPERSE_HPP
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