/*! @file Defines `boost::hana::cycle`. @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_CYCLE_HPP #define BOOST_HANA_CYCLE_HPP #include <boost/hana/fwd/cycle.hpp> #include <boost/hana/at.hpp> #include <boost/hana/concat.hpp> #include <boost/hana/concept/integral_constant.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/array.hpp> #include <boost/hana/empty.hpp> #include <boost/hana/length.hpp> #include <cstddef> #include <utility> BOOST_HANA_NAMESPACE_BEGIN //! @cond template <typename Xs, typename N> constexpr auto cycle_t::operator()(Xs&& xs, N const& n) const { using M = typename hana::tag_of<Xs>::type; using Cycle = BOOST_HANA_DISPATCH_IF(cycle_impl<M>, hana::MonadPlus<M>::value ); #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS static_assert(hana::MonadPlus<M>::value, "hana::cycle(xs, n) requires 'xs' to be a MonadPlus"); static_assert(hana::IntegralConstant<N>::value, "hana::cycle(xs, n) requires 'n' to be an IntegralConstant"); #endif static_assert(N::value >= 0, "hana::cycle(xs, n) requires 'n' to be non-negative"); return Cycle::apply(static_cast<Xs&&>(xs), n); } //! @endcond namespace detail { template <typename M, std::size_t n, bool = n % 2 == 0> struct cycle_helper; template <typename M> struct cycle_helper<M, 0, true> { template <typename Xs> static constexpr auto apply(Xs const&) { return hana::empty<M>(); } }; template <typename M, std::size_t n> struct cycle_helper<M, n, true> { template <typename Xs> static constexpr auto apply(Xs const& xs) { return cycle_helper<M, n/2>::apply(hana::concat(xs, xs)); } }; template <typename M, std::size_t n> struct cycle_helper<M, n, false> { template <typename Xs> static constexpr auto apply(Xs const& xs) { return hana::concat(xs, cycle_helper<M, n-1>::apply(xs)); } }; } template <typename M, bool condition> struct cycle_impl<M, when<condition>> : default_ { template <typename Xs, typename N> static constexpr auto apply(Xs const& xs, N const&) { constexpr std::size_t n = N::value; return detail::cycle_helper<M, n>::apply(xs); } }; namespace detail { template <std::size_t N, std::size_t Len> struct cycle_indices { static constexpr auto compute_value() { detail::array<std::size_t, N * Len> indices{}; // Avoid (incorrect) Clang warning about remainder by zero // in the loop below. std::size_t len = Len; for (std::size_t i = 0; i < N * Len; ++i) indices[i] = i % len; return indices; } static constexpr auto value = compute_value(); }; } template <typename S> struct cycle_impl<S, when<Sequence<S>::value>> { template <typename Indices, typename Xs, std::size_t ...i> static constexpr auto cycle_helper(Xs&& xs, std::index_sequence<i...>) { constexpr auto indices = Indices::value; (void)indices; // workaround GCC warning when sizeof...(i) == 0 return hana::make<S>(hana::at_c<indices[i]>(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 len = decltype(hana::length(xs))::value; using Indices = detail::cycle_indices<n, len>; return cycle_helper<Indices>(static_cast<Xs&&>(xs), std::make_index_sequence<n * len>{}); } }; BOOST_HANA_NAMESPACE_END #endif // !BOOST_HANA_CYCLE_HPP