128 lines
4.3 KiB
C++
128 lines
4.3 KiB
C++
/*!
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@file
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Defines `boost::hana::cycle`.
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@copyright Louis Dionne 2013-2016
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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_CYCLE_HPP
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#define BOOST_HANA_CYCLE_HPP
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#include <boost/hana/fwd/cycle.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/concat.hpp>
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#include <boost/hana/concept/integral_constant.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/array.hpp>
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#include <boost/hana/empty.hpp>
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#include <boost/hana/length.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 N>
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constexpr auto cycle_t::operator()(Xs&& xs, N const& n) const {
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using M = typename hana::tag_of<Xs>::type;
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using Cycle = BOOST_HANA_DISPATCH_IF(cycle_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::cycle(xs, n) requires 'xs' to be a MonadPlus");
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static_assert(hana::IntegralConstant<N>::value,
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"hana::cycle(xs, n) requires 'n' to be an IntegralConstant");
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#endif
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static_assert(N::value >= 0,
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"hana::cycle(xs, n) requires 'n' to be non-negative");
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return Cycle::apply(static_cast<Xs&&>(xs), n);
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}
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//! @endcond
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namespace detail {
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template <typename M, std::size_t n, bool = n % 2 == 0>
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struct cycle_helper;
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template <typename M>
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struct cycle_helper<M, 0, true> {
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template <typename Xs>
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static constexpr auto apply(Xs const&)
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{ return hana::empty<M>(); }
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};
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template <typename M, std::size_t n>
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struct cycle_helper<M, n, true> {
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template <typename Xs>
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static constexpr auto apply(Xs const& xs)
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{ return cycle_helper<M, n/2>::apply(hana::concat(xs, xs)); }
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};
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template <typename M, std::size_t n>
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struct cycle_helper<M, n, false> {
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template <typename Xs>
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static constexpr auto apply(Xs const& xs)
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{ return hana::concat(xs, cycle_helper<M, n-1>::apply(xs)); }
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};
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}
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template <typename M, bool condition>
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struct cycle_impl<M, when<condition>> : default_ {
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template <typename Xs, typename N>
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static constexpr auto apply(Xs const& xs, N const&) {
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constexpr std::size_t n = N::value;
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return detail::cycle_helper<M, n>::apply(xs);
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}
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};
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namespace detail {
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template <std::size_t N, std::size_t Len>
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struct cycle_indices {
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static constexpr auto compute_value() {
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detail::array<std::size_t, N * Len> indices{};
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// Avoid (incorrect) Clang warning about remainder by zero
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// in the loop below.
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std::size_t len = Len;
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for (std::size_t i = 0; i < N * Len; ++i)
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indices[i] = i % len;
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return indices;
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}
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static constexpr auto value = compute_value();
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};
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}
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template <typename S>
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struct cycle_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 cycle_helper(Xs&& xs, std::index_sequence<i...>) {
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constexpr auto indices = Indices::value;
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(void)indices; // workaround GCC warning when sizeof...(i) == 0
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return hana::make<S>(hana::at_c<indices[i]>(xs)...);
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}
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template <typename Xs, typename N>
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static constexpr auto apply(Xs&& xs, N const&) {
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constexpr std::size_t n = N::value;
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constexpr std::size_t len = decltype(hana::length(xs))::value;
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using Indices = detail::cycle_indices<n, len>;
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return cycle_helper<Indices>(static_cast<Xs&&>(xs),
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std::make_index_sequence<n * len>{});
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_CYCLE_HPP
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