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118 lines
4.5 KiB
C++
118 lines
4.5 KiB
C++
/*!
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@file
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Defines `boost::hana::while_`.
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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_WHILE_HPP
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#define BOOST_HANA_WHILE_HPP
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#include <boost/hana/fwd/while.hpp>
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#include <boost/hana/bool.hpp>
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#include <boost/hana/concept/constant.hpp>
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#include <boost/hana/concept/constant.hpp>
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#include <boost/hana/concept/logical.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/to.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/detail/canonical_constant.hpp>
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#include <type_traits>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Pred, typename State, typename F>
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constexpr decltype(auto) while_t::operator()(Pred&& pred, State&& state, F&& f) const {
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using Cond = decltype(pred(state));
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using Bool = typename hana::tag_of<Cond>::type;
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using While = BOOST_HANA_DISPATCH_IF(while_impl<Bool>,
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hana::Logical<Bool>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Logical<Bool>::value,
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"hana::while_(pred, state, f) requires 'pred(state)' to be a Logical");
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#endif
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return While::apply(static_cast<Pred&&>(pred),
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static_cast<State&&>(state),
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static_cast<F&&>(f));
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}
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//! @endcond
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template <typename L, bool condition>
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struct while_impl<L, hana::when<condition>> : hana::default_ {
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template <typename ...Args>
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static constexpr auto apply(Args&& ...) = delete;
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};
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template <typename L>
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struct while_impl<L, hana::when<std::is_arithmetic<L>::value>> {
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template <typename Pred, typename State, typename F>
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static auto apply(Pred&& pred, State&& state, F&& f)
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-> decltype(
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true ? f(static_cast<State&&>(state))
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: static_cast<State&&>(state)
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)
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{
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if (pred(state)) {
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decltype(auto) r = f(static_cast<State&&>(state));
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return hana::while_(static_cast<Pred&&>(pred),
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static_cast<decltype(r)&&>(r),
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static_cast<F&&>(f));
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}
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else {
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return static_cast<State&&>(state);
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}
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}
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};
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template <typename C>
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struct while_impl<C, hana::when<
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hana::Constant<C>::value &&
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hana::Logical<typename C::value_type>::value
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>> {
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template <typename Pred, typename State, typename F>
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static constexpr State
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while_helper(hana::false_, Pred&&, State&& state, F&&) {
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return static_cast<State&&>(state);
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}
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template <typename Pred, typename State, typename F>
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static constexpr decltype(auto)
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while_helper(hana::true_, Pred&& pred, State&& state, F&& f) {
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decltype(auto) r = f(static_cast<State&&>(state));
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return hana::while_(static_cast<Pred&&>(pred),
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static_cast<decltype(r)&&>(r),
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static_cast<F&&>(f));
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}
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template <typename Pred, typename State, typename F>
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static constexpr decltype(auto)
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apply(Pred&& pred, State&& state, F&& f) {
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// Since `pred(state)` returns a `Constant`, we do not actually
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// need to call it; we only need its decltype. However, we still
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// call it to run potential side effects. I'm not sure whether
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// that is desirable, since we pretty much take for granted that
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// functions are pure, but we'll do it like this for now. Also, I
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// think there is something rather deep hidden behind this, and
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// understanding what must be done here should give us a better
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// understanding of something non-trivial.
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auto cond_ = pred(state);
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constexpr auto cond = hana::value(cond_);
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constexpr bool truth_value = hana::if_(cond, true, false);
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return while_helper(hana::bool_c<truth_value>,
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static_cast<Pred&&>(pred),
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static_cast<State&&>(state),
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static_cast<F&&>(f));
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_WHILE_HPP
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