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/*=============================================================================
Copyright (c) 2015 Paul Fultz II
repeat_while.h
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
==============================================================================*/
#ifndef BOOST_HOF_GUARD_REPEAT_WHILE_H
#define BOOST_HOF_GUARD_REPEAT_WHILE_H
/// repeat_while
/// ======
///
/// Description
/// -----------
///
/// The `repeat_while` function decorator will repeatedly apply a function while
/// the predicate returns a boolean that is true. If the predicate returns an
/// `IntergralConstant` then the predicate is only evaluated at compile-time.
///
///
/// Synopsis
/// --------
///
/// template<class Predicate>
/// constexpr auto repeat_while(Predicate predicate);
///
/// Requirements
/// ------------
///
/// Predicate must be:
///
/// * [ConstFunctionObject](ConstFunctionObject)
/// * MoveConstructible
///
/// Example
/// -------
///
/// #include <boost/hof.hpp>
/// #include <cassert>
///
/// struct increment
/// {
/// template<class T>
/// constexpr std::integral_constant<int, T::value + 1> operator()(T) const
/// {
/// return std::integral_constant<int, T::value + 1>();
/// }
/// };
///
/// struct not_6
/// {
/// template<class T>
/// constexpr std::integral_constant<bool, (T::value != 6)>
/// operator()(T) const
/// {
/// return std::integral_constant<bool, (T::value != 6)>();
/// }
/// };
///
/// typedef std::integral_constant<int, 1> one;
/// typedef std::integral_constant<int, 6> six;
///
/// int main() {
/// auto increment_until_6 = boost::hof::repeat_while(not_6())(increment());
/// static_assert(std::is_same<six, decltype(increment_until_6(one()))>::value, "Error");
/// }
///
#include <boost/hof/always.hpp>
#include <boost/hof/detail/delegate.hpp>
#include <boost/hof/detail/result_of.hpp>
#include <boost/hof/detail/move.hpp>
#include <boost/hof/decorate.hpp>
#include <boost/hof/detail/static_const_var.hpp>
#include <boost/hof/first_of.hpp>
#include <boost/hof/detail/recursive_constexpr_depth.hpp>
namespace boost { namespace hof { namespace detail {
template<class P, class... Ts>
struct compute_predicate
{
typedef decltype(std::declval<P>()(std::declval<Ts>()...)) type;
};
template<bool B>
struct while_repeater
{
template<class F, class P, class... Ts>
constexpr BOOST_HOF_SFINAE_RESULT(while_repeater<
compute_predicate<P, typename result_of<const F&, id_<Ts>...>::type>::type::value
>, id_<const F&>, id_<const P&>, result_of<const F&, id_<Ts>...>)
operator()(const F& f, const P& p, Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
(
while_repeater<
compute_predicate<P, decltype(f(BOOST_HOF_FORWARD(Ts)(xs)...))>::type::value
>()(f, p, f(BOOST_HOF_FORWARD(Ts)(xs)...))
);
};
template<>
struct while_repeater<false>
{
template<class F, class P, class T>
constexpr T operator()(const F&, const P&, T&& x) const
BOOST_HOF_RETURNS_DEDUCE_NOEXCEPT(x)
{
return x;
}
};
struct repeat_while_constant_decorator
{
template<class P, class F, class... Ts>
constexpr auto operator()(const P& p, const F& f, Ts&&... xs) const BOOST_HOF_RETURNS
(
detail::while_repeater<
detail::compute_predicate<P, decltype(std::declval<F>()(BOOST_HOF_FORWARD(Ts)(xs)...))>::type::value
>()
(
f,
p,
BOOST_HOF_FORWARD(Ts)(xs)...
)
);
};
template<int Depth>
struct repeat_while_integral_decorator
{
template<class P, class F, class T, class... Ts, class Self=repeat_while_integral_decorator<Depth-1>>
constexpr auto operator()(const P& p, const F& f, T&& x, Ts&&... xs) const BOOST_HOF_RETURNS
(
(p(x, BOOST_HOF_FORWARD(Ts)(xs)...)) ?
Self()(
p,
f,
f(x, BOOST_HOF_FORWARD(Ts)(xs)...)
) :
BOOST_HOF_FORWARD(T)(x)
);
};
template<>
struct repeat_while_integral_decorator<0>
{
template<class P, class F, class T, class Self=repeat_while_integral_decorator<0>>
#if BOOST_HOF_HAS_RELAXED_CONSTEXPR
constexpr
#endif
auto operator()(const P& p, const F& f, T x) const
BOOST_HOF_RETURNS_DEDUCE_NOEXCEPT((p(x), f(x)))
-> decltype(f(x))
{
while(p(x))
{
// TODO: Should move?
x = f(x);
}
return x;
}
};
}
#if BOOST_HOF_HAS_RELAXED_CONSTEXPR
#define BOOST_HOF_REPEAT_WHILE_CONSTEXPR_DEPTH 1
#else
#define BOOST_HOF_REPEAT_WHILE_CONSTEXPR_DEPTH BOOST_HOF_RECURSIVE_CONSTEXPR_DEPTH
#endif
BOOST_HOF_DECLARE_STATIC_VAR(repeat_while, decorate_adaptor<
boost::hof::first_of_adaptor<
detail::repeat_while_constant_decorator,
detail::repeat_while_integral_decorator<BOOST_HOF_REPEAT_WHILE_CONSTEXPR_DEPTH>
>
>);
}} // namespace boost::hof
#endif
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