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+/*=============================================================================
+ Copyright (c) 2015 Paul Fultz II
+ fold.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_FOLD_H
+#define BOOST_HOF_GUARD_FOLD_H
+
+/// fold
+/// ========
+///
+/// Description
+/// -----------
+///
+/// The `fold` function adaptor uses a binary function to apply a
+/// [fold](https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29)
+/// operation to the arguments passed to the function. Additionally, an
+/// optional initial state can be provided, otherwise the first argument is
+/// used as the initial state.
+///
+/// The arguments to the binary function, take first the state and then the
+/// argument.
+///
+/// Synopsis
+/// --------
+///
+/// template<class F, class State>
+/// constexpr fold_adaptor<F, State> fold(F f, State s);
+///
+/// template<class F>
+/// constexpr fold_adaptor<F> fold(F f);
+///
+/// Semantics
+/// ---------
+///
+/// assert(fold(f, z)() == z);
+/// assert(fold(f, z)(x, xs...) == fold(f, f(z, x))(xs...));
+/// assert(fold(f)(x) == x);
+/// assert(fold(f)(x, y, xs...) == fold(f)(f(x, y), xs...));
+///
+/// Requirements
+/// ------------
+///
+/// State must be:
+///
+/// * CopyConstructible
+///
+/// F must be:
+///
+/// * [BinaryInvocable](BinaryInvocable)
+/// * MoveConstructible
+///
+/// Example
+/// -------
+///
+/// #include <boost/hof.hpp>
+/// #include <cassert>
+///
+/// struct max_f
+/// {
+/// template<class T, class U>
+/// constexpr T operator()(T x, U y) const
+/// {
+/// return x > y ? x : y;
+/// }
+/// };
+/// int main() {
+/// assert(boost::hof::fold(max_f())(2, 3, 4, 5) == 5);
+/// }
+///
+/// References
+/// ----------
+///
+/// * [Fold](https://en.wikipedia.org/wiki/Fold_(higher-order_function))
+/// * [Variadic sum](<Variadic sum>)
+///
+
+#include <boost/hof/detail/callable_base.hpp>
+#include <boost/hof/detail/delegate.hpp>
+#include <boost/hof/detail/compressed_pair.hpp>
+#include <boost/hof/detail/move.hpp>
+#include <boost/hof/detail/make.hpp>
+#include <boost/hof/detail/static_const_var.hpp>
+
+namespace boost { namespace hof { namespace detail {
+
+struct v_fold
+{
+ BOOST_HOF_RETURNS_CLASS(v_fold);
+ template<class F, class State, class T, class... Ts>
+ constexpr BOOST_HOF_SFINAE_MANUAL_RESULT(const v_fold&, id_<const F&>, result_of<const F&, id_<State>, id_<T>>, id_<Ts>...)
+ operator()(const F& f, State&& state, T&& x, Ts&&... xs) const BOOST_HOF_SFINAE_MANUAL_RETURNS
+ (
+ (*BOOST_HOF_CONST_THIS)(f, f(BOOST_HOF_FORWARD(State)(state), BOOST_HOF_FORWARD(T)(x)), BOOST_HOF_FORWARD(Ts)(xs)...)
+ );
+
+ template<class F, class State>
+ constexpr State operator()(const F&, State&& state) const noexcept
+ {
+ return BOOST_HOF_FORWARD(State)(state);
+ }
+};
+
+}
+
+template<class F, class State=void>
+struct fold_adaptor
+: detail::compressed_pair<detail::callable_base<F>, State>
+{
+ typedef detail::compressed_pair<detail::callable_base<F>, State> base_type;
+ BOOST_HOF_INHERIT_CONSTRUCTOR(fold_adaptor, base_type)
+
+ template<class... Ts>
+ constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const noexcept
+ {
+ return this->first(xs...);
+ }
+
+ template<class... Ts>
+ constexpr State get_state(Ts&&... xs) const noexcept
+ {
+ return this->second(xs...);
+ }
+
+ BOOST_HOF_RETURNS_CLASS(fold_adaptor);
+
+ template<class... Ts>
+ constexpr BOOST_HOF_SFINAE_RESULT(detail::v_fold, id_<const detail::callable_base<F>&>, id_<State>, id_<Ts>...)
+ operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
+ (
+ detail::v_fold()(
+ BOOST_HOF_MANGLE_CAST(const detail::callable_base<F>&)(BOOST_HOF_CONST_THIS->base_function(xs...)),
+ BOOST_HOF_MANGLE_CAST(State)(BOOST_HOF_CONST_THIS->get_state(xs...)),
+ BOOST_HOF_FORWARD(Ts)(xs)...
+ )
+ )
+};
+
+
+template<class F>
+struct fold_adaptor<F, void>
+: detail::callable_base<F>
+{
+ BOOST_HOF_INHERIT_CONSTRUCTOR(fold_adaptor, detail::callable_base<F>)
+
+ template<class... Ts>
+ constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const noexcept
+ {
+ return boost::hof::always_ref(*this)(xs...);
+ }
+
+ BOOST_HOF_RETURNS_CLASS(fold_adaptor);
+
+ template<class... Ts>
+ constexpr BOOST_HOF_SFINAE_RESULT(detail::v_fold, id_<const detail::callable_base<F>&>, id_<Ts>...)
+ operator()(Ts&&... xs) const BOOST_HOF_SFINAE_RETURNS
+ (
+ detail::v_fold()(
+ BOOST_HOF_MANGLE_CAST(const detail::callable_base<F>&)(BOOST_HOF_CONST_THIS->base_function(xs...)),
+ BOOST_HOF_FORWARD(Ts)(xs)...
+ )
+ )
+};
+
+BOOST_HOF_DECLARE_STATIC_VAR(fold, detail::make<fold_adaptor>);
+
+}} // namespace boost::hof
+
+#endif