fold.hpp

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                /**/

            #define BOOST_PP_ITERATION_PARAMS_1 (3, (1, BOOST_PROTO_MAX_ARITY, <boost/proto/transform/fold.hpp>))
            #include BOOST_PP_ITERATE()

            #undef BOOST_PROTO_REVERSE_FOLD_STATE
            #undef BOOST_PROTO_REVERSE_FOLD_STATE_TYPE
            #undef BOOST_PROTO_FOLD_STATE
            #undef BOOST_PROTO_FOLD_STATE_TYPE
            #undef BOOST_PROTO_CHILD_N_TYPE

        } // namespace detail

        /// \brief A PrimitiveTransform that invokes the <tt>fusion::fold\<\></tt>
        /// algorithm to accumulate
        template<typename Sequence, typename State0, typename Fun>
        struct fold : transform<fold<Sequence, State0, Fun> >
        {
            template<typename Expr, typename State, typename Data>
            struct impl : transform_impl<Expr, State, Data>
            {
                /// \brief A Fusion sequence.
                typedef
                    typename remove_reference<
                        typename when<_, Sequence>::template impl<Expr, State, Data>::result_type
                    >::type
                sequence;

                /// \brief An initial state for the fold.
                typedef
                    typename remove_reference<
                        typename when<_, State0>::template impl<Expr, State, Data>::result_type
                    >::type
                state0;

                /// \brief <tt>fun(v)(e,s) == when\<_,Fun\>()(e,s,v)</tt>
                typedef
                    detail::as_callable<Fun, Data>
                fun;

                typedef
                    typename fusion::BOOST_PROTO_FUSION_RESULT_OF::fold<
                        BOOST_PROTO_AS_FUSION_SEQUENCE_TYPE(sequence)
                      , state0
                      , fun
                    >::type
                result_type;

                /// Let \c seq be <tt>when\<_, Sequence\>()(expr, state, data)</tt>, let
                /// \c state0 be <tt>when\<_, State0\>()(expr, state, data)</tt>, and
                /// let \c fun(data) be an object such that <tt>fun(data)(expr, state)</tt>
                /// is equivalent to <tt>when\<_, Fun\>()(expr, state, data)</tt>. Then, this
                /// function returns <tt>fusion::fold(seq, state0, fun(data))</tt>.
                ///
                /// \param expr The current expression
                /// \param state The current state
                /// \param data An arbitrary data
                result_type operator ()(
                    typename impl::expr_param   expr
                  , typename impl::state_param  state
                  , typename impl::data_param   data
                ) const
                {
                    typename when<_, Sequence>::template impl<Expr, State, Data> sequence;
                    detail::as_callable<Fun, Data> fun(data);
                    return fusion::fold(
                        BOOST_PROTO_AS_FUSION_SEQUENCE(sequence(expr, state, data))
                      , typename when<_, State0>::template impl<Expr, State, Data>()(expr, state, data)
                      , fun
                    );
                }
            };
        };

        /// \brief A PrimitiveTransform that is the same as the
        /// <tt>fold\<\></tt> transform, except that it folds
        /// back-to-front instead of front-to-back. It uses
        /// the \c _reverse callable PolymorphicFunctionObject
        /// to create a <tt>fusion::reverse_view\<\></tt> of the
        /// sequence before invoking <tt>fusion::fold\<\></tt>.
        template<typename Sequence, typename State0, typename Fun>
        struct reverse_fold
          : fold<call<_reverse(Sequence)>, State0, Fun>
        {};

        // This specialization is only for improved compile-time performance
        // in the commom case when the Sequence transform is \c proto::_.
        //
        /// INTERNAL ONLY
        ///
        template<typename State0, typename Fun>
        struct fold<_, State0, Fun> : transform<fold<_, State0, Fun> >
        {
            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::fold_impl<State0, Fun, Expr, State, Data>
            {};
        };

        // This specialization is only for improved compile-time performance
        // in the commom case when the Sequence transform is \c proto::_.
        //
        /// INTERNAL ONLY
        ///
        template<typename State0, typename Fun>
        struct reverse_fold<_, State0, Fun> : transform<reverse_fold<_, State0, Fun> >
        {
            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::reverse_fold_impl<State0, Fun, Expr, State, Data>
            {};
        };

        /// INTERNAL ONLY
        ///
        template<typename Sequence, typename State, typename Fun>
        struct is_callable<fold<Sequence, State, Fun> >
          : mpl::true_
        {};

        /// INTERNAL ONLY
        ///
        template<typename Sequence, typename State, typename Fun>
        struct is_callable<reverse_fold<Sequence, State, Fun> >
          : mpl::true_
        {};

    }}

    #endif

#else

    #define N BOOST_PP_ITERATION()

            template<typename State0, typename Fun, typename Expr, typename State, typename Data>
            struct fold_impl<State0, Fun, Expr, State, Data, N>
              : transform_impl<Expr, State, Data>
            {
                typedef typename when<_, State0>::template impl<Expr, State, Data>::result_type state0;
                BOOST_PP_REPEAT(N, BOOST_PROTO_FOLD_STATE_TYPE, N)
                typedef BOOST_PP_CAT(state, N) result_type;

                result_type operator ()(
                    typename fold_impl::expr_param expr
                  , typename fold_impl::state_param state
                  , typename fold_impl::data_param data
                ) const
                {
                    state0 s0 =
                        typename when<_, State0>::template impl<Expr, State, Data>()(expr, state, data);
                    BOOST_PP_REPEAT(N, BOOST_PROTO_FOLD_STATE, N)
                    return BOOST_PP_CAT(s, N);
                }
            };

            template<typename State0, typename Fun, typename Expr, typename State, typename Data>
            struct reverse_fold_impl<State0, Fun, Expr, State, Data, N>
              : transform_impl<Expr, State, Data>
            {
                typedef typename when<_, State0>::template impl<Expr, State, Data>::result_type BOOST_PP_CAT(state, N);
                BOOST_PP_REPEAT(N, BOOST_PROTO_REVERSE_FOLD_STATE_TYPE, N)
                typedef state0 result_type;

                result_type operator ()(
                    typename reverse_fold_impl::expr_param expr
                  , typename reverse_fold_impl::state_param state
                  , typename reverse_fold_impl::data_param data
                ) const
                {
                    BOOST_PP_CAT(state, N) BOOST_PP_CAT(s, N) =
                        typename when<_, State0>::template impl<Expr, State, Data>()(expr, state, data);
                    BOOST_PP_REPEAT(N, BOOST_PROTO_REVERSE_FOLD_STATE, N)
                    return s0;
                }
            };

    #undef N

#endif