epsilon.hpp

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/*=============================================================================
    Copyright (c) 1998-2003 Joel de Guzman
    Copyright (c) 2002-2003 Martin Wille
    http://spirit.sourceforge.net/

  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_SPIRIT_EPSILON_HPP
#define BOOST_SPIRIT_EPSILON_HPP

////////////////////////////////////////////////////////////////////////////////
#include <boost/spirit/home/classic/namespace.hpp>
#include <boost/spirit/home/classic/core/parser.hpp>
#include <boost/spirit/home/classic/meta/parser_traits.hpp>
#include <boost/spirit/home/classic/core/composite/composite.hpp>
#include <boost/spirit/home/classic/core/composite/no_actions.hpp>

////////////////////////////////////////////////////////////////////////////////
namespace boost { namespace spirit {

BOOST_SPIRIT_CLASSIC_NAMESPACE_BEGIN

///////////////////////////////////////////////////////////////////////////////
//
//  condition_parser class
//
//      handles expresions of the form
//
//          epsilon_p(cond)
//
//      where cond is a function or a functor that returns a value suitable
//      to be used in boolean context. The expression returns a parser that
//      returns an empty match when the condition evaluates to true.
//
///////////////////////////////////////////////////////////////////////////////
    template <typename CondT, bool positive_ = true>
    struct condition_parser : parser<condition_parser<CondT, positive_> >
    {
        typedef condition_parser<CondT, positive_> self_t;

        // not explicit! (needed for implementation of if_p et al.)
        condition_parser(CondT const& cond_) : cond(cond_) {}

        template <typename ScannerT>
        typename parser_result<self_t, ScannerT>::type
        parse(ScannerT const& scan) const
        {
            if (positive_ == bool(cond())) // allow cond to return int
                return scan.empty_match();
            else
                return scan.no_match();
        }

        condition_parser<CondT, !positive_>
        negate() const
        { return condition_parser<CondT, !positive_>(cond); }

    private:

        CondT cond;
    };

#if BOOST_WORKAROUND(BOOST_MSVC, == 1310) || \
    BOOST_WORKAROUND(BOOST_MSVC, == 1400) || \
    BOOST_WORKAROUND(__SUNPRO_CC, <= 0x580)
// VC 7.1, VC8 and Sun CC <= 5.8 do not support general
// expressions of non-type template parameters in instantiations
    template <typename CondT>
    inline condition_parser<CondT, false>
    operator~(condition_parser<CondT, true> const& p)
    { return p.negate(); }

    template <typename CondT>
    inline condition_parser<CondT, true>
    operator~(condition_parser<CondT, false> const& p)
    { return p.negate(); }
#else // BOOST_WORKAROUND(BOOST_MSVC, == 1310) || == 1400
    template <typename CondT, bool positive>
    inline condition_parser<CondT, !positive>
    operator~(condition_parser<CondT, positive> const& p)
    { return p.negate(); }
#endif // BOOST_WORKAROUND(BOOST_MSVC, == 1310) || == 1400

///////////////////////////////////////////////////////////////////////////////
//
//  empty_match_parser class
//
//      handles expressions of the form
//          epsilon_p(subject)
//      where subject is a parser. The expresion returns a composite
//      parser that returns an empty match if the subject parser matches.
//
///////////////////////////////////////////////////////////////////////////////
    struct empty_match_parser_gen;
    struct negated_empty_match_parser_gen;

    template <typename SubjectT>
    struct negated_empty_match_parser; // Forward declaration

    template<typename SubjectT>
    struct empty_match_parser
    : unary<SubjectT, parser<empty_match_parser<SubjectT> > >
    {
        typedef empty_match_parser<SubjectT>        self_t;
        typedef unary<SubjectT, parser<self_t> >    base_t;
        typedef unary_parser_category               parser_category_t;
        typedef empty_match_parser_gen              parser_genererator_t;
        typedef self_t embed_t;

        explicit empty_match_parser(SubjectT const& p) : base_t(p) {}

        template <typename ScannerT>
        struct result
        { typedef typename match_result<ScannerT, nil_t>::type type; };

        template <typename ScannerT>
        typename parser_result<self_t, ScannerT>::type
        parse(ScannerT const& scan) const
        {
            typename ScannerT::iterator_t save(scan.first);
            
            typedef typename no_actions_scanner<ScannerT>::policies_t
                policies_t;

            bool matches = this->subject().parse(
                scan.change_policies(policies_t(scan)));
            if (matches)
            {
                scan.first = save; // reset the position
                return scan.empty_match();
            }
            else
            {
                return scan.no_match();
            }
        }

        negated_empty_match_parser<SubjectT>
        negate() const
        { return negated_empty_match_parser<SubjectT>(this->subject()); }
    };

    template<typename SubjectT>
    struct negated_empty_match_parser
    : public unary<SubjectT, parser<negated_empty_match_parser<SubjectT> > >
    {
        typedef negated_empty_match_parser<SubjectT>    self_t;
        typedef unary<SubjectT, parser<self_t> >        base_t;
        typedef unary_parser_category                   parser_category_t;
        typedef negated_empty_match_parser_gen          parser_genererator_t;

        explicit negated_empty_match_parser(SubjectT const& p) : base_t(p) {}

        template <typename ScannerT>
        struct result
        { typedef typename match_result<ScannerT, nil_t>::type type; };

        template <typename ScannerT>
        typename parser_result<self_t, ScannerT>::type
        parse(ScannerT const& scan) const
        {
            typename ScannerT::iterator_t save(scan.first);

            typedef typename no_actions_scanner<ScannerT>::policies_t
                policies_t;

            bool matches = this->subject().parse(
                scan.change_policies(policies_t(scan)));
            if (!matches)
            {
                scan.first = save; // reset the position
                return scan.empty_match();
            }
            else
            {
                return scan.no_match();
            }
        }

        empty_match_parser<SubjectT>
        negate() const
        { return empty_match_parser<SubjectT>(this->subject()); }
    };

    struct empty_match_parser_gen
    {
        template <typename SubjectT>
        struct result
        { typedef empty_match_parser<SubjectT> type; };

        template <typename SubjectT>
        static empty_match_parser<SubjectT>
        generate(parser<SubjectT> const& subject)
        { return empty_match_parser<SubjectT>(subject.derived()); }
    };

    struct negated_empty_match_parser_gen
    {
        template <typename SubjectT>
        struct result
        { typedef negated_empty_match_parser<SubjectT> type; };

        template <typename SubjectT>
        static negated_empty_match_parser<SubjectT>
        generate(parser<SubjectT> const& subject)
        { return negated_empty_match_parser<SubjectT>(subject.derived()); }
    };

    //////////////////////////////
    template <typename SubjectT>
    inline negated_empty_match_parser<SubjectT>
    operator~(empty_match_parser<SubjectT> const& p)
    { return p.negate(); }

    template <typename SubjectT>
    inline empty_match_parser<SubjectT>
    operator~(negated_empty_match_parser<SubjectT> const& p)
    { return p.negate(); }

///////////////////////////////////////////////////////////////////////////////
//
//  epsilon_ parser and parser generator class
//
//      Operates as primitive parser that always matches an empty sequence.
//
//      Also operates as a parser generator. According to the type of the
//      argument an instance of empty_match_parser<> (when the argument is
//      a parser) or condition_parser<> (when the argument is not a parser)
//      is returned by operator().
//
///////////////////////////////////////////////////////////////////////////////
    namespace impl
    {
        template <typename SubjectT>
        struct epsilon_selector
        {
            typedef typename as_parser<SubjectT>::type subject_t;
            typedef typename
                mpl::if_<
                    is_parser<subject_t>
                    ,empty_match_parser<subject_t>
                    ,condition_parser<subject_t>
                >::type type;
        };
    }

    struct epsilon_parser : public parser<epsilon_parser>
    {
        typedef epsilon_parser self_t;

        epsilon_parser() {}

        template <typename ScannerT>
        typename parser_result<self_t, ScannerT>::type
        parse(ScannerT const& scan) const
        { return scan.empty_match(); }

        template <typename SubjectT>
        typename impl::epsilon_selector<SubjectT>::type
        operator()(SubjectT const& subject) const
        {
            typedef typename impl::epsilon_selector<SubjectT>::type result_t;
            return result_t(subject);
        }
    };

    epsilon_parser const epsilon_p = epsilon_parser();
    epsilon_parser const eps_p = epsilon_parser();

///////////////////////////////////////////////////////////////////////////////
BOOST_SPIRIT_CLASSIC_NAMESPACE_END

}} // namespace BOOST_SPIRIT_CLASSIC_NS

#endif