text: ///////////////////////////////////////////////////////////////////////////////
  // keeper_matcher.hpp
  //
  //  Copyright 2008 Eric Niebler. 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_XPRESSIVE_DETAIL_CORE_MATCHER_KEEPER_MATCHER_HPP_EAN_10_04_2005
  #define BOOST_XPRESSIVE_DETAIL_CORE_MATCHER_KEEPER_MATCHER_HPP_EAN_10_04_2005
  
  // MS compatible compilers support #pragma once
  #if defined(_MSC_VER) && (_MSC_VER >= 1020)
  # pragma once
  #endif
  
  #include <boost/mpl/bool.hpp>
  #include <boost/xpressive/detail/detail_fwd.hpp>
  #include <boost/xpressive/detail/core/quant_style.hpp>
  #include <boost/xpressive/detail/core/state.hpp>
  
  namespace boost { namespace xpressive { namespace detail
  {
  
      ///////////////////////////////////////////////////////////////////////////////
      // keeper_matcher
      //  Xpr can be either a static_xpression, or a shared_matchable
      template<typename Xpr>
      struct keeper_matcher
        : quant_style<quant_variable_width, unknown_width::value, Xpr::pure>
      {
          keeper_matcher(Xpr const &xpr, bool pure = Xpr::pure)
            : xpr_(xpr)
            , pure_(pure)
          {
          }
  
          template<typename BidiIter, typename Next>
          bool match(match_state<BidiIter> &state, Next const &next) const
          {
              return Xpr::pure || this->pure_
                ? this->match_(state, next, mpl::true_())
                : this->match_(state, next, mpl::false_());
          }
  
          template<typename BidiIter, typename Next>
          bool match_(match_state<BidiIter> &state, Next const &next, mpl::true_) const
          {
              BidiIter const tmp = state.cur_;
  
              // matching xpr is guaranteed to not produce side-effects, don't bother saving state
              if(!this->xpr_.match(state))
              {
                  return false;
              }
              else if(next.match(state))
              {
                  return true;
              }
  
              state.cur_ = tmp;
              return false;
          }
  
          template<typename BidiIter, typename Next>
          bool match_(match_state<BidiIter> &state, Next const &next, mpl::false_) const
          {
              BidiIter const tmp = state.cur_;
  
              // matching xpr could produce side-effects, save state
              memento<BidiIter> mem = save_sub_matches(state);
  
              if(!this->xpr_.match(state))
              {
                  restore_action_queue(mem, state);
                  reclaim_sub_matches(mem, state, false);
                  return false;
              }
              restore_action_queue(mem, state);
              if(next.match(state))
              {
                  reclaim_sub_matches(mem, state, true);
                  return true;
              }
  
              restore_sub_matches(mem, state);
              state.cur_ = tmp;
              return false;
          }
  
          Xpr xpr_;
          bool pure_; // false if matching xpr_ could modify the sub-matches
      };
  
  }}}
  
  #endif
  