*  we begin to discard tokens in attempt to get past the point of error   *  to a point where we can continue parsing.  After each token, we attempt    *  to "parse ahead" though the buffered lookahead tokens.  The "parse ahead"   *  process simulates that actual parse, but does not modify the real    *  parser's configuration, nor execute any actions. If we can  parse all    *  the stored tokens without error, then the recovery is considered a    *  success.  Once a successful recovery point is determined, we do an   *  actual parse over the stored input -- modifying the real parse    *  configuration and executing all actions.  Finally, we return the the    *  normal parser to continue with the overall parse.   *   * @param debug should we produce debugging messages as we parse.   */  protected boolean error_recovery(boolean debug)    throws java.lang.Exception    {      if (debug) debug_message("# Attempting error recovery");      /* first pop the stack back into a state that can shift on error and 	 do that shift (if that fails, we fail) */      if (!find_recovery_config(debug))	{	  if (debug) debug_message("# Error recovery fails");	  return false;	}      /* read ahead to create lookahead we can parse multiple times */      read_lookahead();      /* repeatedly try to parse forward until we make it the required dist */      for (;;)	{	  /* try to parse forward, if it makes it, bail out of loop */	  if (debug) debug_message("# Trying to parse ahead");	  if (try_parse_ahead(debug))	    {	      break;	    }	  /* if we are now at EOF, we have failed */	  if (lookahead[0].sym == EOF_sym()) 	    {	      if (debug) debug_message("# Error recovery fails at EOF");	      return false;	    }	  /* otherwise, we consume another token and try again */	  if (debug) 	  debug_message("# Consuming token #" + cur_err_token().sym);	  restart_lookahead();	}      /* we have consumed to a point where we can parse forward */      if (debug) debug_message("# Parse-ahead ok, going back to normal parse");      /* do the real parse (including actions) across the lookahead */      parse_lookahead(debug);      /* we have success */      return true;    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Determine if we can shift under the special error symbol out of the    *  state currently on the top of the (real) parse stack.    */  protected boolean shift_under_error()    {      /* is there a shift under error symbol */      return get_action(((symbol)stack.peek()).parse_state, error_sym()) > 0;    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Put the (real) parse stack into error recovery configuration by    *  popping the stack down to a state that can shift on the special    *  error symbol, then doing the shift.  If no suitable state exists on    *  the stack we return false    *   * @param debug should we produce debugging messages as we parse.   */  protected boolean find_recovery_config(boolean debug)    {      token error_token;      int act;      if (debug) debug_message("# Finding recovery state on stack");      /* pop down until we can shift under error token */      while (!shift_under_error())	{	  /* pop the stack */	  if (debug) 	    debug_message("# Pop stack by one, state was # " +	                  ((symbol)stack.peek()).parse_state);          stack.pop();		  tos--;	  /* if we have hit bottom, we fail */	  if (stack.empty()) 	    {	      if (debug) debug_message("# No recovery state found on stack");	      return false;	    }	}      /* state on top of the stack can shift under error, find the shift */      act = get_action(((symbol)stack.peek()).parse_state, error_sym());      if (debug) 	{	  debug_message("# Recover state found (#" + 			((symbol)stack.peek()).parse_state + ")");	  debug_message("# Shifting on error to state #" + (act-1));	}      /* build and shift a special error token */      error_token = new token(error_sym());      error_token.parse_state = act-1;      stack.push(error_token);      tos++;      return true;    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Lookahead tokens used for attempting error recovery "parse aheads". */  protected token lookahead[];  /** Position in lookahead input buffer used for "parse ahead". */  protected int lookahead_pos;  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Read from input to establish our buffer of "parse ahead" lookahead    *  symbols.    */  protected void read_lookahead() throws java.lang.Exception    {      /* create the lookahead array */      lookahead = new token[error_sync_size()];      /* fill in the array */      for (int i = 0; i < error_sync_size(); i++)	{	  lookahead[i] = cur_token;	  cur_token = scan();	}      /* start at the beginning */      lookahead_pos = 0;    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Return the current lookahead in our error "parse ahead" buffer. */  protected token cur_err_token() { return lookahead[lookahead_pos]; }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Advance to next "parse ahead" input symbol. Return true if we have    *  input to advance to, false otherwise.    */  protected boolean advance_lookahead()    {      /* advance the input location */      lookahead_pos++;      /* return true if we didn't go off the end */      return lookahead_pos < error_sync_size();    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Reset the parse ahead input to one token past where we started error    *  recovery (this consumes one new token from the real input).    */  protected void restart_lookahead() throws java.lang.Exception    {      /* move all the existing input over */      for (int i = 1; i < error_sync_size(); i++)	lookahead[i-1] = lookahead[i];      /* read a new token into the last spot */      cur_token = scan();      lookahead[error_sync_size()-1] = cur_token;      /* reset our internal position marker */      lookahead_pos = 0;    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Do a simulated parse forward (a "parse ahead") from the current    *  stack configuration using stored lookahead input and a virtual parse   *  stack.  Return true if we make it all the way through the stored    *  lookahead input without error. This basically simulates the action of    *  parse() using only our saved "parse ahead" input, and not executing any    *  actions.   *   * @param debug should we produce debugging messages as we parse.   */  protected boolean try_parse_ahead(boolean debug)    throws java.lang.Exception    {      int act;      short lhs, rhs_size;      /* create a virtual stack from the real parse stack */      virtual_parse_stack vstack = new virtual_parse_stack(stack);      /* parse until we fail or get past the lookahead input */      for (;;)	{	  /* look up the action from the current state (on top of stack) */	  act = get_action(vstack.top(), cur_err_token().sym);	  /* if its an error, we fail */	  if (act == 0) return false;	  /* > 0 encodes a shift */	  if (act > 0)	    {	      /* push the new state on the stack */	      vstack.push(act-1);	      if (debug) debug_message("# Parse-ahead shifts token #" + 		       cur_err_token().sym + " into state #" + (act-1));	      /* advance simulated input, if we run off the end, we are done */	      if (!advance_lookahead()) return true;	    }	  /* < 0 encodes a reduce */	  else	    {	      /* if this is a reduce with the start production we are done */	      if ((-act)-1 == start_production()) 		{		  if (debug) debug_message("# Parse-ahead accepts");		  return true;		}	      /* get the lhs symbol and the rhs size */	      lhs = production_tab[(-act)-1][0];	      rhs_size = production_tab[(-act)-1][1];	      /* pop handle off the stack */	      for (int i = 0; i < rhs_size; i++)		vstack.pop();	      if (debug) 		debug_message("# Parse-ahead reduces: handle size = " + 	          rhs_size + " lhs = #" + lhs + " from state #" + vstack.top());	      /* look up goto and push it onto the stack */	      vstack.push(get_reduce(vstack.top(), lhs));	      if (debug) 		debug_message("# Goto state #" + vstack.top());	    }	}    }  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/  /** Parse forward using stored lookahead symbols.  In this case we have   *  already verified that parsing will make it through the stored lookahead   *  symbols and we are now getting back to the point at which we can hand   *  control back to the normal parser.  Consequently, this version of the   *  parser performs all actions and modifies the real parse configuration.     *  This returns once we have consumed all the stored input or we accept.   *   * @param debug should we produce debugging messages as we parse.   */  protected void parse_lookahead(boolean debug)    throws java.lang.Exception    {      /* the current action code */      int act;      /* the symbol/stack element returned by a reduce */      symbol lhs_sym;      /* information about production being reduced with */      short handle_size, lhs_sym_num;      /* restart the saved input at the beginning */      lookahead_pos = 0;      if (debug) 	{	  debug_message("# Reparsing saved input with actions");	  debug_message("# Current token is #" + cur_err_token().sym);	  debug_message("# Current state is #" + 			((symbol)stack.peek()).parse_state);	}      /* continue until we accept or have read all lookahead input */      while(!_done_parsing)	{	  /* current state is always on the top of the stack */	  /* look up action out of the current state with the current input */	  act = 	    get_action(((symbol)stack.peek()).parse_state, cur_err_token().sym);	  /* decode the action -- > 0 encodes shift */	  if (act > 0)	    {	      /* shift to the encoded state by pushing it on the stack */	      cur_err_token().parse_state = act-1;	      if (debug) debug_shift(cur_err_token());	      stack.push(cur_err_token());	      tos++;	      /* advance to the next token, if there is none, we are done */	      if (!advance_lookahead()) 		{		  if (debug) debug_message("# Completed reparse");		  /* scan next token so we can continue parse */		  cur_token = scan();		  /* go back to normal parser */		  return;		}	      	      if (debug) 		debug_message("# Current token is #" + cur_err_token().sym);	    }	  /* if its less than zero, then it encodes a reduce action */	  else if (act < 0)	    {	      /* perform the action for the reduce */	      lhs_sym = do_action((-act)-1, this, stack, tos);	      /* look up information about the production */	      lhs_sym_num = production_tab[(-act)-1][0];	      handle_size = production_tab[(-act)-1][1];	      if (debug) debug_reduce((-act)-1, lhs_sym_num, handle_size);	      /* pop the handle off the stack */	      for (int i = 0; i < handle_size; i++)		{		  stack.pop();		  tos--;		}	      	      /* look up the state to go to from the one popped back to */	      act = get_reduce(((symbol)stack.peek()).parse_state, lhs_sym_num);	      /* shift to that state */	      lhs_sym.parse_state = act;	      stack.push(lhs_sym);	      tos++;	       	      if (debug) debug_message("# Goto state #" + act);	    }	  /* finally if the entry is zero, we have an error 	     (shouldn't happen here, but...)*/	  else if (act == 0)	    {	      report_fatal_error("Syntax error", null);	      return;	    }	}    }  /*-----------------------------------------------------------*/};