
package org.jacorb.idl.runtime;

import java.util.Stack;

/** This class implements a skeleton table driven LR parser.  In general,
 *  LR parsers are a form of bottom up shift-reduce parsers.  Shift-reduce
 *  parsers act by shifting input onto a parse stack until the symbols
 *  matching the right hand side of a production appear on the top of the
 *  stack.  Once this occurs, a reduce is performed.  This involves removing
 *  the symbols corresponding to the right hand side of the production
 *  (the so called "handle") and replacing them with the non-terminal from
 *  the left hand side of the production.  <p>
 *
 *  To control the decision of whether to shift or reduce at any given point,
 *  the parser uses a state machine (the "viable prefix recognition machine"
 *  built by the parser generator).  The current state of the machine is placed
 *  on top of the parse stack (stored as part of a symbol object representing
 *  a terminal or non terminal).  The parse action table is consulted
 *  (using the current state and the current lookahead token as indexes) to
 *  determine whether to shift or to reduce.  When the parser shifts, it
 *  changes to a new state by pushing a new symbol (containing a new state)
 *  onto the stack.  When the parser reduces, it pops the handle (right hand
 *  side of a production) off the stack.  This leaves the parser in the state
 *  it was in before any of those symbols were matched.  Next the reduce-goto
 *  table is consulted (using the new state and current lookahead token as
 *  indexes) to determine a new state to go to.  The parser then shifts to
 *  this goto state by pushing the left hand side symbol of the production
 *  (also containing the new state) onto the stack.<p>
 *
 *  This class actually provides four LR parsers.  The methods parse() and
 *  debug_parse() provide two versions of the main parser (the only difference
 *  being that debug_parse() emits debugging trace messages as it parses).
 *  In addition to these main parsers, the error recovery mechanism uses two
 *  more.  One of these is used to simulate "parsing ahead" in the input
 *  without carrying out actions (to verify that a potential error recovery
 *  has worked), and the other is used to parse through buffered "parse ahead"
 *  input in order to execute all actions and re-synchronize the actual parser
 *  configuration.<p>
 *
 *  This is an abstract class which is normally filled out by a subclass
 *  generated by the JavaCup parser generator.  In addition to supplying
 *  the actual parse tables, generated code also supplies methods which
 *  invoke various pieces of user supplied code, provide access to certain
 *  special symbols (e.g., EOF and error), etc.  Specifically, the following
 *  abstract methods are normally supplied by generated code:
 *  <dl compact>
 *  <dt> short[][] production_table()
 *  <dd> Provides a reference to the production table (indicating the index of
 *       the left hand side non terminal and the length of the right hand side
 *       for each production in the grammar).
 *  <dt> short[][] action_table()
 *  <dd> Provides a reference to the parse action table.
 *  <dt> short[][] reduce_table()
 *  <dd> Provides a reference to the reduce-goto table.
 *  <dt> int start_state()
 *  <dd> Indicates the index of the start state.
 *  <dt> int start_production()
 *  <dd> Indicates the index of the starting production.
 *  <dt> int EOF_sym()
 *  <dd> Indicates the index of the EOF symbol.
 *  <dt> int error_sym()
 *  <dd> Indicates the index of the error symbol.
 *  <dt> symbol do_action()
 *  <dd> Executes a piece of user supplied action code.  This always comes at
 *       the point of a reduce in the parse, so this code also allocates and
 *       fills in the left hand side non terminal symbol object that is to be
 *       pushed onto the stack for the reduce.
 *  <dt> void init_actions()
 *  <dd> Code to initialize a special object that encapsulates user supplied
 *       actions (this object is used by do_action() to actually carry out the
 *       actions).
 *  <dt> token scan()
 *  <dd> Used to get the next input token from the scanner.
 *  </dl>
 *
 *  In addition to these routines that <i>must</i> be supplied by the
 *  generated subclass there are also a series of routines that <i>may</i>
 *  be supplied.  These include:
 *  <dl>
 *  <dt> int error_sync_size()
 *  <dd> This determines how many tokens past the point of an error
 *       must be parsed without error in order to consider a recovery to
 *       be valid.  This defaults to 3.  Values less than 2 are not
 *       recommended.
 *  <dt> void report_error(String message, Object info)
 *  <dd> This method is called to report an error.  The default implementation
 *       simply prints a message to System.err and ignores its second parameter.
 *       This method is often replaced in order to provide a more sophisticated
 *       error reporting mechanism.
 *  <dt> void report_fatal_error(String message, Object info)
 *  <dd> This method is called when a fatal error that cannot be recovered from
 *       is encountered.  In the default implementation, it calls
 *       report_error() to emit a message, then throws an exception.
 *  <dt> void syntax_error(token cur_token)
 *  <dd> This method is called as soon as syntax error is detected (but
 *       before recovery is attempted).  In the default implementation it
 *       invokes: report_error("Syntax error", null);
 *  <dt> void unrecovered_syntax_error(token cur_token)
 *  <dd> This method is called if syntax error recovery fails.  In the default
 *       implementation it invokes:<br>
 *         report_fatal_error("Couldn't repair and continue parse", null);
 *  </dl>
 *
 * @see     org.jacorb.idl.runtime.symbol
 * @see     org.jacorb.idl.runtime.token
 * @see     org.jacorb.idl.runtime.virtual_parse_stack
 * @version last updated: 11/25/95
 * @author  Scott Hudson
 */

public abstract class lr_parser {

  /*-----------------------------------------------------------*/
  /*--- Constructor(s) ----------------------------------------*/
  /*-----------------------------------------------------------*/

  /** Simple constructor. */
  public lr_parser()
    {
      /* nothing to do here */
    }

  /*-----------------------------------------------------------*/
  /*--- (Access to) Static (Class) Variables ------------------*/
  /*-----------------------------------------------------------*/

  /** The default number of tokens after an error we much match to consider
   *  it recovered from.
   */
  protected final static int _error_sync_size = 3;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The number of tokens after an error we much match to consider it
   *  recovered from.
   */
  protected int error_sync_size() {return _error_sync_size; }

  /*-----------------------------------------------------------*/
  /*--- (Access to) Instance Variables ------------------------*/
  /*-----------------------------------------------------------*/

  /** Table of production information (supplied by generated subclass).
   *  This table contains one entry per production and is indexed by
   *  the negative-encoded values (reduce actions) in the action_table.
   *  Each entry has two parts, the index of the non-terminal on the
   *  left hand side of the production, and the number of symbols
   *  on the right hand side.
   */
  public abstract short[][] production_table();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The action table (supplied by generated subclass).  This table is
   *  indexed by state and terminal number indicating what action is to
   *  be taken when the parser is in the given state (i.e., the given state
   *  is on top of the stack) and the given terminal is next on the input.
   *  States are indexed using the first dimension, however, the entries for
   *  a given state are compacted and stored in adjacent index, value pairs
   *  which are searched for rather than accessed directly (see get_action()).
   *  The actions stored in the table will be either shifts, reduces, or
   *  errors.  Shifts are encoded as positive values (one greater than the
   *  state shifted to).  Reduces are encoded as negative values (one less
   *  than the production reduced by).  Error entries are denoted by zero.
   *
   * @see org.jacorb.idl.runtime.lr_parser#get_action
   */
  public abstract short[][] action_table();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The reduce-goto table (supplied by generated subclass).  This
   *  table is indexed by state and non-terminal number and contains
   *  state numbers.  States are indexed using the first dimension, however,
   *  the entries for a given state are compacted and stored in adjacent
   *  index, value pairs which are searched for rather than accessed
   *  directly (see get_reduce()).  When a reduce occurs, the handle
   *  (corresponding to the RHS of the matched production) is popped off
   *  the stack.  The new top of stack indicates a state.  This table is
   *  then indexed by that state and the LHS of the reducing production to
   *  indicate where to "shift" to.
   *
   * @see org.jacorb.idl.runtime.lr_parser#get_reduce
   */
  public abstract short[][] reduce_table();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The index of the start state (supplied by generated subclass). */
  public abstract int start_state();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The index of the start production (supplied by generated subclass). */
  public abstract int start_production();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The index of the end of file terminal symbol (supplied by generated
   *  subclass).
   */
  public abstract int EOF_sym();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The index of the special error symbol (supplied by generated subclass). */
  public abstract int error_sym();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Internal flag to indicate when parser should quit. */
  protected boolean _done_parsing = false;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** This method is called to indicate that the parser should quit.  This is
   *  normally called by an accept action, but can be used to cancel parsing
   *  early in other circumstances if desired.
   */
  public void done_parsing()
    {
      _done_parsing = true;
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
  /* Global parse state shared by parse(), error recovery, and
   * debugging routines */
  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Indication of the index for top of stack (for use by actions). */
  protected int tos;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The current lookahead token. */
  protected token cur_token;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The parse stack itself. */
  protected Stack stack = new Stack();

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Direct reference to the production table. */
  protected short[][] production_tab;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Direct reference to the action table. */
  protected short[][] action_tab;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Direct reference to the reduce-goto table. */
  protected short[][] reduce_tab;

  /*-----------------------------------------------------------*/
  /*--- General Methods ---------------------------------------*/
  /*-----------------------------------------------------------*/

  /** Perform a bit of user supplied action code (supplied by generated
   *  subclass).  Actions are indexed by an internal action number assigned
   *  at parser generation time.
   *
   * @param act_num   the internal index of the action to be performed.
   * @param parser    the parser object we are acting for.
   * @param stack     the parse stack of that object.
   * @param top       the index of the top element of the parse stack.
   */
  public abstract symbol do_action(
    int       act_num,
    lr_parser parser,
    Stack     stack,
    int       top)
    throws java.lang.Exception;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** User code for initialization inside the parser.  Typically this
   *  initializes the scanner.  This is called before the parser requests
   *  the first token.  Here this is just a placeholder for subclasses that
   *  might need this and we perform no action.   This method is normally
   *  overridden by the generated code using this contents of the "init with"
   *  clause as its body.
   */
  public void user_init() throws java.lang.Exception { }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Initialize the action object.  This is called before the parser does
   *  any parse actions. This is filled in by generated code to create
   *  an object that encapsulates all action code.
   */
  protected abstract void init_actions() throws java.lang.Exception;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Get the next token from the input (supplied by generated subclass).
   *  Once end of file has been reached, all subsequent calls to scan
   *  should return an EOF token (which is symbol number 0).  This method
   *  is supplied by the generator using using the code declared in the
   *  "scan with" clause.
   */
  public abstract token scan() throws java.lang.Exception;

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Report a fatal error.  This method takes a  message string and an
   *  additional object (to be used by specializations implemented in
   *  subclasses).  Here in the base class a very simple implementation
   *  is provided which reports the error then throws an exception.
   *
   * @param message an error message.
   * @param info    an extra object reserved for use by specialized subclasses.
   */
  public void report_fatal_error(
    String   message,
    Object   info)
    throws java.lang.Exception
    {
      /* stop parsing (not really necessary since we throw an exception, but) */
      done_parsing();

      /* use the normal error message reporting to put out the message */
      report_error(message, info);

      /* throw an exception */
      throw new Exception("Can't recover from previous error(s)");
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Report a non fatal error (or warning).  This method takes a message
   *  string and an additional object (to be used by specializations
   *  implemented in subclasses).  Here in the base class a very simple
   *  implementation is provided which simply prints the message to
   *  System.err.
   *
   * @param message an error message.
   * @param info    an extra object reserved for use by specialized subclasses.
   */
  public void report_error(String message, Object info)
    {
      System.err.println(message);
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** This method is called when a syntax error has been detected and recovery
   *  is about to be invoked.  Here in the base class we just emit a
   *  "Syntax error" error message.
   *
   * @param cur_token the current lookahead token.
   */
  public void syntax_error(token cur_token)
    {
      report_error("Syntax error", null);
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** This method is called if it is determined that syntax error recovery
   *  has been unsuccessful.  Here in the base class we report a fatal error.
   *
   * @param cur_token the current lookahead token.
   */
  public void unrecovered_syntax_error(token cur_token)
    throws java.lang.Exception
    {
      report_fatal_error("Couldn't repair and continue parse", null);
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Fetch an action from the action table.  The table is broken up into
   *  rows, one per state (rows are indexed directly by state number).
   *  Within each row, a list of index, value pairs are given (as sequential
   *  entries in the table), and the list is terminated by a default entry
   *  (denoted with a symbol index of -1).  To find the proper entry in a row
   *  we do a linear or binary search (depending on the size of the row).
   *
   * @param state the state index of the action being accessed.
   * @param sym   the symbol index of the action being accessed.
   */
  protected final short get_action(int state, int sym)
    {
      short tag;
      int first, last, probe;
      short[] row = action_tab[state];

      /* linear search if we are < 10 entries */
      if (row.length < 20)
        for (probe = 0; probe < row.length; probe++)
	  {
	    /* is this entry labeled with our symbol or the default? */
	    tag = row[probe++];
	    if (tag == sym || tag == -1)
	      {
	        /* return the next entry */
	        return row[probe];
	      }
	  }
      /* otherwise binary search */
      else
	{
	  first = 0;
	  last = (row.length-1)/2 - 1;  /* leave out trailing default entry */
	  while (first <= last)
	    {
	      probe = (first+last)/2;
	      if (sym == row[probe*2])
		return row[probe*2+1];
	      else if (sym > row[probe*2])
		first = probe+1;
	      else
	        last = probe-1;
	    }

	  /* not found, use the default at the end */
	  return row[row.length-1];
	}

      /* shouldn't happened, but if we run off the end we return the
	 default (error == 0) */
      return 0;
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Fetch a state from the reduce-goto table.  The table is broken up into
   *  rows, one per state (rows are indexed directly by state number).
   *  Within each row, a list of index, value pairs are given (as sequential
   *  entries in the table), and the list is terminated by a default entry
   *  (denoted with a symbol index of -1).  To find the proper entry in a row
   *  we do a linear search.
   *
   * @param state the state index of the entry being accessed.
   * @param sym   the symbol index of the entry being accessed.
   */
  protected final short get_reduce(int state, int sym)
    {
      short tag;
      short[] row = reduce_tab[state];

      /* if we have a null row we go with the default */
      if (row == null)
        return -1;

      for (int probe = 0; probe < row.length; probe++)
	{
	  /* is this entry labeled with our symbol or the default? */
	  tag = row[probe++];
	  if (tag == sym || tag == -1)
	    {
	      /* return the next entry */
	      return row[probe];
	    }
	}
      /* if we run off the end we return the default (error == -1) */
      return -1;
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** This method provides the main parsing routine.  It returns only when
   *  done_parsing() has been called (typically because the parser has
   *  accepted, or a fatal error has been reported).  See the header
   *  documentation for the class regarding how shift/reduce parsers operate
   *  and how the various tables are used.
   */
  public void parse() 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;

      /* set up direct reference to tables to drive the parser */

      production_tab = production_table();
      action_tab     = action_table();
      reduce_tab     = reduce_table();

      /* initialize the action encapsulation object */
      init_actions();

      /* do user initialization */
      user_init();

      /* get the first token */
      cur_token = scan();

      /* push dummy symbol with start state to get us underway */
      stack.push(new symbol(0, start_state()));
      tos = 0;

      /* continue until we are told to stop */
      for (_done_parsing = false; !_done_parsing; )
	{
	  /* current state is always on the top of the stack */

          if (cur_token == null)
          {
              // If previous has returned null ptr then just exit now.
              unrecovered_syntax_error (cur_token);
          }
	  /* look up action out of the current state with the current input */
	  act = get_action(((symbol)stack.peek()).parse_state, cur_token.sym);

	  /* decode the action -- > 0 encodes shift */
	  if (act > 0)
	    {
	      /* shift to the encoded state by pushing it on the stack */
	      cur_token.parse_state = act-1;
	      stack.push(cur_token);
	      tos++;

	      /* advance to the next token */
	      cur_token = scan();
	    }
	  /* 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];

	      /* 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++;
	    }
	  /* finally if the entry is zero, we have an error */
	  else if (act == 0)
	    {
	      /* call user syntax error reporting routine */
	      syntax_error(cur_token);

	      /* try to error recover */
	      if (!error_recovery(false))
		{
		  /* if that fails give up with a fatal syntax error */
		  unrecovered_syntax_error(cur_token);

		  /* just in case that wasn't fatal enough, end parse */
		  done_parsing();
		}
	    }
	}
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Write a debugging message to System.err for the debugging version
   *  of the parser.
   *
   * @param mess the text of the debugging message.
   */
  public void debug_message(String mess)
    {
      System.err.println(mess);
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Dump the parse stack for debugging purposes. */
  public void dump_stack()
    {
      if (stack == null)
	{
	  debug_message("# Stack dump requested, but stack is null");
	  return;
	}

      debug_message("============ Parse Stack Dump ============");

      /* dump the stack */
      for (int i=0; i<stack.size(); i++)
	{
	  debug_message("Symbol: " + ((symbol)stack.elementAt(i)).sym +
			" State: " + ((symbol)stack.elementAt(i)).parse_state);
	}
      debug_message("==========================================");
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Do debug output for a reduce.
   *
   * @param prod_num  the production we are reducing with.
   * @param nt_num    the index of the LHS non terminal.
   * @param rhs_size  the size of the RHS.
   */
  public void debug_reduce(int prod_num, int nt_num, int rhs_size)
    {
      debug_message("# Reduce with prod #" + prod_num + " [NT=" + nt_num +
	            ", " + "SZ=" + rhs_size + "]");
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Do debug output for shift.
   *
   * @param shift_tkn the token being shifted onto the stack.
   */
  public void debug_shift(token shift_tkn)
    {
      debug_message("# Shift under term #" + shift_tkn.sym +
		    " to state #" + shift_tkn.parse_state);
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Perform a parse with debugging output.  This does exactly the
   *  same things as parse(), except that it calls debug_shift() and
   *  debug_reduce() when shift and reduce moves are taken by the parser
   *  and produces various other debugging messages.
   */
  public void debug_parse()
    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;

      /* set up direct reference to tables to drive the parser */
      production_tab = production_table();
      action_tab     = action_table();
      reduce_tab     = reduce_table();

      debug_message("# Initializing parser");

      /* initialize the action encapsulation object */
      init_actions();

      /* do user initialization */
      user_init();

      /* the current token */
      cur_token = scan();

      debug_message("# Current token is #" + cur_token.sym);

      /* push dummy symbol with start state to get us underway */
      stack.push(new symbol(0, start_state()));
      tos = 0;

      /* continue until we are told to stop */
      for (_done_parsing = false; !_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_token.sym);

	  /* decode the action -- > 0 encodes shift */
	  if (act > 0)
	    {
	      /* shift to the encoded state by pushing it on the stack */
	      cur_token.parse_state = act-1;
	      debug_shift(cur_token);
	      stack.push(cur_token);
	      tos++;

	      /* advance to the next token */
	      cur_token = scan();
              debug_message("# Current token is #" + cur_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];

	      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++;

	      debug_message("# Goto state #" + act);
	    }
	  /* finally if the entry is zero, we have an error */
	  else if (act == 0)
	    {
	      /* call user syntax error reporting routine */
	      syntax_error(cur_token);

	      /* try to error recover */
	      if (!error_recovery(true))
		{
		  /* if that fails give up with a fatal syntax error */
		  unrecovered_syntax_error(cur_token);

		  /* just in case that wasn't fatal enough, end parse */
		  done_parsing();
		}
	    }
	}
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
  /* Error recovery code */
  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Attempt to recover from a syntax error.  This returns false if recovery
   *  fails, true if it succeeds.  Recovery happens in 4 steps.  First we
   *  pop the parse stack down to a point at which we have a shift out
   *  of the top-most state on the error symbol.  This represents the
   *  initial error recovery configuration.  If no such configuration is
   *  found, then we fail.  Next a small number of "lookahead" or "parse
   *  ahead" tokens are read into a buffer.  The size of this buffer is
   *  determined by error_sync_size() and determines how many tokens beyond
   *  the error must be matched to consider the recovery a success.  Next,
   *  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;
	    }
	}
    }

  /*-----------------------------------------------------------*/

};
