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lr_parser.java

                            
package java_cup.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 Symbol 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 Symbol 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).
 *  </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> Symbol scan()
 *  <dd> Used to get the next input Symbol from the scanner.
 *  <dt> Scanner getScanner()
 *  <dd> Used to provide a scanner for the default implementation of
 *       scan().
 *  <dt> int error_sync_size()
 *  <dd> This determines how many Symbols 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 where the error occurred.
 *       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(Symbol 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(Symbol 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     java_cup.runtime.Symbol
 * @see     java_cup.runtime.Symbol
 * @see     java_cup.runtime.virtual_parse_stack
 * @version last updated: 7/3/96
 * @author  Frank Flannery
 */

00115 public abstract class lr_parser {

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

  /** Simple constructor. */
00122   public lr_parser()
    {
      /* nothing to do here */
    }

  /** Constructor that sets the default scanner. [CSA/davidm] */
00128   public lr_parser(Scanner s) {
    this(); /* in case default constructor someday does something */
    setScanner(s);
  }

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

  /** The default number of Symbols after an error we much match to consider 
   *  it recovered from. 
   */
00140   protected final static int _error_sync_size = 3;

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

  /** The number of Symbols after an error we much match to consider it 
   *  recovered from. 
   */
00147   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 java_cup.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 java_cup.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. */
00222   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. 
   */
00230   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). */
00241   protected int tos;

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

  /** The current lookahead Symbol. */
00246   protected Symbol cur_token;

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

  /** The parse stack itself. */
00251   protected Stack stack = new Stack();

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

  /** Direct reference to the production table. */ 
00256   protected short[][] production_tab;

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

  /** Direct reference to the action table. */
00261   protected short[][] action_tab;

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

  /** Direct reference to the reduce-goto table. */
00266   protected short[][] reduce_tab;

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

  /** This is the scanner object used by the default implementation
   *  of scan() to get Symbols.  To avoid name conflicts with existing
   *  code, this field is private. [CSA/davidm] */
00273   private Scanner _scanner;

  /**
   * Simple accessor method to set the default scanner.
   */
00278   public void setScanner(Scanner s) { _scanner = s; }

  /**
   * Simple accessor method to get the default scanner.
   */
00283   public Scanner getScanner() { return _scanner; }

  /*-----------------------------------------------------------*/
  /*--- 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 Symbol.  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.
   */
00314   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 Symbol from the input (supplied by generated subclass).
   *  Once end of file has been reached, all subsequent calls to scan 
   *  should return an EOF Symbol (which is Symbol number 0).  By default
   *  this method returns getScanner().next_token(); this implementation
   *  can be overriden by the generated parser using the code declared in
   *  the "scan with" clause.  Do not recycle objects; every call to
   *  scan() should return a fresh object.
   */
00334   public Symbol scan() throws java.lang.Exception {
    Symbol sym = getScanner().next_token();
    return (sym!=null) ? sym : new Symbol(EOF_sym());
  }

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

  /** 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.
   */
00349   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.
   */
00375   public void report_error(String message, Object info)
    {
      System.err.print(message);
      if (info instanceof Symbol)
      if (((Symbol)info).left != -1)
      System.err.println(" at character " + ((Symbol)info).left + 
                     " of input");
      else System.err.println("");
      else System.err.println("");
    }

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

  /** 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 Symbol.
   */
00394   public void syntax_error(Symbol cur_token)
    {
      report_error("Syntax error", cur_token);
    }

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

  /** 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 Symbol.
   */
00406   public void unrecovered_syntax_error(Symbol cur_token)
    throws java.lang.Exception
    {
      report_fatal_error("Couldn't repair and continue parse", cur_token);
    }

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

  /** 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.
   */
00424   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.
   */
00479   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.
   */
00510   public Symbol parse() throws java.lang.Exception
    {
      /* the current action code */
      int act;

      /* the Symbol/stack element returned by a reduce */
      Symbol lhs_sym = null;

      /* 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.removeAllElements();
      stack.push(new Symbol(0, start_state()));
      tos = 0;

      /* continue until we are told to stop */
      for (_done_parsing = false; !_done_parsing; )
      {
        /* Check current token for freshness. */
        if (cur_token.used_by_parser)
          throw new Error("Symbol recycling detected (fix your scanner).");

        /* 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;
            cur_token.used_by_parser = true;
            stack.push(cur_token);
            tos++;

            /* advance to the next Symbol */
            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;
            lhs_sym.used_by_parser = true;
            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();
            } else {
              lhs_sym = (Symbol)stack.peek();
            }
          }
      }
      return lhs_sym;
    }

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

  /** Write a debugging message to System.err for the debugging version 
   *  of the parser. 
   *
   * @param mess the text of the debugging message.
   */
00620   public void debug_message(String mess)
    {
      System.err.println(mess);
    }

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

  /** Dump the parse stack for debugging purposes. */
00628   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.
   */
00655   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 Symbol being shifted onto the stack.
   */
00667   public void debug_shift(Symbol shift_tkn)
    {
      debug_message("# Shift under term #" + shift_tkn.sym + 
                " to state #" + shift_tkn.parse_state);
    }

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

  /** Do debug output for stack state. [CSA]
   */
00677   public void debug_stack() {
      StringBuffer sb=new StringBuffer("## STACK:");
      for (int i=0; i<stack.size(); i++) {
        Symbol s = (Symbol) stack.elementAt(i);
        sb.append(" <state "+s.parse_state+", sym "+s.sym+">");
        if ((i%3)==2 || (i==(stack.size()-1))) {
            debug_message(sb.toString());
            sb = new StringBuffer("         ");
        }
      }
  }

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

  /** 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.  
   */
00696   public Symbol debug_parse()
    throws java.lang.Exception
    {
      /* the current action code */
      int act;

      /* the Symbol/stack element returned by a reduce */
      Symbol lhs_sym = null;

      /* 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 Symbol */
      cur_token = scan(); 

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

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

      /* continue until we are told to stop */
      for (_done_parsing = false; !_done_parsing; )
      {
        /* Check current token for freshness. */
        if (cur_token.used_by_parser)
          throw new Error("Symbol recycling detected (fix your scanner).");

        /* current state is always on the top of the stack */
        //debug_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;
            cur_token.used_by_parser = true;
            debug_shift(cur_token);
            stack.push(cur_token);
            tos++;

            /* advance to the next Symbol */
            cur_token = scan();
              debug_message("# Current token is " + cur_token);
          }
        /* 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);
            debug_message("# Reduce rule: top state " +
                       ((Symbol)stack.peek()).parse_state +
                       ", lhs sym " + lhs_sym_num + " -> state " + act); 

            /* shift to that state */
            lhs_sym.parse_state = act;
            lhs_sym.used_by_parser = true;
            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();
            } else {
              lhs_sym = (Symbol)stack.peek();
            }
          }
      }
      return lhs_sym;
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
  /* 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" Symbols are read into a buffer.  The size of this buffer is 
   *  determined by error_sync_size() and determines how many Symbols beyond
   *  the error must be matched to consider the recovery a success.  Next, 
   *  we begin to discard Symbols in attempt to get past the point of error
   *  to a point where we can continue parsing.  After each Symbol, we attempt 
   *  to "parse ahead" though the buffered lookahead Symbols.  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 Symbols 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.
   */
00839   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 Symbol and try again */
        // BUG FIX by Bruce Hutton
        // Computer Science Department, University of Auckland,
        // Auckland, New Zealand.
        // It is the first token that is being consumed, not the one 
        // we were up to parsing
        if (debug) 
            debug_message("# Consuming Symbol #" + lookahead[ 0 ].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. 
   */
00898   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.
   */
00913   protected boolean find_recovery_config(boolean debug)
    {
      Symbol error_token;
      int act;

      if (debug) debug_message("# Finding recovery state on stack");

      /* Remember the right-position of the top symbol on the stack */
      int right_pos = ((Symbol)stack.peek()).right;
      int left_pos  = ((Symbol)stack.peek()).left;

      /* pop down until we can shift under error Symbol */
      while (!shift_under_error())
      {
        /* pop the stack */
        if (debug) 
          debug_message("# Pop stack by one, state was # " +
                        ((Symbol)stack.peek()).parse_state);
          left_pos = ((Symbol)stack.pop()).left;      
        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 Symbol */
      error_token = new Symbol(error_sym(), left_pos, right_pos);
      error_token.parse_state = act-1;
      error_token.used_by_parser = true;
      stack.push(error_token);
      tos++;

      return true;
    }

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

  /** Lookahead Symbols used for attempting error recovery "parse aheads". */
00964   protected Symbol lookahead[];

  /** Position in lookahead input buffer used for "parse ahead". */
00967   protected int lookahead_pos;

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

  /** Read from input to establish our buffer of "parse ahead" lookahead 
   *  Symbols. 
   */
00974   protected void read_lookahead() throws java.lang.Exception
    {
      /* create the lookahead array */
      lookahead = new Symbol[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. */
00993   protected Symbol 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. 
   */
01000   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 Symbol past where we started error 
   *  recovery (this consumes one new Symbol from the real input). 
   */
01014   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 Symbol into the last spot */
      // BUG Fix by Bruce Hutton
      // Computer Science Department, University of Auckland,
      // Auckland, New Zealand. [applied 5-sep-1999 by csa]
      // The following two lines were out of order!!
      lookahead[error_sync_size()-1] = cur_token;
      cur_token = scan();

      /* 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.
   */
01043   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 Symbol #" + 
                   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.
   */
01114   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 = null;

      /* 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 Symbol 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;
            cur_err_token().used_by_parser = true;
            if (debug) debug_shift(cur_err_token());
            stack.push(cur_err_token());
            tos++;

            /* advance to the next Symbol, if there is none, we are done */
            if (!advance_lookahead()) 
            {
              if (debug) debug_message("# Completed reparse");

              /* scan next Symbol so we can continue parse */
              // BUGFIX by Chris Harris <ckharris@ucsd.edu>:
              //   correct a one-off error by commenting out
              //   this next line.
              /*cur_token = scan();*/

              /* go back to normal parser */
              return;
            }
            
            if (debug) 
            debug_message("# Current Symbol 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;
            lhs_sym.used_by_parser = true;
            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", lhs_sym);
            return;
          }
      }

      
    }

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

  /** Utility function: unpacks parse tables from strings */
01220   protected static short[][] unpackFromStrings(String[] sa)
    {
      // Concatanate initialization strings.
      StringBuffer sb = new StringBuffer(sa[0]);
      for (int i=1; i<sa.length; i++)
      sb.append(sa[i]);
      int n=0; // location in initialization string
      int size1 = (((int)sb.charAt(n))<<16) | ((int)sb.charAt(n+1)); n+=2;
      short[][] result = new short[size1][];
      for (int i=0; i<size1; i++) {
        int size2 = (((int)sb.charAt(n))<<16) | ((int)sb.charAt(n+1)); n+=2;
        result[i] = new short[size2];
        for (int j=0; j<size2; j++)
          result[i][j] = (short) (sb.charAt(n++)-2);
      }
      return result;
    }
}


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