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1   /*
2    * Copyright (c) 2003, 2005, Oracle and/or its affiliates. All rights reserved.
3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4    *
5    * This code is free software; you can redistribute it and/or modify it
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7    * published by the Free Software Foundation.  Oracle designates this
8    * particular file as subject to the "Classpath" exception as provided
9    * by Oracle in the LICENSE file that accompanied this code.
10   *
11   * This code is distributed in the hope that it will be useful, but WITHOUT
12   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14   * version 2 for more details (a copy is included in the LICENSE file that
15   * accompanied this code).
16   *
17   * You should have received a copy of the GNU General Public License version
18   * 2 along with this work; if not, write to the Free Software Foundation,
19   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20   *
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25  
26  
27  package com.sun.java_cup.internal.runtime;
28  
29  import java.util.Stack;
30  
31  /** This class implements a skeleton table driven LR parser.  In general,
32   *  LR parsers are a form of bottom up shift-reduce parsers.  Shift-reduce
33   *  parsers act by shifting input onto a parse stack until the Symbols
34   *  matching the right hand side of a production appear on the top of the
35   *  stack.  Once this occurs, a reduce is performed.  This involves removing
36   *  the Symbols corresponding to the right hand side of the production
37   *  (the so called "handle") and replacing them with the non-terminal from
38   *  the left hand side of the production.  <p>
39   *
40   *  To control the decision of whether to shift or reduce at any given point,
41   *  the parser uses a state machine (the "viable prefix recognition machine"
42   *  built by the parser generator).  The current state of the machine is placed
43   *  on top of the parse stack (stored as part of a Symbol object representing
44   *  a terminal or non terminal).  The parse action table is consulted
45   *  (using the current state and the current lookahead Symbol as indexes) to
46   *  determine whether to shift or to reduce.  When the parser shifts, it
47   *  changes to a new state by pushing a new Symbol (containing a new state)
48   *  onto the stack.  When the parser reduces, it pops the handle (right hand
49   *  side of a production) off the stack.  This leaves the parser in the state
50   *  it was in before any of those Symbols were matched.  Next the reduce-goto
51   *  table is consulted (using the new state and current lookahead Symbol as
52   *  indexes) to determine a new state to go to.  The parser then shifts to
53   *  this goto state by pushing the left hand side Symbol of the production
54   *  (also containing the new state) onto the stack.<p>
55   *
56   *  This class actually provides four LR parsers.  The methods parse() and
57   *  debug_parse() provide two versions of the main parser (the only difference
58   *  being that debug_parse() emits debugging trace messages as it parses).
59   *  In addition to these main parsers, the error recovery mechanism uses two
60   *  more.  One of these is used to simulate "parsing ahead" in the input
61   *  without carrying out actions (to verify that a potential error recovery
62   *  has worked), and the other is used to parse through buffered "parse ahead"
63   *  input in order to execute all actions and re-synchronize the actual parser
64   *  configuration.<p>
65   *
66   *  This is an abstract class which is normally filled out by a subclass
67   *  generated by the JavaCup parser generator.  In addition to supplying
68   *  the actual parse tables, generated code also supplies methods which
69   *  invoke various pieces of user supplied code, provide access to certain
70   *  special Symbols (e.g., EOF and error), etc.  Specifically, the following
71   *  abstract methods are normally supplied by generated code:
72   *  <dl compact>
73   *  <dt> short[][] production_table()
74   *  <dd> Provides a reference to the production table (indicating the index of
75   *       the left hand side non terminal and the length of the right hand side
76   *       for each production in the grammar).
77   *  <dt> short[][] action_table()
78   *  <dd> Provides a reference to the parse action table.
79   *  <dt> short[][] reduce_table()
80   *  <dd> Provides a reference to the reduce-goto table.
81   *  <dt> int start_state()
82   *  <dd> Indicates the index of the start state.
83   *  <dt> int start_production()
84   *  <dd> Indicates the index of the starting production.
85   *  <dt> int EOF_sym()
86   *  <dd> Indicates the index of the EOF Symbol.
87   *  <dt> int error_sym()
88   *  <dd> Indicates the index of the error Symbol.
89   *  <dt> Symbol do_action()
90   *  <dd> Executes a piece of user supplied action code.  This always comes at
91   *       the point of a reduce in the parse, so this code also allocates and
92   *       fills in the left hand side non terminal Symbol object that is to be
93   *       pushed onto the stack for the reduce.
94   *  <dt> void init_actions()
95   *  <dd> Code to initialize a special object that encapsulates user supplied
96   *       actions (this object is used by do_action() to actually carry out the
97   *       actions).
98   *  </dl>
99   *
100  *  In addition to these routines that <i>must</i> be supplied by the
101  *  generated subclass there are also a series of routines that <i>may</i>
102  *  be supplied.  These include:
103  *  <dl>
104  *  <dt> Symbol scan()
105  *  <dd> Used to get the next input Symbol from the scanner.
106  *  <dt> Scanner getScanner()
107  *  <dd> Used to provide a scanner for the default implementation of
108  *       scan().
109  *  <dt> int error_sync_size()
110  *  <dd> This determines how many Symbols past the point of an error
111  *       must be parsed without error in order to consider a recovery to
112  *       be valid.  This defaults to 3.  Values less than 2 are not
113  *       recommended.
114  *  <dt> void report_error(String message, Object info)
115  *  <dd> This method is called to report an error.  The default implementation
116  *       simply prints a message to System.err and where the error occurred.
117  *       This method is often replaced in order to provide a more sophisticated
118  *       error reporting mechanism.
119  *  <dt> void report_fatal_error(String message, Object info)
120  *  <dd> This method is called when a fatal error that cannot be recovered from
121  *       is encountered.  In the default implementation, it calls
122  *       report_error() to emit a message, then throws an exception.
123  *  <dt> void syntax_error(Symbol cur_token)
124  *  <dd> This method is called as soon as syntax error is detected (but
125  *       before recovery is attempted).  In the default implementation it
126  *       invokes: report_error("Syntax error", null);
127  *  <dt> void unrecovered_syntax_error(Symbol cur_token)
128  *  <dd> This method is called if syntax error recovery fails.  In the default
129  *       implementation it invokes:<br>
130  *         report_fatal_error("Couldn't repair and continue parse", null);
131  *  </dl>
132  *
133  * @see     com.sun.java_cup.internal.runtime.Symbol
134  * @see     com.sun.java_cup.internal.runtime.Symbol
135  * @see     com.sun.java_cup.internal.runtime.virtual_parse_stack
136  * @author  Frank Flannery
137  */
138 
139 public abstract class lr_parser {
140 
141   /*-----------------------------------------------------------*/
142   /*--- Constructor(s) ----------------------------------------*/
143   /*-----------------------------------------------------------*/
144 
145   /** Simple constructor. */
146   public lr_parser()
147     {
148       /* nothing to do here */
149     }
150 
151   /** Constructor that sets the default scanner. [CSA/davidm] */
152   public lr_parser(Scanner s) {
153     this(); /* in case default constructor someday does something */
154     setScanner(s);
155   }
156 
157   /*-----------------------------------------------------------*/
158   /*--- (Access to) Static (Class) Variables ------------------*/
159   /*-----------------------------------------------------------*/
160 
161   /** The default number of Symbols after an error we much match to consider
162    *  it recovered from.
163    */
164   protected final static int _error_sync_size = 3;
165 
166   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
167 
168   /** The number of Symbols after an error we much match to consider it
169    *  recovered from.
170    */
171   protected int error_sync_size() {return _error_sync_size; }
172 
173   /*-----------------------------------------------------------*/
174   /*--- (Access to) Instance Variables ------------------------*/
175   /*-----------------------------------------------------------*/
176 
177   /** Table of production information (supplied by generated subclass).
178    *  This table contains one entry per production and is indexed by
179    *  the negative-encoded values (reduce actions) in the action_table.
180    *  Each entry has two parts, the index of the non-terminal on the
181    *  left hand side of the production, and the number of Symbols
182    *  on the right hand side.
183    */
184   public abstract short[][] production_table();
185 
186   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
187 
188   /** The action table (supplied by generated subclass).  This table is
189    *  indexed by state and terminal number indicating what action is to
190    *  be taken when the parser is in the given state (i.e., the given state
191    *  is on top of the stack) and the given terminal is next on the input.
192    *  States are indexed using the first dimension, however, the entries for
193    *  a given state are compacted and stored in adjacent index, value pairs
194    *  which are searched for rather than accessed directly (see get_action()).
195    *  The actions stored in the table will be either shifts, reduces, or
196    *  errors.  Shifts are encoded as positive values (one greater than the
197    *  state shifted to).  Reduces are encoded as negative values (one less
198    *  than the production reduced by).  Error entries are denoted by zero.
199    *
200    * @see com.sun.java_cup.internal.runtime.lr_parser#get_action
201    */
202   public abstract short[][] action_table();
203 
204   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
205 
206   /** The reduce-goto table (supplied by generated subclass).  This
207    *  table is indexed by state and non-terminal number and contains
208    *  state numbers.  States are indexed using the first dimension, however,
209    *  the entries for a given state are compacted and stored in adjacent
210    *  index, value pairs which are searched for rather than accessed
211    *  directly (see get_reduce()).  When a reduce occurs, the handle
212    *  (corresponding to the RHS of the matched production) is popped off
213    *  the stack.  The new top of stack indicates a state.  This table is
214    *  then indexed by that state and the LHS of the reducing production to
215    *  indicate where to "shift" to.
216    *
217    * @see com.sun.java_cup.internal.runtime.lr_parser#get_reduce
218    */
219   public abstract short[][] reduce_table();
220 
221   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
222 
223   /** The index of the start state (supplied by generated subclass). */
224   public abstract int start_state();
225 
226   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
227 
228   /** The index of the start production (supplied by generated subclass). */
229   public abstract int start_production();
230 
231   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
232 
233   /** The index of the end of file terminal Symbol (supplied by generated
234    *  subclass).
235    */
236   public abstract int EOF_sym();
237 
238   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
239 
240   /** The index of the special error Symbol (supplied by generated subclass). */
241   public abstract int error_sym();
242 
243   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
244 
245   /** Internal flag to indicate when parser should quit. */
246   protected boolean _done_parsing = false;
247 
248   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
249 
250   /** This method is called to indicate that the parser should quit.  This is
251    *  normally called by an accept action, but can be used to cancel parsing
252    *  early in other circumstances if desired.
253    */
254   public void done_parsing()
255     {
256       _done_parsing = true;
257     }
258 
259   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
260   /* Global parse state shared by parse(), error recovery, and
261    * debugging routines */
262   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
263 
264   /** Indication of the index for top of stack (for use by actions). */
265   protected int tos;
266 
267   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
268 
269   /** The current lookahead Symbol. */
270   protected Symbol cur_token;
271 
272   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
273 
274   /** The parse stack itself. */
275   protected Stack stack = new Stack();
276 
277   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
278 
279   /** Direct reference to the production table. */
280   protected short[][] production_tab;
281 
282   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
283 
284   /** Direct reference to the action table. */
285   protected short[][] action_tab;
286 
287   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
288 
289   /** Direct reference to the reduce-goto table. */
290   protected short[][] reduce_tab;
291 
292   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
293 
294   /** This is the scanner object used by the default implementation
295    *  of scan() to get Symbols.  To avoid name conflicts with existing
296    *  code, this field is private. [CSA/davidm] */
297   private Scanner _scanner;
298 
299   /**
300    * Simple accessor method to set the default scanner.
301    */
302   public void setScanner(Scanner s) { _scanner = s; }
303 
304   /**
305    * Simple accessor method to get the default scanner.
306    */
307   public Scanner getScanner() { return _scanner; }
308 
309   /*-----------------------------------------------------------*/
310   /*--- General Methods ---------------------------------------*/
311   /*-----------------------------------------------------------*/
312 
313   /** Perform a bit of user supplied action code (supplied by generated
314    *  subclass).  Actions are indexed by an internal action number assigned
315    *  at parser generation time.
316    *
317    * @param act_num   the internal index of the action to be performed.
318    * @param parser    the parser object we are acting for.
319    * @param stack     the parse stack of that object.
320    * @param top       the index of the top element of the parse stack.
321    */
322   public abstract Symbol do_action(
323     int       act_num,
324     lr_parser parser,
325     Stack     stack,
326     int       top)
327     throws java.lang.Exception;
328 
329   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
330 
331   /** User code for initialization inside the parser.  Typically this
332    *  initializes the scanner.  This is called before the parser requests
333    *  the first Symbol.  Here this is just a placeholder for subclasses that
334    *  might need this and we perform no action.   This method is normally
335    *  overridden by the generated code using this contents of the "init with"
336    *  clause as its body.
337    */
338   public void user_init() throws java.lang.Exception { }
339 
340   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
341 
342   /** Initialize the action object.  This is called before the parser does
343    *  any parse actions. This is filled in by generated code to create
344    *  an object that encapsulates all action code.
345    */
346   protected abstract void init_actions() throws java.lang.Exception;
347 
348   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
349 
350   /** Get the next Symbol from the input (supplied by generated subclass).
351    *  Once end of file has been reached, all subsequent calls to scan
352    *  should return an EOF Symbol (which is Symbol number 0).  By default
353    *  this method returns getScanner().next_token(); this implementation
354    *  can be overriden by the generated parser using the code declared in
355    *  the "scan with" clause.  Do not recycle objects; every call to
356    *  scan() should return a fresh object.
357    */
358   public Symbol scan() throws java.lang.Exception {
359     return getScanner().next_token();
360   }
361 
362   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
363 
364   /** Report a fatal error.  This method takes a  message string and an
365    *  additional object (to be used by specializations implemented in
366    *  subclasses).  Here in the base class a very simple implementation
367    *  is provided which reports the error then throws an exception.
368    *
369    * @param message an error message.
370    * @param info    an extra object reserved for use by specialized subclasses.
371    */
372   public void report_fatal_error(
373     String   message,
374     Object   info)
375     throws java.lang.Exception
376     {
377       /* stop parsing (not really necessary since we throw an exception, but) */
378       done_parsing();
379 
380       /* use the normal error message reporting to put out the message */
381       report_error(message, info);
382 
383       /* throw an exception */
384       throw new Exception("Can't recover from previous error(s)");
385     }
386 
387   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
388 
389   /** Report a non fatal error (or warning).  This method takes a message
390    *  string and an additional object (to be used by specializations
391    *  implemented in subclasses).  Here in the base class a very simple
392    *  implementation is provided which simply prints the message to
393    *  System.err.
394    *
395    * @param message an error message.
396    * @param info    an extra object reserved for use by specialized subclasses.
397    */
398   public void report_error(String message, Object info)
399     {
400       System.err.print(message);
401       if (info instanceof Symbol)
402         if (((Symbol)info).left != -1)
403         System.err.println(" at character " + ((Symbol)info).left +
404                            " of input");
405         else System.err.println("");
406       else System.err.println("");
407     }
408 
409   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
410 
411   /** This method is called when a syntax error has been detected and recovery
412    *  is about to be invoked.  Here in the base class we just emit a
413    *  "Syntax error" error message.
414    *
415    * @param cur_token the current lookahead Symbol.
416    */
417   public void syntax_error(Symbol cur_token)
418     {
419       report_error("Syntax error", cur_token);
420     }
421 
422   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
423 
424   /** This method is called if it is determined that syntax error recovery
425    *  has been unsuccessful.  Here in the base class we report a fatal error.
426    *
427    * @param cur_token the current lookahead Symbol.
428    */
429   public void unrecovered_syntax_error(Symbol cur_token)
430     throws java.lang.Exception
431     {
432       report_fatal_error("Couldn't repair and continue parse", cur_token);
433     }
434 
435   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
436 
437   /** Fetch an action from the action table.  The table is broken up into
438    *  rows, one per state (rows are indexed directly by state number).
439    *  Within each row, a list of index, value pairs are given (as sequential
440    *  entries in the table), and the list is terminated by a default entry
441    *  (denoted with a Symbol index of -1).  To find the proper entry in a row
442    *  we do a linear or binary search (depending on the size of the row).
443    *
444    * @param state the state index of the action being accessed.
445    * @param sym   the Symbol index of the action being accessed.
446    */
447   protected final short get_action(int state, int sym)
448     {
449       short tag;
450       int first, last, probe;
451       short[] row = action_tab[state];
452 
453       /* linear search if we are < 10 entries */
454       if (row.length < 20)
455         for (probe = 0; probe < row.length; probe++)
456           {
457             /* is this entry labeled with our Symbol or the default? */
458             tag = row[probe++];
459             if (tag == sym || tag == -1)
460               {
461                 /* return the next entry */
462                 return row[probe];
463               }
464           }
465       /* otherwise binary search */
466       else
467         {
468           first = 0;
469           last = (row.length-1)/2 - 1;  /* leave out trailing default entry */
470           while (first <= last)
471             {
472               probe = (first+last)/2;
473               if (sym == row[probe*2])
474                 return row[probe*2+1];
475               else if (sym > row[probe*2])
476                 first = probe+1;
477               else
478                 last = probe-1;
479             }
480 
481           /* not found, use the default at the end */
482           return row[row.length-1];
483         }
484 
485       /* shouldn't happened, but if we run off the end we return the
486          default (error == 0) */
487       return 0;
488     }
489 
490   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
491 
492   /** Fetch a state from the reduce-goto table.  The table is broken up into
493    *  rows, one per state (rows are indexed directly by state number).
494    *  Within each row, a list of index, value pairs are given (as sequential
495    *  entries in the table), and the list is terminated by a default entry
496    *  (denoted with a Symbol index of -1).  To find the proper entry in a row
497    *  we do a linear search.
498    *
499    * @param state the state index of the entry being accessed.
500    * @param sym   the Symbol index of the entry being accessed.
501    */
502   protected final short get_reduce(int state, int sym)
503     {
504       short tag;
505       short[] row = reduce_tab[state];
506 
507       /* if we have a null row we go with the default */
508       if (row == null)
509         return -1;
510 
511       for (int probe = 0; probe < row.length; probe++)
512         {
513           /* is this entry labeled with our Symbol or the default? */
514           tag = row[probe++];
515           if (tag == sym || tag == -1)
516             {
517               /* return the next entry */
518               return row[probe];
519             }
520         }
521       /* if we run off the end we return the default (error == -1) */
522       return -1;
523     }
524 
525   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
526 
527   /** This method provides the main parsing routine.  It returns only when
528    *  done_parsing() has been called (typically because the parser has
529    *  accepted, or a fatal error has been reported).  See the header
530    *  documentation for the class regarding how shift/reduce parsers operate
531    *  and how the various tables are used.
532    */
533   public Symbol parse() throws java.lang.Exception
534     {
535       /* the current action code */
536       int act;
537 
538       /* the Symbol/stack element returned by a reduce */
539       Symbol lhs_sym = null;
540 
541       /* information about production being reduced with */
542       short handle_size, lhs_sym_num;
543 
544       /* set up direct reference to tables to drive the parser */
545 
546       production_tab = production_table();
547       action_tab     = action_table();
548       reduce_tab     = reduce_table();
549 
550       /* initialize the action encapsulation object */
551       init_actions();
552 
553       /* do user initialization */
554       user_init();
555 
556       /* get the first token */
557       cur_token = scan();
558 
559       /* push dummy Symbol with start state to get us underway */
560       stack.removeAllElements();
561       stack.push(new Symbol(0, start_state()));
562       tos = 0;
563 
564       /* continue until we are told to stop */
565       for (_done_parsing = false; !_done_parsing; )
566         {
567           /* Check current token for freshness. */
568           if (cur_token.used_by_parser)
569             throw new Error("Symbol recycling detected (fix your scanner).");
570 
571           /* current state is always on the top of the stack */
572 
573           /* look up action out of the current state with the current input */
574           act = get_action(((Symbol)stack.peek()).parse_state, cur_token.sym);
575 
576           /* decode the action -- > 0 encodes shift */
577           if (act > 0)
578             {
579               /* shift to the encoded state by pushing it on the stack */
580               cur_token.parse_state = act-1;
581               cur_token.used_by_parser = true;
582               stack.push(cur_token);
583               tos++;
584 
585               /* advance to the next Symbol */
586               cur_token = scan();
587             }
588           /* if its less than zero, then it encodes a reduce action */
589           else if (act < 0)
590             {
591               /* perform the action for the reduce */
592               lhs_sym = do_action((-act)-1, this, stack, tos);
593 
594               /* look up information about the production */
595               lhs_sym_num = production_tab[(-act)-1][0];
596               handle_size = production_tab[(-act)-1][1];
597 
598               /* pop the handle off the stack */
599               for (int i = 0; i < handle_size; i++)
600                 {
601                   stack.pop();
602                   tos--;
603                 }
604 
605               /* look up the state to go to from the one popped back to */
606               act = get_reduce(((Symbol)stack.peek()).parse_state, lhs_sym_num);
607 
608               /* shift to that state */
609               lhs_sym.parse_state = act;
610               lhs_sym.used_by_parser = true;
611               stack.push(lhs_sym);
612               tos++;
613             }
614           /* finally if the entry is zero, we have an error */
615           else if (act == 0)
616             {
617               /* call user syntax error reporting routine */
618               syntax_error(cur_token);
619 
620               /* try to error recover */
621               if (!error_recovery(false))
622                 {
623                   /* if that fails give up with a fatal syntax error */
624                   unrecovered_syntax_error(cur_token);
625 
626                   /* just in case that wasn't fatal enough, end parse */
627                   done_parsing();
628                 } else {
629                   lhs_sym = (Symbol)stack.peek();
630                 }
631             }
632         }
633       return lhs_sym;
634     }
635 
636   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
637 
638   /** Write a debugging message to System.err for the debugging version
639    *  of the parser.
640    *
641    * @param mess the text of the debugging message.
642    */
643   public void debug_message(String mess)
644     {
645       System.err.println(mess);
646     }
647 
648   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
649 
650   /** Dump the parse stack for debugging purposes. */
651   public void dump_stack()
652     {
653       if (stack == null)
654         {
655           debug_message("# Stack dump requested, but stack is null");
656           return;
657         }
658 
659       debug_message("============ Parse Stack Dump ============");
660 
661       /* dump the stack */
662       for (int i=0; i<stack.size(); i++)
663         {
664           debug_message("Symbol: " + ((Symbol)stack.elementAt(i)).sym +
665                         " State: " + ((Symbol)stack.elementAt(i)).parse_state);
666         }
667       debug_message("==========================================");
668     }
669 
670   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
671 
672   /** Do debug output for a reduce.
673    *
674    * @param prod_num  the production we are reducing with.
675    * @param nt_num    the index of the LHS non terminal.
676    * @param rhs_size  the size of the RHS.
677    */
678   public void debug_reduce(int prod_num, int nt_num, int rhs_size)
679     {
680       debug_message("# Reduce with prod #" + prod_num + " [NT=" + nt_num +
681                     ", " + "SZ=" + rhs_size + "]");
682     }
683 
684   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
685 
686   /** Do debug output for shift.
687    *
688    * @param shift_tkn the Symbol being shifted onto the stack.
689    */
690   public void debug_shift(Symbol shift_tkn)
691     {
692       debug_message("# Shift under term #" + shift_tkn.sym +
693                     " to state #" + shift_tkn.parse_state);
694     }
695 
696   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
697 
698   /** Do debug output for stack state. [CSA]
699    */
700   public void debug_stack() {
701       StringBuffer sb=new StringBuffer("## STACK:");
702       for (int i=0; i<stack.size(); i++) {
703           Symbol s = (Symbol) stack.elementAt(i);
704           sb.append(" <state "+s.parse_state+", sym "+s.sym+">");
705           if ((i%3)==2 || (i==(stack.size()-1))) {
706               debug_message(sb.toString());
707               sb = new StringBuffer("         ");
708           }
709       }
710   }
711 
712   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
713 
714   /** Perform a parse with debugging output.  This does exactly the
715    *  same things as parse(), except that it calls debug_shift() and
716    *  debug_reduce() when shift and reduce moves are taken by the parser
717    *  and produces various other debugging messages.
718    */
719   public Symbol debug_parse()
720     throws java.lang.Exception
721     {
722       /* the current action code */
723       int act;
724 
725       /* the Symbol/stack element returned by a reduce */
726       Symbol lhs_sym = null;
727 
728       /* information about production being reduced with */
729       short handle_size, lhs_sym_num;
730 
731       /* set up direct reference to tables to drive the parser */
732       production_tab = production_table();
733       action_tab     = action_table();
734       reduce_tab     = reduce_table();
735 
736       debug_message("# Initializing parser");
737 
738       /* initialize the action encapsulation object */
739       init_actions();
740 
741       /* do user initialization */
742       user_init();
743 
744       /* the current Symbol */
745       cur_token = scan();
746 
747       debug_message("# Current Symbol is #" + cur_token.sym);
748 
749       /* push dummy Symbol with start state to get us underway */
750       stack.removeAllElements();
751       stack.push(new Symbol(0, start_state()));
752       tos = 0;
753 
754       /* continue until we are told to stop */
755       for (_done_parsing = false; !_done_parsing; )
756         {
757           /* Check current token for freshness. */
758           if (cur_token.used_by_parser)
759             throw new Error("Symbol recycling detected (fix your scanner).");
760 
761           /* current state is always on the top of the stack */
762           //debug_stack();
763 
764           /* look up action out of the current state with the current input */
765           act = get_action(((Symbol)stack.peek()).parse_state, cur_token.sym);
766 
767           /* decode the action -- > 0 encodes shift */
768           if (act > 0)
769             {
770               /* shift to the encoded state by pushing it on the stack */
771               cur_token.parse_state = act-1;
772               cur_token.used_by_parser = true;
773               debug_shift(cur_token);
774               stack.push(cur_token);
775               tos++;
776 
777               /* advance to the next Symbol */
778               cur_token = scan();
779               debug_message("# Current token is " + cur_token);
780             }
781           /* if its less than zero, then it encodes a reduce action */
782           else if (act < 0)
783             {
784               /* perform the action for the reduce */
785               lhs_sym = do_action((-act)-1, this, stack, tos);
786 
787               /* look up information about the production */
788               lhs_sym_num = production_tab[(-act)-1][0];
789               handle_size = production_tab[(-act)-1][1];
790 
791               debug_reduce((-act)-1, lhs_sym_num, handle_size);
792 
793               /* pop the handle off the stack */
794               for (int i = 0; i < handle_size; i++)
795                 {
796                   stack.pop();
797                   tos--;
798                 }
799 
800               /* look up the state to go to from the one popped back to */
801               act = get_reduce(((Symbol)stack.peek()).parse_state, lhs_sym_num);
802               debug_message("# Reduce rule: top state " +
803                              ((Symbol)stack.peek()).parse_state +
804                              ", lhs sym " + lhs_sym_num + " -> state " + act);
805 
806               /* shift to that state */
807               lhs_sym.parse_state = act;
808               lhs_sym.used_by_parser = true;
809               stack.push(lhs_sym);
810               tos++;
811 
812               debug_message("# Goto state #" + act);
813             }
814           /* finally if the entry is zero, we have an error */
815           else if (act == 0)
816             {
817               /* call user syntax error reporting routine */
818               syntax_error(cur_token);
819 
820               /* try to error recover */
821               if (!error_recovery(true))
822                 {
823                   /* if that fails give up with a fatal syntax error */
824                   unrecovered_syntax_error(cur_token);
825 
826                   /* just in case that wasn't fatal enough, end parse */
827                   done_parsing();
828                 } else {
829                   lhs_sym = (Symbol)stack.peek();
830                 }
831             }
832         }
833       return lhs_sym;
834     }
835 
836   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
837   /* Error recovery code */
838   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
839 
840   /** Attempt to recover from a syntax error.  This returns false if recovery
841    *  fails, true if it succeeds.  Recovery happens in 4 steps.  First we
842    *  pop the parse stack down to a point at which we have a shift out
843    *  of the top-most state on the error Symbol.  This represents the
844    *  initial error recovery configuration.  If no such configuration is
845    *  found, then we fail.  Next a small number of "lookahead" or "parse
846    *  ahead" Symbols are read into a buffer.  The size of this buffer is
847    *  determined by error_sync_size() and determines how many Symbols beyond
848    *  the error must be matched to consider the recovery a success.  Next,
849    *  we begin to discard Symbols in attempt to get past the point of error
850    *  to a point where we can continue parsing.  After each Symbol, we attempt
851    *  to "parse ahead" though the buffered lookahead Symbols.  The "parse ahead"
852    *  process simulates that actual parse, but does not modify the real
853    *  parser's configuration, nor execute any actions. If we can  parse all
854    *  the stored Symbols without error, then the recovery is considered a
855    *  success.  Once a successful recovery point is determined, we do an
856    *  actual parse over the stored input -- modifying the real parse
857    *  configuration and executing all actions.  Finally, we return the the
858    *  normal parser to continue with the overall parse.
859    *
860    * @param debug should we produce debugging messages as we parse.
861    */
862   protected boolean error_recovery(boolean debug)
863     throws java.lang.Exception
864     {
865       if (debug) debug_message("# Attempting error recovery");
866 
867       /* first pop the stack back into a state that can shift on error and
868          do that shift (if that fails, we fail) */
869       if (!find_recovery_config(debug))
870         {
871           if (debug) debug_message("# Error recovery fails");
872           return false;
873         }
874 
875       /* read ahead to create lookahead we can parse multiple times */
876       read_lookahead();
877 
878       /* repeatedly try to parse forward until we make it the required dist */
879       for (;;)
880         {
881           /* try to parse forward, if it makes it, bail out of loop */
882           if (debug) debug_message("# Trying to parse ahead");
883           if (try_parse_ahead(debug))
884             {
885               break;
886             }
887 
888           /* if we are now at EOF, we have failed */
889           if (lookahead[0].sym == EOF_sym())
890             {
891               if (debug) debug_message("# Error recovery fails at EOF");
892               return false;
893             }
894 
895           /* otherwise, we consume another Symbol and try again */
896           if (debug)
897           debug_message("# Consuming Symbol #" + cur_err_token().sym);
898           restart_lookahead();
899         }
900 
901       /* we have consumed to a point where we can parse forward */
902       if (debug) debug_message("# Parse-ahead ok, going back to normal parse");
903 
904       /* do the real parse (including actions) across the lookahead */
905       parse_lookahead(debug);
906 
907       /* we have success */
908       return true;
909     }
910 
911   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
912 
913   /** Determine if we can shift under the special error Symbol out of the
914    *  state currently on the top of the (real) parse stack.
915    */
916   protected boolean shift_under_error()
917     {
918       /* is there a shift under error Symbol */
919       return get_action(((Symbol)stack.peek()).parse_state, error_sym()) > 0;
920     }
921 
922   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
923 
924   /** Put the (real) parse stack into error recovery configuration by
925    *  popping the stack down to a state that can shift on the special
926    *  error Symbol, then doing the shift.  If no suitable state exists on
927    *  the stack we return false
928    *
929    * @param debug should we produce debugging messages as we parse.
930    */
931   protected boolean find_recovery_config(boolean debug)
932     {
933       Symbol error_token;
934       int act;
935 
936       if (debug) debug_message("# Finding recovery state on stack");
937 
938       /* Remember the right-position of the top symbol on the stack */
939       int right_pos = ((Symbol)stack.peek()).right;
940       int left_pos  = ((Symbol)stack.peek()).left;
941 
942       /* pop down until we can shift under error Symbol */
943       while (!shift_under_error())
944         {
945           /* pop the stack */
946           if (debug)
947             debug_message("# Pop stack by one, state was # " +
948                           ((Symbol)stack.peek()).parse_state);
949           left_pos = ((Symbol)stack.pop()).left;
950           tos--;
951 
952           /* if we have hit bottom, we fail */
953           if (stack.empty())
954             {
955               if (debug) debug_message("# No recovery state found on stack");
956               return false;
957             }
958         }
959 
960       /* state on top of the stack can shift under error, find the shift */
961       act = get_action(((Symbol)stack.peek()).parse_state, error_sym());
962       if (debug)
963         {
964           debug_message("# Recover state found (#" +
965                         ((Symbol)stack.peek()).parse_state + ")");
966           debug_message("# Shifting on error to state #" + (act-1));
967         }
968 
969       /* build and shift a special error Symbol */
970       error_token = new Symbol(error_sym(), left_pos, right_pos);
971       error_token.parse_state = act-1;
972       error_token.used_by_parser = true;
973       stack.push(error_token);
974       tos++;
975 
976       return true;
977     }
978 
979   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
980 
981   /** Lookahead Symbols used for attempting error recovery "parse aheads". */
982   protected Symbol lookahead[];
983 
984   /** Position in lookahead input buffer used for "parse ahead". */
985   protected int lookahead_pos;
986 
987   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
988 
989   /** Read from input to establish our buffer of "parse ahead" lookahead
990    *  Symbols.
991    */
992   protected void read_lookahead() throws java.lang.Exception
993     {
994       /* create the lookahead array */
995       lookahead = new Symbol[error_sync_size()];
996 
997       /* fill in the array */
998       for (int i = 0; i < error_sync_size(); i++)
999         {
1000           lookahead[i] = cur_token;
1001           cur_token = scan();
1002         }
1003 
1004       /* start at the beginning */
1005       lookahead_pos = 0;
1006     }
1007 
1008   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1009 
1010   /** Return the current lookahead in our error "parse ahead" buffer. */
1011   protected Symbol cur_err_token() { return lookahead[lookahead_pos]; }
1012 
1013   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1014 
1015   /** Advance to next "parse ahead" input Symbol. Return true if we have
1016    *  input to advance to, false otherwise.
1017    */
1018   protected boolean advance_lookahead()
1019     {
1020       /* advance the input location */
1021       lookahead_pos++;
1022 
1023       /* return true if we didn't go off the end */
1024       return lookahead_pos < error_sync_size();
1025     }
1026 
1027   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1028 
1029   /** Reset the parse ahead input to one Symbol past where we started error
1030    *  recovery (this consumes one new Symbol from the real input).
1031    */
1032   protected void restart_lookahead() throws java.lang.Exception
1033     {
1034       /* move all the existing input over */
1035       for (int i = 1; i < error_sync_size(); i++)
1036         lookahead[i-1] = lookahead[i];
1037 
1038       /* read a new Symbol into the last spot */
1039       cur_token = scan();
1040       lookahead[error_sync_size()-1] = cur_token;
1041 
1042       /* reset our internal position marker */
1043       lookahead_pos = 0;
1044     }
1045 
1046   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1047 
1048   /** Do a simulated parse forward (a "parse ahead") from the current
1049    *  stack configuration using stored lookahead input and a virtual parse
1050    *  stack.  Return true if we make it all the way through the stored
1051    *  lookahead input without error. This basically simulates the action of
1052    *  parse() using only our saved "parse ahead" input, and not executing any
1053    *  actions.
1054    *
1055    * @param debug should we produce debugging messages as we parse.
1056    */
1057   protected boolean try_parse_ahead(boolean debug)
1058     throws java.lang.Exception
1059     {
1060       int act;
1061       short lhs, rhs_size;
1062 
1063       /* create a virtual stack from the real parse stack */
1064       virtual_parse_stack vstack = new virtual_parse_stack(stack);
1065 
1066       /* parse until we fail or get past the lookahead input */
1067       for (;;)
1068         {
1069           /* look up the action from the current state (on top of stack) */
1070           act = get_action(vstack.top(), cur_err_token().sym);
1071 
1072           /* if its an error, we fail */
1073           if (act == 0) return false;
1074 
1075           /* > 0 encodes a shift */
1076           if (act > 0)
1077             {
1078               /* push the new state on the stack */
1079               vstack.push(act-1);
1080 
1081               if (debug) debug_message("# Parse-ahead shifts Symbol #" +
1082                        cur_err_token().sym + " into state #" + (act-1));
1083 
1084               /* advance simulated input, if we run off the end, we are done */
1085               if (!advance_lookahead()) return true;
1086             }
1087           /* < 0 encodes a reduce */
1088           else
1089             {
1090               /* if this is a reduce with the start production we are done */
1091               if ((-act)-1 == start_production())
1092                 {
1093                   if (debug) debug_message("# Parse-ahead accepts");
1094                   return true;
1095                 }
1096 
1097               /* get the lhs Symbol and the rhs size */
1098               lhs = production_tab[(-act)-1][0];
1099               rhs_size = production_tab[(-act)-1][1];
1100 
1101               /* pop handle off the stack */
1102               for (int i = 0; i < rhs_size; i++)
1103                 vstack.pop();
1104 
1105               if (debug)
1106                 debug_message("# Parse-ahead reduces: handle size = " +
1107                   rhs_size + " lhs = #" + lhs + " from state #" + vstack.top());
1108 
1109               /* look up goto and push it onto the stack */
1110               vstack.push(get_reduce(vstack.top(), lhs));
1111               if (debug)
1112                 debug_message("# Goto state #" + vstack.top());
1113             }
1114         }
1115     }
1116 
1117   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1118 
1119   /** Parse forward using stored lookahead Symbols.  In this case we have
1120    *  already verified that parsing will make it through the stored lookahead
1121    *  Symbols and we are now getting back to the point at which we can hand
1122    *  control back to the normal parser.  Consequently, this version of the
1123    *  parser performs all actions and modifies the real parse configuration.
1124    *  This returns once we have consumed all the stored input or we accept.
1125    *
1126    * @param debug should we produce debugging messages as we parse.
1127    */
1128   protected void parse_lookahead(boolean debug)
1129     throws java.lang.Exception
1130     {
1131       /* the current action code */
1132       int act;
1133 
1134       /* the Symbol/stack element returned by a reduce */
1135       Symbol lhs_sym = null;
1136 
1137       /* information about production being reduced with */
1138       short handle_size, lhs_sym_num;
1139 
1140       /* restart the saved input at the beginning */
1141       lookahead_pos = 0;
1142 
1143       if (debug)
1144         {
1145           debug_message("# Reparsing saved input with actions");
1146           debug_message("# Current Symbol is #" + cur_err_token().sym);
1147           debug_message("# Current state is #" +
1148                         ((Symbol)stack.peek()).parse_state);
1149         }
1150 
1151       /* continue until we accept or have read all lookahead input */
1152       while(!_done_parsing)
1153         {
1154           /* current state is always on the top of the stack */
1155 
1156           /* look up action out of the current state with the current input */
1157           act =
1158             get_action(((Symbol)stack.peek()).parse_state, cur_err_token().sym);
1159 
1160           /* decode the action -- > 0 encodes shift */
1161           if (act > 0)
1162             {
1163               /* shift to the encoded state by pushing it on the stack */
1164               cur_err_token().parse_state = act-1;
1165               cur_err_token().used_by_parser = true;
1166               if (debug) debug_shift(cur_err_token());
1167               stack.push(cur_err_token());
1168               tos++;
1169 
1170               /* advance to the next Symbol, if there is none, we are done */
1171               if (!advance_lookahead())
1172                 {
1173                   if (debug) debug_message("# Completed reparse");
1174 
1175                   /* scan next Symbol so we can continue parse */
1176                   // BUGFIX by Chris Harris <ckharris@ucsd.edu>:
1177                   //   correct a one-off error by commenting out
1178                   //   this next line.
1179                   /*cur_token = scan();*/
1180 
1181                   /* go back to normal parser */
1182                   return;
1183                 }
1184 
1185               if (debug)
1186                 debug_message("# Current Symbol is #" + cur_err_token().sym);
1187             }
1188           /* if its less than zero, then it encodes a reduce action */
1189           else if (act < 0)
1190             {
1191               /* perform the action for the reduce */
1192               lhs_sym = do_action((-act)-1, this, stack, tos);
1193 
1194               /* look up information about the production */
1195               lhs_sym_num = production_tab[(-act)-1][0];
1196               handle_size = production_tab[(-act)-1][1];
1197 
1198               if (debug) debug_reduce((-act)-1, lhs_sym_num, handle_size);
1199 
1200               /* pop the handle off the stack */
1201               for (int i = 0; i < handle_size; i++)
1202                 {
1203                   stack.pop();
1204                   tos--;
1205                 }
1206 
1207               /* look up the state to go to from the one popped back to */
1208               act = get_reduce(((Symbol)stack.peek()).parse_state, lhs_sym_num);
1209 
1210               /* shift to that state */
1211               lhs_sym.parse_state = act;
1212               lhs_sym.used_by_parser = true;
1213               stack.push(lhs_sym);
1214               tos++;
1215 
1216               if (debug) debug_message("# Goto state #" + act);
1217 
1218             }
1219           /* finally if the entry is zero, we have an error
1220              (shouldn't happen here, but...)*/
1221           else if (act == 0)
1222             {
1223               report_fatal_error("Syntax error", lhs_sym);
1224               return;
1225             }
1226         }
1227 
1228 
1229     }
1230 
1231   /*-----------------------------------------------------------*/
1232 
1233   /** Utility function: unpacks parse tables from strings */
1234   protected static short[][] unpackFromStrings(String[] sa)
1235     {
1236       // Concatanate initialization strings.
1237       StringBuffer sb = new StringBuffer(sa[0]);
1238       for (int i=1; i<sa.length; i++)
1239         sb.append(sa[i]);
1240       int n=0; // location in initialization string
1241       int size1 = (((int)sb.charAt(n))<<16) | ((int)sb.charAt(n+1)); n+=2;
1242       short[][] result = new short[size1][];
1243       for (int i=0; i<size1; i++) {
1244         int size2 = (((int)sb.charAt(n))<<16) | ((int)sb.charAt(n+1)); n+=2;
1245         result[i] = new short[size2];
1246         for (int j=0; j<size2; j++)
1247           result[i][j] = (short) (sb.charAt(n++)-2);
1248       }
1249       return result;
1250     }
1251 }