Java > Open Source Codes > java_cup > runtime > lr_parser


1                     
2 package java_cup.runtime;
3
4 import java.util.Stack ;
5
6 /** This class implements a skeleton table driven LR parser. In general,
7  * LR parsers are a form of bottom up shift-reduce parsers. Shift-reduce
8  * parsers act by shifting input onto a parse stack until the Symbols
9  * matching the right hand side of a production appear on the top of the
10  * stack. Once this occurs, a reduce is performed. This involves removing
11  * the Symbols corresponding to the right hand side of the production
12  * (the so called "handle") and replacing them with the non-terminal from
13  * the left hand side of the production. <p>
14  *
15  * To control the decision of whether to shift or reduce at any given point,
16  * the parser uses a state machine (the "viable prefix recognition machine"
17  * built by the parser generator). The current state of the machine is placed
18  * on top of the parse stack (stored as part of a Symbol object representing
19  * a terminal or non terminal). The parse action table is consulted
20  * (using the current state and the current lookahead Symbol as indexes) to
21  * determine whether to shift or to reduce. When the parser shifts, it
22  * changes to a new state by pushing a new Symbol (containing a new state)
23  * onto the stack. When the parser reduces, it pops the handle (right hand
24  * side of a production) off the stack. This leaves the parser in the state
25  * it was in before any of those Symbols were matched. Next the reduce-goto
26  * table is consulted (using the new state and current lookahead Symbol as
27  * indexes) to determine a new state to go to. The parser then shifts to
28  * this goto state by pushing the left hand side Symbol of the production
29  * (also containing the new state) onto the stack.<p>
30  *
31  * This class actually provides four LR parsers. The methods parse() and
32  * debug_parse() provide two versions of the main parser (the only difference
33  * being that debug_parse() emits debugging trace messages as it parses).
34  * In addition to these main parsers, the error recovery mechanism uses two
35  * more. One of these is used to simulate "parsing ahead" in the input
36  * without carrying out actions (to verify that a potential error recovery
37  * has worked), and the other is used to parse through buffered "parse ahead"
38  * input in order to execute all actions and re-synchronize the actual parser
39  * configuration.<p>
40  *
41  * This is an abstract class which is normally filled out by a subclass
42  * generated by the JavaCup parser generator. In addition to supplying
43  * the actual parse tables, generated code also supplies methods which
44  * invoke various pieces of user supplied code, provide access to certain
45  * special Symbols (e.g., EOF and error), etc. Specifically, the following
46  * abstract methods are normally supplied by generated code:
47  * <dl compact>
48  * <dt> short[][] production_table()
49  * <dd> Provides a reference to the production table (indicating the index of
50  * the left hand side non terminal and the length of the right hand side
51  * for each production in the grammar).
52  * <dt> short[][] action_table()
53  * <dd> Provides a reference to the parse action table.
54  * <dt> short[][] reduce_table()
55  * <dd> Provides a reference to the reduce-goto table.
56  * <dt> int start_state()
57  * <dd> Indicates the index of the start state.
58  * <dt> int start_production()
59  * <dd> Indicates the index of the starting production.
60  * <dt> int EOF_sym()
61  * <dd> Indicates the index of the EOF Symbol.
62  * <dt> int error_sym()
63  * <dd> Indicates the index of the error Symbol.
64  * <dt> Symbol do_action()
65  * <dd> Executes a piece of user supplied action code. This always comes at
66  * the point of a reduce in the parse, so this code also allocates and
67  * fills in the left hand side non terminal Symbol object that is to be
68  * pushed onto the stack for the reduce.
69  * <dt> void init_actions()
70  * <dd> Code to initialize a special object that encapsulates user supplied
71  * actions (this object is used by do_action() to actually carry out the
72  * actions).
73  * </dl>
74  *
75  * In addition to these routines that <i>must</i> be supplied by the
76  * generated subclass there are also a series of routines that <i>may</i>
77  * be supplied. These include:
78  * <dl>
79  * <dt> Symbol scan()
80  * <dd> Used to get the next input Symbol from the scanner.
81  * <dt> Scanner getScanner()
82  * <dd> Used to provide a scanner for the default implementation of
83  * scan().
84  * <dt> int error_sync_size()
85  * <dd> This determines how many Symbols past the point of an error
86  * must be parsed without error in order to consider a recovery to
87  * be valid. This defaults to 3. Values less than 2 are not
88  * recommended.
89  * <dt> void report_error(String message, Object info)
90  * <dd> This method is called to report an error. The default implementation
91  * simply prints a message to System.err and where the error occurred.
92  * This method is often replaced in order to provide a more sophisticated
93  * error reporting mechanism.
94  * <dt> void report_fatal_error(String message, Object info)
95  * <dd> This method is called when a fatal error that cannot be recovered from
96  * is encountered. In the default implementation, it calls
97  * report_error() to emit a message, then throws an exception.
98  * <dt> void syntax_error(Symbol cur_token)
99  * <dd> This method is called as soon as syntax error is detected (but
100  * before recovery is attempted). In the default implementation it
101  * invokes: report_error("Syntax error", null);
102  * <dt> void unrecovered_syntax_error(Symbol cur_token)
103  * <dd> This method is called if syntax error recovery fails. In the default
104  * implementation it invokes:<br>
105  * report_fatal_error("Couldn't repair and continue parse", null);
106  * </dl>
107  *
108  * @see java_cup.runtime.Symbol
109  * @see java_cup.runtime.Symbol
110  * @see java_cup.runtime.virtual_parse_stack
111  * @version last updated: 7/3/96
112  * @author Frank Flannery
113  */
114
115 public abstract class lr_parser {
116
117   /*-----------------------------------------------------------*/
118   /*--- Constructor(s) ----------------------------------------*/
119   /*-----------------------------------------------------------*/
120
121   /** Simple constructor. */
122   public lr_parser()
123     {
124       /* nothing to do here */
125     }
126
127   /** Constructor that sets the default scanner. [CSA/davidm] */
128   public lr_parser(Scanner s) {
129     this(); /* in case default constructor someday does something */
130     setScanner(s);
131   }
132
133   /*-----------------------------------------------------------*/
134   /*--- (Access to) Static (Class) Variables ------------------*/
135   /*-----------------------------------------------------------*/
136
137   /** The default number of Symbols after an error we much match to consider
138    * it recovered from.
139    */
140   protected final static int _error_sync_size = 3;
141
142   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
143
144   /** The number of Symbols after an error we much match to consider it
145    * recovered from.
146    */
147   protected int error_sync_size() {return _error_sync_size; }
148
149   /*-----------------------------------------------------------*/
150   /*--- (Access to) Instance Variables ------------------------*/
151   /*-----------------------------------------------------------*/
152
153   /** Table of production information (supplied by generated subclass).
154    * This table contains one entry per production and is indexed by
155    * the negative-encoded values (reduce actions) in the action_table.
156    * Each entry has two parts, the index of the non-terminal on the
157    * left hand side of the production, and the number of Symbols
158    * on the right hand side.
159    */
160   public abstract short[][] production_table();
161
162   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
163
164   /** The action table (supplied by generated subclass). This table is
165    * indexed by state and terminal number indicating what action is to
166    * be taken when the parser is in the given state (i.e., the given state
167    * is on top of the stack) and the given terminal is next on the input.
168    * States are indexed using the first dimension, however, the entries for
169    * a given state are compacted and stored in adjacent index, value pairs
170    * which are searched for rather than accessed directly (see get_action()).
171    * The actions stored in the table will be either shifts, reduces, or
172    * errors. Shifts are encoded as positive values (one greater than the
173    * state shifted to). Reduces are encoded as negative values (one less
174    * than the production reduced by). Error entries are denoted by zero.
175    *
176    * @see java_cup.runtime.lr_parser#get_action
177    */
178   public abstract short[][] action_table();
179
180   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
181
182   /** The reduce-goto table (supplied by generated subclass). This
183    * table is indexed by state and non-terminal number and contains
184    * state numbers. States are indexed using the first dimension, however,
185    * the entries for a given state are compacted and stored in adjacent
186    * index, value pairs which are searched for rather than accessed
187    * directly (see get_reduce()). When a reduce occurs, the handle
188    * (corresponding to the RHS of the matched production) is popped off
189    * the stack. The new top of stack indicates a state. This table is
190    * then indexed by that state and the LHS of the reducing production to
191    * indicate where to "shift" to.
192    *
193    * @see java_cup.runtime.lr_parser#get_reduce
194    */
195   public abstract short[][] reduce_table();
196
197   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
198
199   /** The index of the start state (supplied by generated subclass). */
200   public abstract int start_state();
201
202   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
203
204   /** The index of the start production (supplied by generated subclass). */
205   public abstract int start_production();
206
207   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
208
209   /** The index of the end of file terminal Symbol (supplied by generated
210    * subclass).
211    */
212   public abstract int EOF_sym();
213
214   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
215
216   /** The index of the special error Symbol (supplied by generated subclass). */
217   public abstract int error_sym();
218
219   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
220
221   /** Internal flag to indicate when parser should quit. */
222   protected boolean _done_parsing = false;
223
224   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
225
226   /** This method is called to indicate that the parser should quit. This is
227    * normally called by an accept action, but can be used to cancel parsing
228    * early in other circumstances if desired.
229    */
230   public void done_parsing()
231     {
232       _done_parsing = true;
233     }
234
235   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
236   /* Global parse state shared by parse(), error recovery, and
237    * debugging routines */
238   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
239
240   /** Indication of the index for top of stack (for use by actions). */
241   protected int tos;
242
243   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
244
245   /** The current lookahead Symbol. */
246   protected Symbol cur_token;
247
248   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
249
250   /** The parse stack itself. */
251   protected Stack stack = new Stack ();
252
253   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
254
255   /** Direct reference to the production table. */
256   protected short[][] production_tab;
257
258   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
259
260   /** Direct reference to the action table. */
261   protected short[][] action_tab;
262
263   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
264
265   /** Direct reference to the reduce-goto table. */
266   protected short[][] reduce_tab;
267
268   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
269
270   /** This is the scanner object used by the default implementation
271    * of scan() to get Symbols. To avoid name conflicts with existing
272    * code, this field is private. [CSA/davidm] */
273   private Scanner _scanner;
274
275   /**
276    * Simple accessor method to set the default scanner.
277    */
278   public void setScanner(Scanner s) { _scanner = s; }
279
280   /**
281    * Simple accessor method to get the default scanner.
282    */
283   public Scanner getScanner() { return _scanner; }
284
285   /*-----------------------------------------------------------*/
286   /*--- General Methods ---------------------------------------*/
287   /*-----------------------------------------------------------*/
288
289   /** Perform a bit of user supplied action code (supplied by generated
290    * subclass). Actions are indexed by an internal action number assigned
291    * at parser generation time.
292    *
293    * @param act_num the internal index of the action to be performed.
294    * @param parser the parser object we are acting for.
295    * @param stack the parse stack of that object.
296    * @param top the index of the top element of the parse stack.
297    */
298   public abstract Symbol do_action(
299     int act_num,
300     lr_parser parser,
301     Stack stack,
302     int top)
303     throws java.lang.Exception ;
304
305   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
306
307   /** User code for initialization inside the parser. Typically this
308    * initializes the scanner. This is called before the parser requests
309    * the first Symbol. Here this is just a placeholder for subclasses that
310    * might need this and we perform no action. This method is normally
311    * overridden by the generated code using this contents of the "init with"
312    * clause as its body.
313    */
314   public void user_init() throws java.lang.Exception { }
315
316   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
317
318   /** Initialize the action object. This is called before the parser does
319    * any parse actions. This is filled in by generated code to create
320    * an object that encapsulates all action code.
321    */
322   protected abstract void init_actions() throws java.lang.Exception ;
323
324   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
325
326   /** Get the next Symbol from the input (supplied by generated subclass).
327    * Once end of file has been reached, all subsequent calls to scan
328    * should return an EOF Symbol (which is Symbol number 0). By default
329    * this method returns getScanner().next_token(); this implementation
330    * can be overriden by the generated parser using the code declared in
331    * the "scan with" clause. Do not recycle objects; every call to
332    * scan() should return a fresh object.
333    */
334   public Symbol scan() throws java.lang.Exception {
335     Symbol sym = getScanner().next_token();
336     return (sym!=null) ? sym : new Symbol(EOF_sym());
337   }
338
339   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
340
341   /** Report a fatal error. This method takes a message string and an
342    * additional object (to be used by specializations implemented in
343    * subclasses). Here in the base class a very simple implementation
344    * is provided which reports the error then throws an exception.
345    *
346    * @param message an error message.
347    * @param info an extra object reserved for use by specialized subclasses.
348    */
349   public void report_fatal_error(
350     String message,
351     Object info)
352     throws java.lang.Exception
353     {
354       /* stop parsing (not really necessary since we throw an exception, but) */
355       done_parsing();
356
357       /* use the normal error message reporting to put out the message */
358       report_error(message, info);
359
360       /* throw an exception */
361       throw new Exception ("Can't recover from previous error(s)");
362     }
363
364   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
365
366   /** Report a non fatal error (or warning). This method takes a message
367    * string and an additional object (to be used by specializations
368    * implemented in subclasses). Here in the base class a very simple
369    * implementation is provided which simply prints the message to
370    * System.err.
371    *
372    * @param message an error message.
373    * @param info an extra object reserved for use by specialized subclasses.
374    */
375   public void report_error(String message, Object info)
376     {
377       System.err.print(message);
378       if (info instanceof Symbol)
379     if (((Symbol)info).left != -1)
380     System.err.println(" at character " + ((Symbol)info).left +
381                " of input");
382     else System.err.println("");
383       else System.err.println("");
384     }
385
386   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
387
388   /** This method is called when a syntax error has been detected and recovery
389    * is about to be invoked. Here in the base class we just emit a
390    * "Syntax error" error message.
391    *
392    * @param cur_token the current lookahead Symbol.
393    */
394   public void syntax_error(Symbol cur_token)
395     {
396       report_error("Syntax error", cur_token);
397     }
398
399   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
400
401   /** This method is called if it is determined that syntax error recovery
402    * has been unsuccessful. Here in the base class we report a fatal error.
403    *
404    * @param cur_token the current lookahead Symbol.
405    */
406   public void unrecovered_syntax_error(Symbol cur_token)
407     throws java.lang.Exception
408     {
409       report_fatal_error("Couldn't repair and continue parse", cur_token);
410     }
411
412   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
413
414   /** Fetch an action from the action table. The table is broken up into
415    * rows, one per state (rows are indexed directly by state number).
416    * Within each row, a list of index, value pairs are given (as sequential
417    * entries in the table), and the list is terminated by a default entry
418    * (denoted with a Symbol index of -1). To find the proper entry in a row
419    * we do a linear or binary search (depending on the size of the row).
420    *
421    * @param state the state index of the action being accessed.
422    * @param sym the Symbol index of the action being accessed.
423    */
424   protected final short get_action(int state, int sym)
425     {
426       short tag;
427       int first, last, probe;
428       short[] row = action_tab[state];
429
430       /* linear search if we are < 10 entries */
431       if (row.length < 20)
432         for (probe = 0; probe < row.length; probe++)
433       {
434         /* is this entry labeled with our Symbol or the default? */
435         tag = row[probe++];
436         if (tag == sym || tag == -1)
437           {
438             /* return the next entry */
439             return row[probe];
440           }
441       }
442       /* otherwise binary search */
443       else
444     {
445       first = 0;
446       last = (row.length-1)/2 - 1; /* leave out trailing default entry */
447       while (first <= last)
448         {
449           probe = (first+last)/2;
450           if (sym == row[probe*2])
451         return row[probe*2+1];
452           else if (sym > row[probe*2])
453         first = probe+1;
454           else
455             last = probe-1;
456         }
457
458       /* not found, use the default at the end */
459       return row[row.length-1];
460     }
461
462       /* shouldn't happened, but if we run off the end we return the
463      default (error == 0) */
464       return 0;
465     }
466
467   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
468
469   /** Fetch a state from the reduce-goto table. The table is broken up into
470    * rows, one per state (rows are indexed directly by state number).
471    * Within each row, a list of index, value pairs are given (as sequential
472    * entries in the table), and the list is terminated by a default entry
473    * (denoted with a Symbol index of -1). To find the proper entry in a row
474    * we do a linear search.
475    *
476    * @param state the state index of the entry being accessed.
477    * @param sym the Symbol index of the entry being accessed.
478    */
479   protected final short get_reduce(int state, int sym)
480     {
481       short tag;
482       short[] row = reduce_tab[state];
483
484       /* if we have a null row we go with the default */
485       if (row == null)
486         return -1;
487
488       for (int probe = 0; probe < row.length; probe++)
489     {
490       /* is this entry labeled with our Symbol or the default? */
491       tag = row[probe++];
492       if (tag == sym || tag == -1)
493         {
494           /* return the next entry */
495           return row[probe];
496         }
497     }
498       /* if we run off the end we return the default (error == -1) */
499       return -1;
500     }
501
502   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
503
504   /** This method provides the main parsing routine. It returns only when
505    * done_parsing() has been called (typically because the parser has
506    * accepted, or a fatal error has been reported). See the header
507    * documentation for the class regarding how shift/reduce parsers operate
508    * and how the various tables are used.
509    */
510   public Symbol parse() throws java.lang.Exception
511     {
512       /* the current action code */
513       int act;
514
515       /* the Symbol/stack element returned by a reduce */
516       Symbol lhs_sym = null;
517
518       /* information about production being reduced with */
519       short handle_size, lhs_sym_num;
520
521       /* set up direct reference to tables to drive the parser */
522
523       production_tab = production_table();
524       action_tab = action_table();
525       reduce_tab = reduce_table();
526
527       /* initialize the action encapsulation object */
528       init_actions();
529
530       /* do user initialization */
531       user_init();
532
533       /* get the first token */
534       cur_token = scan();
535
536       /* push dummy Symbol with start state to get us underway */
537       stack.removeAllElements();
538       stack.push(new Symbol(0, start_state()));
539       tos = 0;
540
541       /* continue until we are told to stop */
542       for (_done_parsing = false; !_done_parsing; )
543     {
544       /* Check current token for freshness. */
545       if (cur_token.used_by_parser)
546         throw new Error ("Symbol recycling detected (fix your scanner).");
547
548       /* current state is always on the top of the stack */
549
550       /* look up action out of the current state with the current input */
551       act = get_action(((Symbol)stack.peek()).parse_state, cur_token.sym);
552
553       /* decode the action -- > 0 encodes shift */
554       if (act > 0)
555         {
556           /* shift to the encoded state by pushing it on the stack */
557           cur_token.parse_state = act-1;
558           cur_token.used_by_parser = true;
559           stack.push(cur_token);
560           tos++;
561
562           /* advance to the next Symbol */
563           cur_token = scan();
564         }
565       /* if its less than zero, then it encodes a reduce action */
566       else if (act < 0)
567         {
568           /* perform the action for the reduce */
569           lhs_sym = do_action((-act)-1, this, stack, tos);
570
571           /* look up information about the production */
572           lhs_sym_num = production_tab[(-act)-1][0];
573           handle_size = production_tab[(-act)-1][1];
574
575           /* pop the handle off the stack */
576           for (int i = 0; i < handle_size; i++)
577         {
578           stack.pop();
579           tos--;
580         }
581           
582           /* look up the state to go to from the one popped back to */
583           act = get_reduce(((Symbol)stack.peek()).parse_state, lhs_sym_num);
584
585           /* shift to that state */
586           lhs_sym.parse_state = act;
587           lhs_sym.used_by_parser = true;
588           stack.push(lhs_sym);
589           tos++;
590         }
591       /* finally if the entry is zero, we have an error */
592       else if (act == 0)
593         {
594           /* call user syntax error reporting routine */
595           syntax_error(cur_token);
596
597           /* try to error recover */
598           if (!error_recovery(false))
599         {
600           /* if that fails give up with a fatal syntax error */
601           unrecovered_syntax_error(cur_token);
602
603           /* just in case that wasn't fatal enough, end parse */
604           done_parsing();
605         } else {
606           lhs_sym = (Symbol)stack.peek();
607         }
608         }
609     }
610       return lhs_sym;
611     }
612
613   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
614
615   /** Write a debugging message to System.err for the debugging version
616    * of the parser.
617    *
618    * @param mess the text of the debugging message.
619    */
620   public void debug_message(String mess)
621     {
622       System.err.println(mess);
623     }
624
625   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
626
627   /** Dump the parse stack for debugging purposes. */
628   public void dump_stack()
629     {
630       if (stack == null)
631     {
632       debug_message("# Stack dump requested, but stack is null");
633       return;
634     }
635
636       debug_message("============ Parse Stack Dump ============");
637
638       /* dump the stack */
639       for (int i=0; i<stack.size(); i++)
640     {
641       debug_message("Symbol: " + ((Symbol)stack.elementAt(i)).sym +
642             " State: " + ((Symbol)stack.elementAt(i)).parse_state);
643     }
644       debug_message("==========================================");
645     }
646
647   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
648
649   /** Do debug output for a reduce.
650    *
651    * @param prod_num the production we are reducing with.
652    * @param nt_num the index of the LHS non terminal.
653    * @param rhs_size the size of the RHS.
654    */
655   public void debug_reduce(int prod_num, int nt_num, int rhs_size)
656     {
657       debug_message("# Reduce with prod #" + prod_num + " [NT=" + nt_num +
658                 ", " + "SZ=" + rhs_size + "]");
659     }
660
661   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
662
663   /** Do debug output for shift.
664    *
665    * @param shift_tkn the Symbol being shifted onto the stack.
666    */
667   public void debug_shift(Symbol shift_tkn)
668     {
669       debug_message("# Shift under term #" + shift_tkn.sym +
670             " to state #" + shift_tkn.parse_state);
671     }
672
673   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
674
675   /** Do debug output for stack state. [CSA]
676    */
677   public void debug_stack() {
678       StringBuffer sb=new StringBuffer ("## STACK:");
679       for (int i=0; i<stack.size(); i++) {
680       Symbol s = (Symbol) stack.elementAt(i);
681       sb.append(" <state "+s.parse_state+", sym "+s.sym+">");
682       if ((i%3)==2 || (i==(stack.size()-1))) {
683           debug_message(sb.toString());
684           sb = new StringBuffer (" ");
685       }
686       }
687   }
688
689   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
690
691   /** Perform a parse with debugging output. This does exactly the
692    * same things as parse(), except that it calls debug_shift() and
693    * debug_reduce() when shift and reduce moves are taken by the parser
694    * and produces various other debugging messages.
695    */
696   public Symbol debug_parse()
697     throws java.lang.Exception
698     {
699       /* the current action code */
700       int act;
701
702       /* the Symbol/stack element returned by a reduce */
703       Symbol lhs_sym = null;
704
705       /* information about production being reduced with */
706       short handle_size, lhs_sym_num;
707
708       /* set up direct reference to tables to drive the parser */
709       production_tab = production_table();
710       action_tab = action_table();
711       reduce_tab = reduce_table();
712
713       debug_message("# Initializing parser");
714
715       /* initialize the action encapsulation object */
716       init_actions();
717
718       /* do user initialization */
719       user_init();
720
721       /* the current Symbol */
722       cur_token = scan();
723
724       debug_message("# Current Symbol is #" + cur_token.sym);
725
726       /* push dummy Symbol with start state to get us underway */
727       stack.removeAllElements();
728       stack.push(new Symbol(0, start_state()));
729       tos = 0;
730
731       /* continue until we are told to stop */
732       for (_done_parsing = false; !_done_parsing; )
733     {
734       /* Check current token for freshness. */
735       if (cur_token.used_by_parser)
736         throw new Error ("Symbol recycling detected (fix your scanner).");
737
738       /* current state is always on the top of the stack */
739       //debug_stack();
740
741       /* look up action out of the current state with the current input */
742       act = get_action(((Symbol)stack.peek()).parse_state, cur_token.sym);
743
744       /* decode the action -- > 0 encodes shift */
745       if (act > 0)
746         {
747           /* shift to the encoded state by pushing it on the stack */
748           cur_token.parse_state = act-1;
749           cur_token.used_by_parser = true;
750           debug_shift(cur_token);
751           stack.push(cur_token);
752           tos++;
753
754           /* advance to the next Symbol */
755           cur_token = scan();
756               debug_message("# Current token is " + cur_token);
757         }
758       /* if its less than zero, then it encodes a reduce action */
759       else if (act < 0)
760         {
761           /* perform the action for the reduce */
762           lhs_sym = do_action((-act)-1, this, stack, tos);
763
764           /* look up information about the production */
765           lhs_sym_num = production_tab[(-act)-1][0];
766           handle_size = production_tab[(-act)-1][1];
767
768           debug_reduce((-act)-1, lhs_sym_num, handle_size);
769
770           /* pop the handle off the stack */
771           for (int i = 0; i < handle_size; i++)
772         {
773           stack.pop();
774           tos--;
775         }
776           
777           /* look up the state to go to from the one popped back to */
778           act = get_reduce(((Symbol)stack.peek()).parse_state, lhs_sym_num);
779           debug_message("# Reduce rule: top state " +
780                  ((Symbol)stack.peek()).parse_state +
781                  ", lhs sym " + lhs_sym_num + " -> state " + act);
782
783           /* shift to that state */
784           lhs_sym.parse_state = act;
785           lhs_sym.used_by_parser = true;
786           stack.push(lhs_sym);
787           tos++;
788
789           debug_message("# Goto state #" + act);
790         }
791       /* finally if the entry is zero, we have an error */
792       else if (act == 0)
793         {
794           /* call user syntax error reporting routine */
795           syntax_error(cur_token);
796
797           /* try to error recover */
798           if (!error_recovery(true))
799         {
800           /* if that fails give up with a fatal syntax error */
801           unrecovered_syntax_error(cur_token);
802
803           /* just in case that wasn't fatal enough, end parse */
804           done_parsing();
805         } else {
806           lhs_sym = (Symbol)stack.peek();
807         }
808         }
809     }
810       return lhs_sym;
811     }
812
813   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
814   /* Error recovery code */
815   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
816
817   /** Attempt to recover from a syntax error. This returns false if recovery
818    * fails, true if it succeeds. Recovery happens in 4 steps. First we
819    * pop the parse stack down to a point at which we have a shift out
820    * of the top-most state on the error Symbol. This represents the
821    * initial error recovery configuration. If no such configuration is
822    * found, then we fail. Next a small number of "lookahead" or "parse
823    * ahead" Symbols are read into a buffer. The size of this buffer is
824    * determined by error_sync_size() and determines how many Symbols beyond
825    * the error must be matched to consider the recovery a success. Next,
826    * we begin to discard Symbols in attempt to get past the point of error
827    * to a point where we can continue parsing. After each Symbol, we attempt
828    * to "parse ahead" though the buffered lookahead Symbols. The "parse ahead"
829    * process simulates that actual parse, but does not modify the real
830    * parser's configuration, nor execute any actions. If we can parse all
831    * the stored Symbols without error, then the recovery is considered a
832    * success. Once a successful recovery point is determined, we do an
833    * actual parse over the stored input -- modifying the real parse
834    * configuration and executing all actions. Finally, we return the the
835    * normal parser to continue with the overall parse.
836    *
837    * @param debug should we produce debugging messages as we parse.
838    */
839   protected boolean error_recovery(boolean debug)
840     throws java.lang.Exception
841     {
842       if (debug) debug_message("# Attempting error recovery");
843
844       /* first pop the stack back into a state that can shift on error and
845      do that shift (if that fails, we fail) */
846       if (!find_recovery_config(debug))
847     {
848       if (debug) debug_message("# Error recovery fails");
849       return false;
850     }
851
852       /* read ahead to create lookahead we can parse multiple times */
853       read_lookahead();
854
855       /* repeatedly try to parse forward until we make it the required dist */
856       for (;;)
857     {
858       /* try to parse forward, if it makes it, bail out of loop */
859       if (debug) debug_message("# Trying to parse ahead");
860       if (try_parse_ahead(debug))
861         {
862           break;
863         }
864
865       /* if we are now at EOF, we have failed */
866       if (lookahead[0].sym == EOF_sym())
867         {
868           if (debug) debug_message("# Error recovery fails at EOF");
869           return false;
870         }
871
872       /* otherwise, we consume another Symbol and try again */
873       // BUG FIX by Bruce Hutton
874 // Computer Science Department, University of Auckland,
875 // Auckland, New Zealand.
876 // It is the first token that is being consumed, not the one
877 // we were up to parsing
878 if (debug)
879           debug_message("# Consuming Symbol #" + lookahead[ 0 ].sym);
880       restart_lookahead();
881     }
882
883       /* we have consumed to a point where we can parse forward */
884       if (debug) debug_message("# Parse-ahead ok, going back to normal parse");
885
886       /* do the real parse (including actions) across the lookahead */
887       parse_lookahead(debug);
888
889       /* we have success */
890       return true;
891     }
892
893   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
894
895   /** Determine if we can shift under the special error Symbol out of the
896    * state currently on the top of the (real) parse stack.
897    */
898   protected boolean shift_under_error()
899     {
900       /* is there a shift under error Symbol */
901       return get_action(((Symbol)stack.peek()).parse_state, error_sym()) > 0;
902     }
903
904   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
905
906   /** Put the (real) parse stack into error recovery configuration by
907    * popping the stack down to a state that can shift on the special
908    * error Symbol, then doing the shift. If no suitable state exists on
909    * the stack we return false
910    *
911    * @param debug should we produce debugging messages as we parse.
912    */
913   protected boolean find_recovery_config(boolean debug)
914     {
915       Symbol error_token;
916       int act;
917
918       if (debug) debug_message("# Finding recovery state on stack");
919
920       /* Remember the right-position of the top symbol on the stack */
921       int right_pos = ((Symbol)stack.peek()).right;
922       int left_pos = ((Symbol)stack.peek()).left;
923
924       /* pop down until we can shift under error Symbol */
925       while (!shift_under_error())
926     {
927       /* pop the stack */
928       if (debug)
929         debug_message("# Pop stack by one, state was # " +
930                       ((Symbol)stack.peek()).parse_state);
931           left_pos = ((Symbol)stack.pop()).left;
932       tos--;
933
934       /* if we have hit bottom, we fail */
935       if (stack.empty())
936         {
937           if (debug) debug_message("# No recovery state found on stack");
938           return false;
939         }
940     }
941
942       /* state on top of the stack can shift under error, find the shift */
943       act = get_action(((Symbol)stack.peek()).parse_state, error_sym());
944       if (debug)
945     {
946       debug_message("# Recover state found (#" +
947             ((Symbol)stack.peek()).parse_state + ")");
948       debug_message("# Shifting on error to state #" + (act-1));
949     }
950
951       /* build and shift a special error Symbol */
952       error_token = new Symbol(error_sym(), left_pos, right_pos);
953       error_token.parse_state = act-1;
954       error_token.used_by_parser = true;
955       stack.push(error_token);
956       tos++;
957
958       return true;
959     }
960
961   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
962
963   /** Lookahead Symbols used for attempting error recovery "parse aheads". */
964   protected Symbol lookahead[];
965
966   /** Position in lookahead input buffer used for "parse ahead". */
967   protected int lookahead_pos;
968
969   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
970
971   /** Read from input to establish our buffer of "parse ahead" lookahead
972    * Symbols.
973    */
974   protected void read_lookahead() throws java.lang.Exception
975     {
976       /* create the lookahead array */
977       lookahead = new Symbol[error_sync_size()];
978
979       /* fill in the array */
980       for (int i = 0; i < error_sync_size(); i++)
981     {
982       lookahead[i] = cur_token;
983       cur_token = scan();
984     }
985
986       /* start at the beginning */
987       lookahead_pos = 0;
988     }
989
990   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
991
992   /** Return the current lookahead in our error "parse ahead" buffer. */
993   protected Symbol cur_err_token() { return lookahead[lookahead_pos]; }
994
995   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
996
997   /** Advance to next "parse ahead" input Symbol. Return true if we have
998    * input to advance to, false otherwise.
999    */
1000  protected boolean advance_lookahead()
1001    {
1002      /* advance the input location */
1003      lookahead_pos++;
1004
1005      /* return true if we didn't go off the end */
1006      return lookahead_pos < error_sync_size();
1007    }
1008
1009  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1010
1011  /** Reset the parse ahead input to one Symbol past where we started error
1012   * recovery (this consumes one new Symbol from the real input).
1013   */
1014  protected void restart_lookahead() throws java.lang.Exception
1015    {
1016      /* move all the existing input over */
1017      for (int i = 1; i < error_sync_size(); i++)
1018    lookahead[i-1] = lookahead[i];
1019
1020      /* read a new Symbol into the last spot */
1021      // BUG Fix by Bruce Hutton
1022 // Computer Science Department, University of Auckland,
1023 // Auckland, New Zealand. [applied 5-sep-1999 by csa]
1024 // The following two lines were out of order!!
1025 lookahead[error_sync_size()-1] = cur_token;
1026      cur_token = scan();
1027
1028      /* reset our internal position marker */
1029      lookahead_pos = 0;
1030    }
1031
1032  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1033
1034  /** Do a simulated parse forward (a "parse ahead") from the current
1035   * stack configuration using stored lookahead input and a virtual parse
1036   * stack. Return true if we make it all the way through the stored
1037   * lookahead input without error. This basically simulates the action of
1038   * parse() using only our saved "parse ahead" input, and not executing any
1039   * actions.
1040   *
1041   * @param debug should we produce debugging messages as we parse.
1042   */
1043  protected boolean try_parse_ahead(boolean debug)
1044    throws java.lang.Exception
1045    {
1046      int act;
1047      short lhs, rhs_size;
1048
1049      /* create a virtual stack from the real parse stack */
1050      virtual_parse_stack vstack = new virtual_parse_stack(stack);
1051
1052      /* parse until we fail or get past the lookahead input */
1053      for (;;)
1054    {
1055      /* look up the action from the current state (on top of stack) */
1056      act = get_action(vstack.top(), cur_err_token().sym);
1057
1058      /* if its an error, we fail */
1059      if (act == 0) return false;
1060
1061      /* > 0 encodes a shift */
1062      if (act > 0)
1063        {
1064          /* push the new state on the stack */
1065          vstack.push(act-1);
1066
1067          if (debug) debug_message("# Parse-ahead shifts Symbol #" +
1068               cur_err_token().sym + " into state #" + (act-1));
1069
1070          /* advance simulated input, if we run off the end, we are done */
1071          if (!advance_lookahead()) return true;
1072        }
1073      /* < 0 encodes a reduce */
1074      else
1075        {
1076          /* if this is a reduce with the start production we are done */
1077          if ((-act)-1 == start_production())
1078        {
1079          if (debug) debug_message("# Parse-ahead accepts");
1080          return true;
1081        }
1082
1083          /* get the lhs Symbol and the rhs size */
1084          lhs = production_tab[(-act)-1][0];
1085          rhs_size = production_tab[(-act)-1][1];
1086
1087          /* pop handle off the stack */
1088          for (int i = 0; i < rhs_size; i++)
1089        vstack.pop();
1090
1091          if (debug)
1092        debug_message("# Parse-ahead reduces: handle size = " +
1093              rhs_size + " lhs = #" + lhs + " from state #" + vstack.top());
1094
1095          /* look up goto and push it onto the stack */
1096          vstack.push(get_reduce(vstack.top(), lhs));
1097          if (debug)
1098        debug_message("# Goto state #" + vstack.top());
1099        }
1100    }
1101    }
1102
1103  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
1104
1105  /** Parse forward using stored lookahead Symbols. In this case we have
1106   * already verified that parsing will make it through the stored lookahead
1107   * Symbols and we are now getting back to the point at which we can hand
1108   * control back to the normal parser. Consequently, this version of the
1109   * parser performs all actions and modifies the real parse configuration.
1110   * This returns once we have consumed all the stored input or we accept.
1111   *
1112   * @param debug should we produce debugging messages as we parse.
1113   */
1114  protected void parse_lookahead(boolean debug)
1115    throws java.lang.Exception
1116    {
1117      /* the current action code */
1118      int act;
1119
1120      /* the Symbol/stack element returned by a reduce */
1121      Symbol lhs_sym = null;
1122
1123      /* information about production being reduced with */
1124      short handle_size, lhs_sym_num;
1125
1126      /* restart the saved input at the beginning */
1127      lookahead_pos = 0;
1128
1129      if (debug)
1130    {
1131      debug_message("# Reparsing saved input with actions");
1132      debug_message("# Current Symbol is #" + cur_err_token().sym);
1133      debug_message("# Current state is #" +
1134            ((Symbol)stack.peek()).parse_state);
1135    }
1136
1137      /* continue until we accept or have read all lookahead input */
1138      while(!_done_parsing)
1139    {
1140      /* current state is always on the top of the stack */
1141
1142      /* look up action out of the current state with the current input */
1143      act =
1144        get_action(((Symbol)stack.peek()).parse_state, cur_err_token().sym);
1145
1146      /* decode the action -- > 0 encodes shift */
1147      if (act > 0)
1148        {
1149          /* shift to the encoded state by pushing it on the stack */
1150          cur_err_token().parse_state = act-1;
1151          cur_err_token().used_by_parser = true;
1152          if (debug) debug_shift(cur_err_token());
1153          stack.push(cur_err_token());
1154          tos++;
1155
1156          /* advance to the next Symbol, if there is none, we are done */
1157          if (!advance_lookahead())
1158        {
1159          if (debug) debug_message("# Completed reparse");
1160
1161          /* scan next Symbol so we can continue parse */
1162          // BUGFIX by Chris Harris <ckharris@ucsd.edu>:
1163 // correct a one-off error by commenting out
1164 // this next line.
1165 /*cur_token = scan();*/
1166
1167          /* go back to normal parser */
1168          return;
1169        }
1170          
1171          if (debug)
1172        debug_message("# Current Symbol is #" + cur_err_token().sym);
1173        }
1174      /* if its less than zero, then it encodes a reduce action */
1175      else if (act < 0)
1176        {
1177          /* perform the action for the reduce */
1178          lhs_sym = do_action((-act)-1, this, stack, tos);
1179
1180          /* look up information about the production */
1181          lhs_sym_num = production_tab[(-act)-1][0];
1182          handle_size = production_tab[(-act)-1][1];
1183
1184          if (debug) debug_reduce((-act)-1, lhs_sym_num, handle_size);
1185
1186          /* pop the handle off the stack */
1187          for (int i = 0; i < handle_size; i++)
1188        {
1189          stack.pop();
1190          tos--;
1191        }
1192          
1193          /* look up the state to go to from the one popped back to */
1194          act = get_reduce(((Symbol)stack.peek()).parse_state, lhs_sym_num);
1195
1196          /* shift to that state */
1197          lhs_sym.parse_state = act;
1198          lhs_sym.used_by_parser = true;
1199          stack.push(lhs_sym);
1200          tos++;
1201           
1202          if (debug) debug_message("# Goto state #" + act);
1203
1204        }
1205      /* finally if the entry is zero, we have an error
1206         (shouldn't happen here, but...)*/
1207      else if (act == 0)
1208        {
1209          report_fatal_error("Syntax error", lhs_sym);
1210          return;
1211        }
1212    }
1213
1214    
1215    }
1216
1217  /*-----------------------------------------------------------*/
1218
1219  /** Utility function: unpacks parse tables from strings */
1220  protected static short[][] unpackFromStrings(String [] sa)
1221    {
1222      // Concatanate initialization strings.
1223 StringBuffer sb = new StringBuffer (sa[0]);
1224      for (int i=1; i<sa.length; i++)
1225    sb.append(sa[i]);
1226      int n=0; // location in initialization string
1227 int size1 = (((int)sb.charAt(n))<<16) | ((int)sb.charAt(n+1)); n+=2;
1228      short[][] result = new short[size1][];
1229      for (int i=0; i<size1; i++) {
1230        int size2 = (((int)sb.charAt(n))<<16) | ((int)sb.charAt(n+1)); n+=2;
1231        result[i] = new short[size2];
1232        for (int j=0; j<size2; j++)
1233          result[i][j] = (short) (sb.charAt(n++)-2);
1234      }
1235      return result;
1236    }
1237}
1238
1239

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