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Java > Open Source Codes > java_cup > runtime > virtual_parse_stack


1
2 package java_cup.runtime;
3
4 import java.util.Stack JavaDoc;
5
6 /** This class implements a temporary or "virtual" parse stack that
7  * replaces the top portion of the actual parse stack (the part that
8  * has been changed by some set of operations) while maintaining its
9  * original contents. This data structure is used when the parse needs
10  * to "parse ahead" to determine if a given error recovery attempt will
11  * allow the parse to continue far enough to consider it successful. Once
12  * success or failure of parse ahead is determined the system then
13  * reverts to the original parse stack (which has not actually been
14  * modified). Since parse ahead does not execute actions, only parse
15  * state is maintained on the virtual stack, not full Symbol objects.
16  *
17  * @see java_cup.runtime.lr_parser
18  * @version last updated: 7/3/96
19  * @author Frank Flannery
20  */

21
22 public class virtual_parse_stack {
23   /*-----------------------------------------------------------*/
24   /*--- Constructor(s) ----------------------------------------*/
25   /*-----------------------------------------------------------*/
26
27   /** Constructor to build a virtual stack out of a real stack. */
28   public virtual_parse_stack(Stack JavaDoc shadowing_stack) throws java.lang.Exception JavaDoc
29     {
30       /* sanity check */
31       if (shadowing_stack == null)
32     throw new Exception JavaDoc(
33       "Internal parser error: attempt to create null virtual stack");
34
35       /* set up our internals */
36       real_stack = shadowing_stack;
37       vstack = new Stack JavaDoc();
38       real_next = 0;
39
40       /* get one element onto the virtual portion of the stack */
41       get_from_real();
42     }
43
44   /*-----------------------------------------------------------*/
45   /*--- (Access to) Instance Variables ------------------------*/
46   /*-----------------------------------------------------------*/
47        
48   /** The real stack that we shadow. This is accessed when we move off
49    * the bottom of the virtual portion of the stack, but is always left
50    * unmodified.
51    */

52   protected Stack JavaDoc real_stack;
53
54   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
55
56   /** Top of stack indicator for where we leave off in the real stack.
57    * This is measured from top of stack, so 0 would indicate that no
58    * elements have been "moved" from the real to virtual stack.
59    */

60   protected int real_next;
61
62   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
63
64   /** The virtual top portion of the stack. This stack contains Integer
65    * objects with state numbers. This stack shadows the top portion
66    * of the real stack within the area that has been modified (via operations
67    * on the virtual stack). When this portion of the stack becomes empty we
68    * transfer elements from the underlying stack onto this stack.
69    */

70   protected Stack JavaDoc vstack;
71
72   /*-----------------------------------------------------------*/
73   /*--- General Methods ---------------------------------------*/
74   /*-----------------------------------------------------------*/
75
76   /** Transfer an element from the real to the virtual stack. This assumes
77    * that the virtual stack is currently empty.
78    */

79   protected void get_from_real()
80     {
81       Symbol stack_sym;
82
83       /* don't transfer if the real stack is empty */
84       if (real_next >= real_stack.size()) return;
85
86       /* get a copy of the first Symbol we have not transfered */
87       stack_sym = (Symbol)real_stack.elementAt(real_stack.size()-1-real_next);
88
89       /* record the transfer */
90       real_next++;
91
92       /* put the state number from the Symbol onto the virtual stack */
93       vstack.push(new Integer JavaDoc(stack_sym.parse_state));
94     }
95
96   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
97
98   /** Indicate whether the stack is empty. */
99   public boolean empty()
100     {
101       /* if vstack is empty then we were unable to transfer onto it and
102      the whole thing is empty. */

103       return vstack.empty();
104     }
105
106   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
107       
108   /** Return value on the top of the stack (without popping it). */
109   public int top() throws java.lang.Exception JavaDoc
110     {
111       if (vstack.empty())
112     throw new Exception JavaDoc(
113           "Internal parser error: top() called on empty virtual stack");
114
115       return ((Integer JavaDoc)vstack.peek()).intValue();
116     }
117
118   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
119
120   /** Pop the stack. */
121   public void pop() throws java.lang.Exception JavaDoc
122     {
123       if (vstack.empty())
124     throw new Exception JavaDoc(
125           "Internal parser error: pop from empty virtual stack");
126
127       /* pop it */
128       vstack.pop();
129
130       /* if we are now empty transfer an element (if there is one) */
131       if (vstack.empty())
132         get_from_real();
133     }
134
135   /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
136
137   /** Push a state number onto the stack. */
138   public void push(int state_num)
139     {
140       vstack.push(new Integer JavaDoc(state_num));
141     }
142
143   /*-----------------------------------------------------------*/
144
145 }
146
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