Source code: java_cup/runtime/virtual_parse_stack.java
1
2 package java_cup.runtime;
3
4 import java.util.Stack;
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 shadowing_stack) throws java.lang.Exception
29 {
30 /* sanity check */
31 if (shadowing_stack == null)
32 throw new Exception(
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();
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 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 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(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
110 {
111 if (vstack.empty())
112 throw new Exception(
113 "Internal parser error: top() called on empty virtual stack");
114
115 return ((Integer)vstack.peek()).intValue();
116 }
117
118 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
119
120 /** Pop the stack. */
121 public void pop() throws java.lang.Exception
122 {
123 if (vstack.empty())
124 throw new Exception(
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(state_num));
141 }
142
143 /*-----------------------------------------------------------*/
144
145 }