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: 11/25/95
19  * @author  Scott Hudson
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. */
virtual_parse_stack(Stack shadowing_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    */
get_from_real()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. */
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). */
top()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. */
pop()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. */
push(int state_num)138   public void push(int state_num)
139     {
140       vstack.push(new Integer(state_num));
141     }
142 
143   /*-----------------------------------------------------------*/
144 
145 };
146