virtual_parse_stack


1   
2   package com.sun.java_cup.internal.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     com.sun.java_cup.internal.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 }
146
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