2 package java_cup.runtime;
4 import java.util.Stack;
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.
17 * @see java_cup.runtime.lr_parser
18 * @version last updated: 7/3/96
19 * @author Frank Flannery
22 public class virtual_parse_stack {
23 /*-----------------------------------------------------------*/
24 /*--- Constructor(s) ----------------------------------------*/
25 /*-----------------------------------------------------------*/
27 /** Constructor to build a virtual stack out of a real stack. */
28 public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception
31 if (shadowing_stack == null)
33 "Internal parser error: attempt to create null virtual stack");
35 /* set up our internals */
36 real_stack = shadowing_stack;
40 /* get one element onto the virtual portion of the stack */
44 /*-----------------------------------------------------------*/
45 /*--- (Access to) Instance Variables ------------------------*/
46 /*-----------------------------------------------------------*/
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
52 protected Stack real_stack;
54 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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.
60 protected int real_next;
62 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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.
70 protected Stack vstack;
72 /*-----------------------------------------------------------*/
73 /*--- General Methods ---------------------------------------*/
74 /*-----------------------------------------------------------*/
76 /** Transfer an element from the real to the virtual stack. This assumes
77 * that the virtual stack is currently empty.
79 protected void get_from_real()
83 /* don't transfer if the real stack is empty */
84 if (real_next >= real_stack.size()) return;
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);
89 /* record the transfer */
92 /* put the state number from the Symbol onto the virtual stack */
93 vstack.push(new Integer(stack_sym.parse_state));
96 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
98 /** Indicate whether the stack is empty. */
99 public boolean empty()
101 /* if vstack is empty then we were unable to transfer onto it and
102 the whole thing is empty. */
103 return vstack.empty();
106 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
108 /** Return value on the top of the stack (without popping it). */
109 public int top() throws java.lang.Exception
113 "Internal parser error: top() called on empty virtual stack");
115 return ((Integer)vstack.peek()).intValue();
118 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
120 /** Pop the stack. */
121 public void pop() throws java.lang.Exception
125 "Internal parser error: pop from empty virtual stack");
130 /* if we are now empty transfer an element (if there is one) */
135 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
137 /** Push a state number onto the stack. */
138 public void push(int state_num)
140 vstack.push(new Integer(state_num));
143 /*-----------------------------------------------------------*/