001: package java_cup.runtime;
002:
003: import java.util.Stack;
004:
005: /** This class implements a temporary or "virtual" parse stack that
006: * replaces the top portion of the actual parse stack (the part that
007: * has been changed by some set of operations) while maintaining its
008: * original contents. This data structure is used when the parse needs
009: * to "parse ahead" to determine if a given error recovery attempt will
010: * allow the parse to continue far enough to consider it successful. Once
011: * success or failure of parse ahead is determined the system then
012: * reverts to the original parse stack (which has not actually been
013: * modified). Since parse ahead does not execute actions, only parse
014: * state is maintained on the virtual stack, not full Symbol objects.
015: *
016: * @see java_cup.runtime.lr_parser
017: * @version last updated: 7/3/96
018: * @author Frank Flannery
019: */
020:
021: public class virtual_parse_stack {
022: /*-----------------------------------------------------------*/
023: /*--- Constructor(s) ----------------------------------------*/
024: /*-----------------------------------------------------------*/
025:
026: /** Constructor to build a virtual stack out of a real stack. */
027: public virtual_parse_stack(Stack shadowing_stack)
028: throws java.lang.Exception {
029: /* sanity check */
030: if (shadowing_stack == null)
031: throw new Exception(
032: "Internal parser error: attempt to create null virtual stack");
033:
034: /* set up our internals */
035: real_stack = shadowing_stack;
036: vstack = new Stack();
037: real_next = 0;
038:
039: /* get one element onto the virtual portion of the stack */
040: get_from_real();
041: }
042:
043: /*-----------------------------------------------------------*/
044: /*--- (Access to) Instance Variables ------------------------*/
045: /*-----------------------------------------------------------*/
046:
047: /** The real stack that we shadow. This is accessed when we move off
048: * the bottom of the virtual portion of the stack, but is always left
049: * unmodified.
050: */
051: protected Stack real_stack;
052:
053: /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
054:
055: /** Top of stack indicator for where we leave off in the real stack.
056: * This is measured from top of stack, so 0 would indicate that no
057: * elements have been "moved" from the real to virtual stack.
058: */
059: protected int real_next;
060:
061: /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
062:
063: /** The virtual top portion of the stack. This stack contains Integer
064: * objects with state numbers. This stack shadows the top portion
065: * of the real stack within the area that has been modified (via operations
066: * on the virtual stack). When this portion of the stack becomes empty we
067: * transfer elements from the underlying stack onto this stack.
068: */
069: protected Stack vstack;
070:
071: /*-----------------------------------------------------------*/
072: /*--- General Methods ---------------------------------------*/
073: /*-----------------------------------------------------------*/
074:
075: /** Transfer an element from the real to the virtual stack. This assumes
076: * that the virtual stack is currently empty.
077: */
078: protected void get_from_real() {
079: Symbol stack_sym;
080:
081: /* don't transfer if the real stack is empty */
082: if (real_next >= real_stack.size())
083: return;
084:
085: /* get a copy of the first Symbol we have not transfered */
086: stack_sym = (Symbol) real_stack.elementAt(real_stack.size() - 1
087: - real_next);
088:
089: /* record the transfer */
090: real_next++;
091:
092: /* put the state number from the Symbol onto the virtual stack */
093: vstack.push(new Integer(stack_sym.parse_state));
094: }
095:
096: /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
097:
098: /** Indicate whether the stack is empty. */
099: public boolean empty() {
100: /* if vstack is empty then we were unable to transfer onto it and
101: the whole thing is empty. */
102: return vstack.empty();
103: }
104:
105: /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
106:
107: /** Return value on the top of the stack (without popping it). */
108: public int top() throws java.lang.Exception {
109: if (vstack.empty())
110: throw new Exception(
111: "Internal parser error: top() called on empty virtual stack");
112:
113: return ((Integer) vstack.peek()).intValue();
114: }
115:
116: /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
117:
118: /** Pop the stack. */
119: public void pop() throws java.lang.Exception {
120: if (vstack.empty())
121: throw new Exception(
122: "Internal parser error: pop from empty virtual stack");
123:
124: /* pop it */
125: vstack.pop();
126:
127: /* if we are now empty transfer an element (if there is one) */
128: if (vstack.empty())
129: get_from_real();
130: }
131:
132: /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
133:
134: /** Push a state number onto the stack. */
135: public void push(int state_num) {
136: vstack.push(new Integer(state_num));
137: }
138:
139: /*-----------------------------------------------------------*/
140:
141: }
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