+++ /dev/null
-
-package java_cup.runtime;
-
-import java.util.Stack;
-
-/** This class implements a temporary or "virtual" parse stack that
- * replaces the top portion of the actual parse stack (the part that
- * has been changed by some set of operations) while maintaining its
- * original contents. This data structure is used when the parse needs
- * to "parse ahead" to determine if a given error recovery attempt will
- * allow the parse to continue far enough to consider it successful. Once
- * success or failure of parse ahead is determined the system then
- * reverts to the original parse stack (which has not actually been
- * modified). Since parse ahead does not execute actions, only parse
- * state is maintained on the virtual stack, not full Symbol objects.
- *
- * @see java_cup.runtime.lr_parser
- * @version last updated: 7/3/96
- * @author Frank Flannery
- */
-
-public class virtual_parse_stack {
- /*-----------------------------------------------------------*/
- /*--- Constructor(s) ----------------------------------------*/
- /*-----------------------------------------------------------*/
-
- /** Constructor to build a virtual stack out of a real stack. */
- public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception
- {
- /* sanity check */
- if (shadowing_stack == null)
- throw new Exception(
- "Internal parser error: attempt to create null virtual stack");
-
- /* set up our internals */
- real_stack = shadowing_stack;
- vstack = new Stack();
- real_next = 0;
-
- /* get one element onto the virtual portion of the stack */
- get_from_real();
- }
-
- /*-----------------------------------------------------------*/
- /*--- (Access to) Instance Variables ------------------------*/
- /*-----------------------------------------------------------*/
-
- /** The real stack that we shadow. This is accessed when we move off
- * the bottom of the virtual portion of the stack, but is always left
- * unmodified.
- */
- protected Stack real_stack;
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** Top of stack indicator for where we leave off in the real stack.
- * This is measured from top of stack, so 0 would indicate that no
- * elements have been "moved" from the real to virtual stack.
- */
- protected int real_next;
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** The virtual top portion of the stack. This stack contains Integer
- * objects with state numbers. This stack shadows the top portion
- * of the real stack within the area that has been modified (via operations
- * on the virtual stack). When this portion of the stack becomes empty we
- * transfer elements from the underlying stack onto this stack.
- */
- protected Stack vstack;
-
- /*-----------------------------------------------------------*/
- /*--- General Methods ---------------------------------------*/
- /*-----------------------------------------------------------*/
-
- /** Transfer an element from the real to the virtual stack. This assumes
- * that the virtual stack is currently empty.
- */
- protected void get_from_real()
- {
- Symbol stack_sym;
-
- /* don't transfer if the real stack is empty */
- if (real_next >= real_stack.size()) return;
-
- /* get a copy of the first Symbol we have not transfered */
- stack_sym = (Symbol)real_stack.elementAt(real_stack.size()-1-real_next);
-
- /* record the transfer */
- real_next++;
-
- /* put the state number from the Symbol onto the virtual stack */
- vstack.push(new Integer(stack_sym.parse_state));
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** Indicate whether the stack is empty. */
- public boolean empty()
- {
- /* if vstack is empty then we were unable to transfer onto it and
- the whole thing is empty. */
- return vstack.empty();
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** Return value on the top of the stack (without popping it). */
- public int top() throws java.lang.Exception
- {
- if (vstack.empty())
- throw new Exception(
- "Internal parser error: top() called on empty virtual stack");
-
- return ((Integer)vstack.peek()).intValue();
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** Pop the stack. */
- public void pop() throws java.lang.Exception
- {
- if (vstack.empty())
- throw new Exception(
- "Internal parser error: pop from empty virtual stack");
-
- /* pop it */
- vstack.pop();
-
- /* if we are now empty transfer an element (if there is one) */
- if (vstack.empty())
- get_from_real();
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** Push a state number onto the stack. */
- public void push(int state_num)
- {
- vstack.push(new Integer(state_num));
- }
-
- /*-----------------------------------------------------------*/
-
-}