content: 
  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));
      }
  
    /*-----------------------------------------------------------*/
  
  }
  