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));    }  /*-----------------------------------------------------------*/}