virtual_parse_stack.java

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
 */
@SuppressWarnings (
{ "all" } )
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 ) );
  }
  /*-----------------------------------------------------------*/
}