/*

 * Copyright (c) 2003, 2005, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package com.sun.java_cup.internal.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     com.sun.java_cup.internal.runtime.lr_parser

 * @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));

    }

  /*-----------------------------------------------------------*/

}