
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<Symbol> 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<Symbol> 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<Integer> 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 = 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(Integer.valueOf(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 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(Integer.valueOf(state_num));
  }

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

}
