/*
 * POSITION.java
 *
 * Created on March 24, 2008, 1:06 PM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */

package list_purge;

/**
 *
 * @author suchenek
 */
//Copyright Dr. Marek A. Suchenek, 2005, 2006, 2007, 2008 - 2015
//All rights reserved

// SLL implementation

// POSITION is a list of the form (head, tail).

// Definition of position of an element x in a list L: a subtail N of list L such that
// x is the head of tail of N. (It exists for every element; in particular, use of the header
// ensures the existence of first position.)

// Definition of subtail of list L: Subtail of a list L is either L itself or
// (if L is non-empty) a subtail of tail of L

// Interpretation: Position always points to the previous element (for an extra level of
// indirection), which is also needed to accomodate "end" position (next after last).

// This makes the trailer (an element the represents the end position) superflous, but
// requires introduction of an element (the header) that is pointed to by the first position.

public class POSITION<E>
{
	public E head;
	public POSITION<E> tail;

	public boolean isEqual(POSITION<E> p)
	{
		return (this == p); // don't be deceived by the simplicity of this - it's not as obvious as it seems!
	}


        public boolean isValidPosition(LIST<E> L)
	{
            if (this.isEqual(L.End())) return true;
            for (POSITION<E> q = L.First(); q != L.End(); q = L.Next(q))
		if (this.isEqual(q)) return true;
            return false;
	}


}