/*
 * POSITION_DLL.java
 *
 * Created on February 6, 2008, 1:51 PM
 *
 *
 * MORE UP TO DATE VERSION IS IN LIST_DLL_purge with classname POSITION
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */

package list_dll_purge;

/**
 *
 * @author suchenek
 */
//Copyright Dr. Marek A. Suchenek, 2005, 2006, 2007, 2008 ,2009, 2010, 2011 - 2025

// Interpretation: Always points to the previous element (for an extra level of indirection);
// needed to accomodate "end" position (next after last).

// 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 header
// ensures the existence of first position.)

// Definition of nose for a non-empty subtail N of list L: the inverse function of tail, that is,
// nose of tail of N is identical with N

// Property: nose of subtail N of list L is position of the head of N in L.

// 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

public class POSITION_DLL
{
	public Object head;
	public POSITION_DLL tail;
	public POSITION_DLL nose;

	public boolean isEqual(POSITION_DLL p)
	{
		return (this == p); 	// test (this.tail == p.tail) doesn't work
					// for this and p being end positions on
					// different lists (returns true, while false
					// is correct)
	}

        public boolean isValidPosition(LIST_DLL L)
	{
            if (this.isEqual(L.End())) return true;
            for (POSITION_DLL q = L.First(); q != L.End(); q = L.Next(q))
		if (this.isEqual(q)) return true;
            return false;
	}

}