package plp.collection;

/**
 * Cells are used for different kind of linked structures.
 */

class Cell implements Position {
    
    private Object data;
    private Cell prev;
    private Cell next;

    private static final Object INVALID_DATA = new Object();

    Cell() {
	this(null, null, null);
    }

    Cell(Object o) {
	this(null, o, null);
    }

    Cell(Cell prev, Cell next) {
	this(prev, null, next);
    }

    Cell(Cell prev, Object o, Cell next) {
	setPrev(prev);
	setNext(next);
	setData(o);
    }

    Cell prev() {
	return prev;
    }

    void setPrev(Cell prev) {
	this.prev = prev;
	if (prev != null) {
	    prev.next = this;
	}
    }	

    Cell next () {
	return next;
    }

    void setNext(Cell next) {
	this.next = next;
	if (next != null) {
	    next.prev = this;
	}
    }	

    Object data() {
	return data;
    }

    void setData(Object o) {
	data = o;
    }
    
    void remove() {
	if (prev != null) {
	    prev.setNext(next);
	}
	if (next != null) {
	    next.setPrev(prev);
	}
	data = INVALID_DATA;
    }

    public String toString() {
	return "(" + super.toString() + " data : " + data + ")";
    }

    public boolean isValid() {
	return data != INVALID_DATA;
    }
}