doubleLink = function(){	
		this.head = null;
   		this.feild = null;
  		this.size= 0;
    	this.head = {};
        //return a empty double link list
   		this.head.next = this.head;
    	this.head.before = this.head;
    };
    
    doubleLink.prototype.isBefore =  function(a, b){
		//is a before b?
        var listItem = a;
        while (listItem != b) {
            listItem = listItem.next;
            if (listItem == this.head) {
                return false;
            }
        }
        return true;
    };
    
    doubleLink.prototype.insert = function(/*Object*/item,/*Object*/ targetItem,/*Boolen*/ before){

        if (!targetItem) {
            var listItem = this.head.next;
            if (listItem == this.head) {
                //insert first node
                this.head.next = item;
                this.head.before = item;
                item.before = this.head;
                item.next = this.head;
            }
            else {
                //insert node at the last position
                listItem = this.head.before;
                
                listItem.next = item;
                item.before = listItem;
                this.head.before = item;
                item.next = this.head;
            }
        }
        else {
            if (!before) {
                //insert after targetItem
                item.next = targetItem.next;
                item.next.before = item;
                targetItem.next = item;
                item.before = targetItem;
                
            }
            else {
                //insert before targetItem
                item.before = targetItem.before;
                item.before.next = item;
                targetItem.before = item;
                item.next = targetItem;
                
            }
        }
        this.size++;

    };
    doubleLink.prototype.remove = function(/*Object*/item){
    
        item.before.next = item.next;
        item.next.before = item.before;
        item.next = null;
		item.before = null;
        this.size--;
    };
    doubleLink.prototype.removeAll = function(){
        var listItem = this.head.next;
        while (listItem != this.head) {
			listItem.before = null;
            listItem = listItem.next;
			listItem.before.next = null;
        }
		listItem.before.next=null;
		this.head.next=this.head;
		this.head.before=this.head;
        this.size = 0;
    };
    
    doubleLink.prototype.size = function(){
        return this.size;
    };
	
    doubleLink.prototype.showAll = function(){
		//just for test.
        var position = 0;
        var str = '';
        var listItem = this.head.next;
        while (listItem != null) {
            str += position + ':' + listItem.node.id + ';';
            listItem = listItem.next;
            position++;
        }
        console.info('show all:' + str);
    };

//test double linklist

	 function start(){
	 	var ss0={};
		var ss1={};
		var ss2={};
		var ss3={};
		var list = new doubleLink();
		list.insert(ss0);
		list.insert(ss2);
		list.insert(ss3);
		list.insert(ss1);
		console.info(list.isBefore(ss3,ss1));
	 }