我下了一个贪食蛇的源程序，问题在于它是怎么动的？这一点看不明白，主要代码：
bool flag = true;     
int fangx = 2;  // fangx 代表的是什么？？？不懂
 Point dou = new Point()
LinkedList<Point> ll = new LinkedList<Point>()
void F_Move()

        {

            Point first = ll.First.Value;//ll表示双向链表
            switch (fangx)
            {
                case 0:
                    {
                        if (flag)
                        {
                            if (first.Y == 0)//为什么坐标是这样？不懂

                            {
                                first.Y = 40;
                            }
                        }
                        ll.AddFirst(new Point(first.X, --first.Y ));
                        break;
                    }
                case 1:
                    {
                        if (flag)
                        {
                            if (first.X == 0)//为什么坐标是这样？不懂
                            {
                                first.X = 40;
                            }
                        }
                        ll.AddFirst(new Point(--first.X, first.Y));
                        break;
                    }
                case 2:
                    {
                        if (flag)
                        {
                            if (first.X == 39)
                            {
                                first.X = -1;
                            }
                        }
                        ll.AddFirst(new Point(++first.X, first.Y));//不懂？？？？？
                        break;
                    }
                case 3:
                    {
                        if (flag)
                        {
                            if (first.Y == 39)//不懂??
                            {
                                first.Y = -1;
                            }
                        }
                        ll.AddFirst(new Point(first.X, ++first.Y ));
                        break;
                    }
            }
            if (first.Equals(dou))
            {
                F_score();//获得分数
                dou = F_getdou();
            }
            else
            {
                ll.RemoveLast();
            }
            int i = 0;
            foreach (Point p in ll)
            {
                if (p.Equals(first))
                {
                    i++;
                }
            }
            if (first.X > 39 || first.X < 0 || first.Y > 39 || first.Y < 0 || i > 1)//不懂???
            {
                F_Over();//结束
                return;
            }
            F_paint();
        }

      Point  F_getdou()
        {
            int i = new Random().Next(1690);
            return  new Point(i / 40, i % 40);
        }
//