// 程序：雪花粒子
// 范例文件：Snow.java

import java.applet.*;
import java.awt.*;

public class Snow extends Applet implements Runnable
{
   final int Max = 500;
   snowparticle p[]; 
   int AppletWidth,AppletHeight;
   Image        OffScreen;
   Graphics     drawOffScreen;
   Thread pThread;

   public void init()
   {
      setBackground(Color.black); 

      p = new snowparticle[Max]; 


      AppletWidth = getSize().width;
      AppletHeight = getSize().height;


      for(int i=0; i<Max; i++)
          p[i] = new snowparticle(AppletWidth,AppletHeight);


      OffScreen     = createImage(AppletWidth,AppletHeight);
      drawOffScreen = OffScreen.getGraphics();
   }

   public void start()
   {
      pThread = new Thread(this);
      pThread.start();
   }

   public void stop()
   {
      pThread = null;
   }
 
   public void update(Graphics g)
   {
       paint(g);
   }

   public void paint(Graphics g)
   {
      g.drawImage(OffScreen,0,0,this);
   }

   public void run()
   {
      int i,wperiod = 0,speriod = 10;
      double Wx = 0;
      while(true)
      {

         drawOffScreen.clearRect(0,0,AppletWidth,AppletHeight);
         drawOffScreen.setColor(Color.white);
  
         for(i=0; i<Max; i++)
         {

            drawOffScreen.fillOval((int)p[i].X,(int)p[i].Y,
               (int)p[i].Size,(int)p[i].Size);

            p[i].X += Wx/p[i].Size;

            p[i].Y += p[i].Vy * p[i].Size;

        
            if(p[i].Y > AppletHeight)
               p[i].reset(AppletWidth,AppletHeight);
         }


         if(wperiod > 0)
            wperiod--;
         else if(wperiod == 0)
         {

            speriod--;
            Wx =0;
            if(speriod == 0)
            {
               Wx = Math.random()*3 - Math.random()*3;
               wperiod = (int)(Math.random()*10+1); 
               speriod = (int)(Math.random()*10+1); 
            }
         }

         repaint();
 
         try {
             Thread.sleep(200);
         }
         catch (InterruptedException e) { }
      }
   }
}

class snowparticle
{
   double X,Y;     
   double Vx,Vy;    
   double Size;    


   public snowparticle(int x,int y)
   {
      reset(x, y);
   }


   public void reset(int x, int y)
   {

       X = Math.random()*x;
       Y = -Math.random()*y;

       Vx = 0;
       Vy = 1;

       Size = Math.random() * 5 + 1;
   }
}