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

public class Sir extends Applet implements Runnable
{
       Thread       runner;

       Graphics     bG;               // Graphics buffer
       Graphics     sG;               // Display
       Image        bI;

       Color        background =       new Color(0, 0, 0);
       Color        inside =           new Color(255, 255, 255);
       Color        features =         new Color(0, 0, 0);
       Color        susceptibleColor = new Color(255, 96, 96);
       Color        carrierColor =     new Color(192, 255, 192);
       Color        symptomaticColor = new Color(0, 192, 0);
       Color        recoveredColor =   new Color(192, 192, 255);
       Color        infectedgraph =    new Color(0, 192, 0);
       Color        susceptiblegraph = new Color(255, 0, 0);
       Color        recoveredgraph =   new Color(0, 0, 255);
       Color        black =            new Color(0, 0, 0);

       Font         font = new Font("Helvetica", Font.BOLD, 12);

       int          duration = 30;
       int          contacts = 200;
       int          olds     =  95;
       int          oldi     =   5;
       int          oldr     =   0;
       int          news, newi, newr;
       double       prob =   0.75;

       int          day[] = new int[101];
       int          i, j, k, n;
       int          running = 0;

       public void init()
       {      
              bI = createImage(this.size().width, this.size().height);
              bG = bI.getGraphics();
              bG.setFont(font);
              drawbackground();

              for (i = 0; i < 100; i = i + 1)
              {   day[i] = 0;
              }

              for (i = 0; i < 100; i = i + 1)
              {   drawface(i);
              }

              bG.setColor(black);
              bG.drawString("Click anywhere to start", 275, 105);
       }

       public void start()
       {
              if (runner == null)
              {
                 runner = new Thread(this);
                 runner.start();
              }
       }

       public void run()
       {
       }

       public void paint(Graphics g)
       {
              g.drawImage(bI, 0, 0, this);
              sG = getGraphics();
       }

       public void update(Graphics g)
       {
              g.drawImage(bI, 0, 0, this);
       }


       public void drawbackground()
       {
              bG.setColor(background);
              bG.fillRect(0, 0, 501, 201);
              bG.setColor(inside);
              bG.fillRect(10, 10, 181, 181);
              bG.fillRect(201, 10, 291, 181);
              olds = 95;
              oldi = 5;
              oldr = 0;
              bG.setColor(infectedgraph);
              bG.drawString("Infected", 430, 170);
              bG.setColor(recoveredgraph);
              bG.drawString("Recovered", 415, 30);
              bG.setColor(susceptiblegraph);
              bG.drawString("Susceptible", 230, 30);

       }

       public boolean mouseDown(Event evt, int x, int y)
       {      if (running == 0)
                 {   drawbackground();
                     simulate();
                 }
              return true;
       }

       public void simulate()
       {
              running = 1;
       
              for (i = 0; i < 100; i = i + 1)
              {   day[i] = 0;
              }

              for (i = 0; i < 5; i = i + 1)
              {   j = (int) java.lang.Math.round(100 * java.lang.Math.random());
                  day[j] = 1;
              }

              for (i = 0; i < 100; i = i + 1)
              {   drawface(i);
              }

              sG.drawImage(bI, 0, 0, this);

              for (n = 0; n < duration; n = n + 1)
              {
                  try 
                  {   runner.sleep(200);
                  }
                  catch(InterruptedException e) {};
                  for (i = 0; i < 100; i = i + 1)
                  {    if (day[i] > 0)
                       {   day[i] = day[i] + 1;
                       }
                  }

                  for (k = 0; k < contacts; k = k + 1)
                  {    i = (int) java.lang.Math.round(100 * java.lang.Math.random());
                       j = (int) java.lang.Math.round(100 * java.lang.Math.random());
                       if ((day[j] == 0) && (day[i] < 5) && (day[i] > 0) &&
                           (java.lang.Math.random() > prob))
                       {  day[j] = 1;
                       }
                  }

                  news = 0;
                  newi = 0;
                  newr = 0;

                  for (i = 0; i < 100; i = i + 1)
                  {   drawface(i);
                      if (day[i] == 0)
                      {  news = news + 1;
                      }
                      if ((day[i] > 0) && (day[i] < 7))
                      {  newi = newi + 1;
                      }
                      if (day[i] > 6)
                      {  newr = newr + 1;
                      } 
                  }

                  bG.setColor(susceptiblegraph);
                  bG.drawLine(xscale(n), yscale(olds), 
                              xscale(n + 1), yscale(news));
                  bG.drawLine(xscale(n), yscale(olds)-1, 
                              xscale(n + 1), yscale(news)-1);
                  bG.setColor(recoveredgraph);
                  bG.drawLine(xscale(n), yscale(oldr), 
                              xscale(n + 1), yscale(newr));
                  bG.drawLine(xscale(n), yscale(oldr)-1, 
                              xscale(n + 1), yscale(newr)-1);
                  bG.setColor(infectedgraph);
                  bG.drawLine(xscale(n), yscale(oldi), 
                              xscale(n + 1), yscale(newi));
                  bG.drawLine(xscale(n), yscale(oldi)-1, 
                              xscale(n + 1), yscale(newi)-1);

                  olds = news;
                  oldi = newi;
                  oldr = newr;

                  sG.drawImage(bI, 0, 0, this);

              }
                        
              running = 0;
      }

      public int yscale(int ys)
      {      return (int) java.lang.Math.round(190 - ys * 1.8);
      }

      public int xscale(int xs)
      {      return (int) java.lang.Math.round(201 + (280.0/(duration-1)) * xs);
      } 

      public void drawface(int person)
      {
              int     sx, sy;
              sx = person / 10;
              sy = person % 10;
              bG.setColor(recoveredColor);
              if (day[person] < 7) 
                 {     bG.setColor(symptomaticColor);
                 }
              if (day[person] == 0)
                 {     bG.setColor(susceptibleColor);
                 }
              if (day[person] == 1) 
                 {     bG.setColor(carrierColor);
                 }
              bG.fillOval(18 * sy + 14, 18 * sx + 12, 9, 13);
      }
        

}

Copyright c 1995 by Frank Wattenberg Department of Mathematics, Montana State University, Bozeman, MT 59717.