/* file Pendulum.java
     ___________________Bob Terrell 4/16/97_____________ */

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

// ---------- public class Pendulum ----------

public class Pendulum extends java.applet.Applet
{ graphr   animator,
           plotter;
  double   x00,y00,
           tmax,
           g,q,w,
           av,
           max = 0, min = 0;
  double[] x,y;
  final double delt = 0.1;
  int      n, time = 1;
  // "time" runs from 1, representing 0, to n-1, representing tmax
  
  public void init()
  { setLayout(new BorderLayout(5,5)); 
    add("East",new bpanel(this));
    animator= new graphr(this);
    plotter = new graphr(this);
    add("West",plotter);
    add("Center",animator);
    q=10;g=2;w=1;tmax=25;
  }
 
  public void info()
  { System.out.println("\n Pendulum solves x''+(1/q)x'+sin(x) = g cos(wt)"); 
    System.out.println("by the fourth order Runge-Kutta method.          "); 
    System.out.println("The animation shows the pendulum weight together "); 
    System.out.println("with an arrow representing the direction of the  ");
    System.out.println("applied force. The plot shows x versus t, and the");
    System.out.println("effect of a small change in initial conditions or");
    System.out.println("other parameters can be seen if you do not clear ");
    System.out.println("the previous plot, though a large change will    ");
    System.out.println("affect the scale. Mouse clicks within the        "); 
    System.out.println("animator stop and restart it.                    "); 
   // System.out.println("The Poincare' sections show (x,x') at succesive  "); 
   // System.out.println("maxima of the applied force."); 
    System.out.println("Inspiration provided by Baker and Gollub's book. "); 
    System.out.println("-This applet is unfinished. Please send comments "); 
    System.out.println("-to bterrell@math.cornell.edu                    "); 
  }

  public void go() // called when the Go button is pressed
  { int j;
    n = 2 + (int)(tmax/delt);
    x = null; x = new double[n];
    y = null; y = new double[n];
    x[1] = x00;
    y[1] = y00;

    for(time = 1; time < n-1; ++time) 
    {  rkstep(time);
    }
    max = min = x[1];
    for(time = 2; time < n-1; ++time)
    { if(x[time] > max)
       max = x[time];
      if(x[time] < min)
       min = x[time];
    }
    plotter.paint(plotter.getGraphics());
    animator.whereweare = 1;
    animator.start();
  }

  private void rkstep(int j)
  { double k1x,k1y,k2x,k2y,k3x,k3y,k4x,k4y, 
           xj = x[j],
           yj = y[j],
           tj = delt*j,
           xx,yy,tt;
    k1x = yj;
    k1y = -(1/q)*yj - Math.sin(xj) + g*Math.cos(w*tj);
    tt = 0.5*delt; xx = xj+tt*k1x; yy = yj+tt*k1y;
    k2x = yy;
    k2y = -(1/q)*yy - Math.sin(xx) + g*Math.cos(w*(tj+tt));
    xx = xj+tt*k2x; yy = yj+tt*k2y;
    k3x = yy;
    k3y = -(1/q)*yy - Math.sin(xx) + g*Math.cos(w*(tj+tt));
    tt = delt; xx = xj+tt*k3x; yy = yj+tt*k3y;
    k4x = yy;
    k4y = -(1/q)*yy - Math.sin(xx) + g*Math.cos(w*(tj+tt));
    x[j+1] = xj + tt*(k1x+2*k2x+2*k3x+k4x)/6;
    y[j+1] = yj + tt*(k1y+2*k2y+2*k3y+k4y)/6;
    return;
  }  
}

// ----------- class graphr -----------

class graphr extends Canvas implements Runnable 
{ 
  private Pendulum p;
  Thread thr;
  Graphics g;
  double x1, x2, y1, y2, F1, F2;
  int whereweare;
  boolean pleasestop;

  public void start()
  { if(this == p.animator)
    { x1 = Math.sin(0);
      y1 = -Math.cos(0);
      F1 = -.8*(p.g)*Math.cos((p.delt)*(p.w)*1);
      g = getGraphics();
      pleasestop = false;
      if(thr == null)
      { thr = new Thread(this);
        thr.setPriority(Thread.MAX_PRIORITY);
      }
      thr.start();
    }
  }

  public void run()
  { if(this == p.animator)
    { Image offscreen = this.createImage(this.size().width,this.size().height);
      Graphics gw = offscreen.getGraphics();
      if(thr != null)
      { if(whereweare == 1)
        { g.drawOval(tox(x1)-4,toy(y1)-4,8,8);
          g.drawLine(tox(x1),toy(y1),tox(0),toy(0));
        }
        if(F1 > 0)
         g.drawString("<--",tox(0),290);
        else
         g.drawString("-->",tox(0),290);
      }
      while(thr != null)
      { if(pleasestop)
        { g.drawString("t = "
                      +(new Double(p.delt*whereweare)).toString(),tox(0),275);
          thr = null;
          return;
        }
        x2 = Math.sin(p.x[whereweare+1]);
        y2 = -Math.cos(p.x[whereweare+1]);
        F2 = -.8*(p.g)*Math.cos((p.delt)*(p.w)*whereweare);
        gw.clearRect(0,0,size().width,size().height);
        gw.drawOval(tox(x2)-4,toy(y2)-4,8,8);
        gw.drawLine(tox(x2),toy(y2),tox(0),toy(0));
        if(F2 > 0)
         gw.drawString("<--",tox(0),290);
        else
         gw.drawString("-->",tox(0),290);
        //g.drawImage(offscreen,0,0,this.getBackground(),this);
        g.drawImage(offscreen,0,0,getBackground(),this);
        x1 = x2;
        y1 = y2;
        F1 = F2;
        ++whereweare;
        try { thr.sleep(60);} catch(InterruptedException ir) {}
        if(whereweare == p.n-1)
        { whereweare = 1;
          thr.stop();
          thr = null;
        }
      }
    }
  }

  public boolean mouseDown(Event e,int x, int y)
  { if( this != p.animator)
     return true;
    if( thr != null)
     pleasestop = true;
    else
    { pleasestop = false;
      start();
    }
    return true;
  }

  public int tox(double x)
  { double sc = 1.2;
    int    hscale = size().width - 5;
    return (int)(hscale*((x + sc)/(2*sc)));
  }

  public int toy(double y)
  { double sc = 1.5;
    int    vscale = size().height - 1;
    return (int)(vscale - vscale*((y + sc)/(2*sc)));
  }

  public int tov(double y)
  { double sc = p.max - p.min; 
    int    vscale = 200; ////////plotter.size().height - 1;
    return (int)(vscale - vscale*((y + sc)/(2*sc)))+60;
  }

  public graphr(Pendulum p)
  { super();
    this.p = p;
  }

  public Dimension minimumSize()
  { return new Dimension(330,330);
  }
  public Dimension preferredSize()
  { return new Dimension(330,330);
  }
  public void paint(Graphics g)
  { if(this == p.animator)
    { /*g.setColor(Color.blue);
      g.drawString("x'' + (1/q)x' + sin(x) = g cos(wt)",15,20);
      g.setColor(Color.black);
      */
    }
    if(this == p.plotter && p.time == p.n-1)
    { double  t1 = 1,
              x1 = 0, // depends on IC
              t2,x2,
              pmx=p.max,pmn=p.min;
      g.drawLine(5,tov(pmn-1.2*Math.abs(pmn)),5,tov(pmx+1.2*Math.abs(pmx)));
      g.drawString("x",10,tov(pmx)-20);
      g.drawString((new Integer(-1+(int)pmn)).toString(),10,tov(-1+pmn));
      g.drawString((new Integer( 1+(int)pmx)).toString(),10,tov( 1+pmx));
      g.drawLine(2,tov(0),260,tov(0));
      g.drawString("t",260,tov(0)-20);
      g.drawString((new Integer((int)p.tmax)).toString(),260,tov(0)+20);
      for(int tim = 1; tim < p.n-1; ++tim)
      {  t2 = tim + 1;
         x2 = p.x[tim+1];
         g.drawLine((int)((260*t1)/p.n)+5,tov(x1),
                    (int)((260*t2)/p.n)+5,tov(x2));
         t1 = t2;
         x1 = x2;
      }     
    }
  }
}

// ----------- class bpanel --------------

class bpanel extends Panel
{
  Pendulum p; 
  TextField qF, gF, wF, tF, x0F, v0F;
 
  public bpanel(Pendulum p)
  { super();
    this.p = p;
    qF= new TextField("10");
    gF= new TextField("2");
    wF= new TextField("1");
    tF= new TextField("25");
    x0F = new TextField("0");
    v0F = new TextField("0");
    setLayout(new GridLayout(12,2));
    Label l1 = new Label("x''+ (1/q)x'"),
          l2 = new Label("+ sin(x)"),
          l3 = new Label("= g cos(wt)");
    l1.setForeground(Color.blue);
    l2.setForeground(Color.blue);
    l3.setForeground(Color.blue);
    add(l1);                       add(l2);
    add(new Label(""));            add(l3);
    add(new Button("Info"));       add(new Label(""));
    add(new Button("Clear"));      add(new Label(""));
    add(new Button("Go"));         add(new Label(""));
    add(new Button("Stop"));       add(new Label(""));
    add(new Label("  q = "));      add(qF);
    add(new Label("  g = "));      add(gF);
    add(new Label("  w = "));      add(wF);
    add(new Label(" 0 < t <  "));  add(tF);
    add(new Label(" x(0) =  "));   add(x0F);
    add(new Label(" x'(0) = "));   add(v0F);
  }

  public synchronized Dimension minimumSize()
  { return new Dimension(170,240);} 

  public synchronized Dimension preferredSize()
  { return new Dimension(170,240);}
  
  public boolean action(Event e,Object arg)
  { if(e.target instanceof Button)
    { if(((String)arg).equals("Info"))
       p.info();
      else if(((String)arg).equals("Clear"))
      { p.animator.getGraphics().clearRect(0,30,
                   p.animator.size().width,p.animator.size().height);
        p.plotter.getGraphics().clearRect(0,0,
                   p.plotter.size().width,p.plotter.size().height);
      } 
      else if(((String)arg).equals("Go"))
      { try
        { p.q    = (new Double(qF.getText())).doubleValue();
          p.g    = (new Double(gF.getText())).doubleValue();
          p.w    = (new Double(wF.getText())).doubleValue();
          p.tmax = (new Double(tF.getText())).doubleValue();
          p.x00  = (new Double(x0F.getText())).doubleValue();
          p.y00  = (new Double(v0F.getText())).doubleValue();
        }
        catch(NumberFormatException er)
        { System.err.println(
          " There is an error in q, g, w, or tmax.");
          return true;
        }
        p.go();
      } 
      else if(((String)arg).equals("Stop"))
      { p.animator.thr = null;
      }
      return true;
    }
    return false;
  }
}

// end Pendulum.java