import java.awt.*;

public class Bspline {

   private int xpoints[], ypoints[];
   private int limit=4;
   private int count;
   private int width=10;
   private int height=10;
   private int intervals=20;

   // constructor
   public Bspline ()
   {
      xpoints = new int[limit];
      ypoints = new int[limit];
      count = 0;
   }

   public void paintCurve ( Graphics g ) {
      double x, y, xold, yold, A, B, C;
      double t1, t2, t3;
      double deltax1, deltax2, deltax3;
      double deltay1, deltay2, deltay3;

      // must have at least 4 points
      if (count < limit)
         return;
 
      //  This is forward differencing code to draw fast Bspline
      t1 = 1.0/intervals;
      t2 = t1*t1;
      t3 = t2*t1;

      //  For B-spline curve, "D" is the starting x,y coord
      //  So the first x,y coord is the D term from the cubic equation
      x= (xpoints[0]+4.0*xpoints[1]+xpoints[2]) / 6.0;
      y= (ypoints[0]+4.0*ypoints[1]+ypoints[2]) / 6.0;
      xold = x;
      yold = y;

      // set up deltas for the x-coords of B-spline
      A = (-xpoints[0] + 3*xpoints[1] - 3*xpoints[2] + xpoints[3]) / 6.0;
      B = (3*xpoints[0] - 6*xpoints[1] + 3*xpoints[2]) / 6.0;
      C = (-3*xpoints[0] + 3*xpoints[2]) / 6.0;

      deltax1 = A*t3 + B*t2 + C*t1;
      deltax2 = 6*A*t3 + 2*B*t2;
      deltax3 = 6*A*t3;

      // set up deltas for the y-coords
      A = (-ypoints[0] + 3*ypoints[1] - 3*ypoints[2] + ypoints[3]) / 6.0;
      B = (3*ypoints[0] - 6*ypoints[1] + 3*ypoints[2]) / 6.0;
      C = (-3*ypoints[0] + 3*ypoints[2]) / 6.0;

      deltay1 = A*t3 + B*t2 + C*t1;
      deltay2 = 6*A*t3 + 2*B*t2;
      deltay3 = 6*A*t3;

      for (int i = 0; i < intervals; i++) {
            x += deltax1;
            deltax1 += deltax2;
            deltax2 += deltax3;

            y += deltay1;
            deltay1 += deltay2;
            deltay2 += deltay3;

            g.drawLine ((int)xold, (int)yold, (int)x, (int)y);
            xold = x;
            yold = y;
      }

   }

   public void drawPolyline ( Graphics g ) {
      for (int i = 0; i < (count-1); i++) {
            g.fillOval (xpoints[i]-(width/2), ypoints[i]-(height/2), 
                        width, height);
            g.drawLine (xpoints[i], ypoints[i], xpoints[i+1], ypoints[i+1]);
         }
         g.fillOval (xpoints[count-1]-(width/2), 
                     ypoints[count-1]-(height/2), width, height);
   }

   public void addPoint (int x, int y)
   {
      if (count >= limit)
         return;
      
      xpoints[count] = x;
      ypoints[count] = y;
      count++;
   }

   public void resetCurve () {
      count = 0;
   }

   public void setInterval (int i)
   {
      intervals = i;
   }

}