// RKmain.java   test program demonstrates a library class that
//               remains unchanged for various user provided functions.
//               This could be a non "main" class with a method that
//               returned a value or an array of values.

public class RKmain
{
  public RKmain() // solve RKuser ordinary differential equation
                  // using 4th order Runge-Kutta method
  {
    double k1,k2,k3,k4;
    double x,y,h;

    System.out.println("RKmain running");
    System.out.println("solving RKuser ordinary differential equation");
    System.out.println("xmin="+user.xmin+", xmax="+user.xmax+
                       ", y="+user.y+", h="+user.h);

    x=user.xmin;
    y=user.y;
    System.out.println("y' = f(x,y) = "+user.f(x,y));
    h=user.h;
    System.out.println("  x                     y");
    while(x<user.xmax) {
      if(user.printAll) System.out.println(x+"    "+y);
      k1=h*user.f(x,y);
      k2=h*user.f(x+h/2.0,y+k1/2.0);
      k3=h*user.f(x+h/2.0,y+k2/2.0);
      k4=h*user.f(x+h,y+k3);
      y=y+(k1+2.0*k2+2.0*k3+k4)/6.0;
      x=x+h;
    }
    System.out.println(x+"    "+y);
  }

  public static RKuser user=new RKuser();

  public static void main (String[] args)
  {
    new RKmain();
  }
}