/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; and CC Bordeianu, Univ Bucharest, 2007.
   Support by National Science Foundation                            
*/
//  Eqstring.java: Solution of wave equation via time stepping
//                   Output in 3-D gnuplot format                   

import java.io.*; 

public class Eqstring  {
  final static double rho = 0.01, ten = 40., max = 100.; 

  public static void main(String[] argv) 
                         throws IOException, FileNotFoundException  {
    int i, k; 
    double x[][] = new double[101][3]; 
		double ratio, c, c1; 
    PrintWriter w = new PrintWriter              // Save data in file
                        (new FileOutputStream("EqString.dat"), true);
    c = Math.sqrt(ten/rho);                      // Propagation speed
    c1 = c;                                           // CFL criteria
    ratio =  c*c/(c1*c1);                          
		                                         // Initial configuration
    for ( i=0;  i < 81;   i++ ) x[i][0] = 0.00125*i; 
    for ( i=81; i < 101;  i++ ) x[i][0] = 0.1-0.005*(i-80); 
		                                               // First time step
    for ( i=1;  i < 100;  i++ ) {                      
      x[i][1] = x[i][0]+0.5*ratio*(x[i+1][0]+x[i-1][0]-2.*x[i][0]);}
			                                            // Later time steps
      for ( k=1;  k < max;  k++ )  {                   
        for ( i=1;  i < 100;  i++ )  x[i][2] = 2.*x[i][1]
                -x[i][0] + ratio*(x[i+1][1] + x[i-1][1] - 2*x[i][1]);
        for ( i=0;  i < 101;  i++ )  {  
          x[i][0] = x[i][1];     
          x[i][1] = x[i][2];      
        }
        if ((k%5) == 0) {                    // Print every 5th point
          for ( i=0;  i < 101;  i++ ) {
            w.println("" + x[i][2] + ""); }      // Gnuplot 3D format
            w.println("");                  // Empty line for gnuplot
        } 
      }
      System.out.println("data in EqString.dat, gnuplot format");   
  } 
}