/*  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; & CC Bordeianu, Univ Bucharest, 2007
   Support by National Science Foundation                          
*/
// LaplaceSOR.java: Solves Laplace eqn using SOR for convergence

import java.io.*; 


public class LaplaceSOR  { 
  static int  max =  40;                     // Number of grid points

	public static void main(String[] argv) throws IOException,
                                            FileNotFoundException  {
																							
   double x, tol, aux, omega, r, pi  =  3.1415926535;
   double p[][] = new double[max][max];
   int i, j, iter;
	 
   PrintWriter w =new PrintWriter(
	                         new FileOutputStream("laplR.dat"), true);
   omega = 1.8;                                      // SOR parameter
   long timeStart=System.nanoTime();
   System.out.println(""+timeStart+"");
	                                                            // Init
   for (i=0; i<max; i++) for ( j=0; j<max; j++ ) p[i][j]  =  0.;   
   for (i = 0; i<max; i++) p[i][0]  =  +100.0;
   tol  =  1.0;                                          // Tolerance
   iter  =  0; 
	                                                     //  Iterations
   while ( (tol > 0.000001) && (iter <=  140) ) {
		 tol  =  0.0;
		                                          // x, then y directions
     for (i = 1; i<(max-1); i++)  {                          
       for (j = 1; j<(max-1); j++)   {      
				                                            // SOR ALGORITHM
         r  =  omega * ( p[i][j+1] + p[i][j-1] + p[i+1][j] +
                           p[i-1][j] - 4.0 * p[i][j] ) / 4.0;
         p[i][j] +=  r;
         if ( Math.abs(r) > tol ) tol  =  Math.abs(r);
       }
     iter++;
      }
    }
    long timeFinal=System.nanoTime();
    System.out.println(""+timeFinal+"");
    System.out.println("Nanoseconds="+(timeFinal-timeStart));
    for (i = 0; i<max ; i++)  {            
      for (j = 0; j<max; j++) w.println(""+p[i][j]+"");  
      w.println("");                       //  Empty line for gnuplot
    }
    System.out.println("data stored in laplR.dat");
}  }