/* 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                              
   */
//  DLA.java: diffusion limited aggregations

import java.io.*;                          // Location of PrintWriter
import java.util.*;                             // Location of Random
import java.lang.*;                               // Location of Math

public class DLA  {
  
  public static void main(String[] argv) 
                   throws IOException, FileNotFoundException  {
	  PrintWriter q = new PrintWriter(                     // Data file
			                         new FileOutputStream("DLA.DAT"),true);
                  // Initialize 48 bit pseudo-random number generator
		Random randnum = new Random();            // system clock is seed
   	int max = 40000, size = 401, step = 0, trav = 0, hit = 0;
	  double PI = Math.PI;
	  int mem_old[] = new int[2];
	  mem_old[0] = 0;
	  mem_old[1] = 0;
	  double angle = 0.0, rad = 180.0;
	  int i = 0, j = 0, x = 0, y = 0, dist = 0, dir = 0 ;
	  int grid[][] = new int[size][size]; 
                                                        // Initialize
    for (i = 0;i<size;i++)	for (j = 0;j<size;j++) grid[i][j] = 0;
    grid[200][200] = 1;	                    // one particle in center
                                             // choose starting point
	  for (i = 0;i<max;i++) 	{
		  hit = 0;
		  angle = 2.0*PI*randnum.nextDouble();            // random angle
		  x = (200 + (int)(rad*Math.cos(angle)));          // coordinates
		  y = (200 + (int)(rad*Math.sin(angle)));
		  dist = gauss_ran(mem_old);       // random number gaussian dist
		  if ( dist < 0 ) step = -1;        // move forwards or backwards
		  else step = 1;
      trav = 0;
                                          // while loop does building
		  while ( (hit==0) && (x<399) && (x>1) && (y<399) &&(y>1) &&
			                                   (trav<Math.abs(dist)))  {
			  if( (grid[x+1][y] + grid[x-1][y] + grid[x][y+1] 
                                         + grid[x][y-1]) >= 1 )  {
				  hit = 1;                           // one neighbor occupied
				  grid[x][y] = 1;               // particle sticks, walk over
				}
			 			                                     // move horizontally
			  else if (randnum.nextDouble() <0.5 ) x += step; 
			                                              //move vertically
			  else y+=step; trav++;
			}
		}
                                              // print resulting grid
	  for ( i = 0; i<size; i++ )
	  for ( j = 0; j<size; j++ )
	  if ( grid[i][j] == 1 ) q.println(i+"  "+j);
    System.out.println(" ");
	  System.out.println("DLA Program Complete.");
	  System.out.println("Data stored in DLA.DAT");
	  System.out.println(" ");
 }
 
  public static int gauss_ran(int mem_old[]) {
                    // gaussian random numbers via Box-Mueller method
	  Random randnum = new Random();             //System clock is seed
    double fac = 0.0, rr = 0.0, r1 = 0.0, r2 = 0.0;
    if ( mem_old[1] == 0 )  {	  //no random number from previous call
		  do  {
			                           //choose random point in unit circle
			  r1 = 2.0*randnum.nextDouble()-1.0;
			  r2 = 2.0*randnum.nextDouble()-1.0;
			  rr = r1*r1+r2*r2;
			}
		  while ( (rr>=1) || (rr==0) );
		    fac = Math.sqrt(-2.0*Math.log(rr)/rr);
		    mem_old[0] = (int)(5000.0*r1*fac);      // save for next call
		    mem_old[1] = 1;         		                      // set flag
 		   
      return((int)(5000.0*r2*fac));
	  }
	  else {		          			 // return second number from last call
			mem_old[1] = 0;			                              // unset flag
		  return mem_old[0];		          // return number from last call
		}
	}
}   // End of class