SymmetricDLA.java

Below is the syntax highlighted version of SymmetricDLA.java from §9.8 Data Analysis.

/******************************************************************************
 *  Compilation:  javac DLA.java
 *  Execution:    java DLA
 *  Dependencies: StdDraw.java
 *
 *  Diffusion limited aggregation.
 *
 *  % java SymmetricDLA 600
 *
 ******************************************************************************/

import java.awt.Color;

public class SymmetricDLA { 

   // test client
    public static void main(String[] args) {
        int N = Integer.parseInt(args[0]);    // N-by-N grid
        int x, y;                             // current (x, y) location
        double radius = 10;                   // radius
        double dist;                          // distance squared of (x, y) from seed
        boolean[][] dla = new boolean[N][N];  // is cell (x, y) occupied
        int particles = 0;

        // create rainbow of colors
        Color[] colors = new Color[256];
        for (int i = 0; i < 256; i++)
           colors[i] = Color.getHSBColor(1.0f * i / 255, .8f, .8f);


        // create graphics canvas and set seed
        Picture pic = new Picture(N, N);

        dla[N/2][N/2] = true;
        pic.set(N/2, N/2, Color.BLUE);

        while (radius < (N/2 - 2)) {

            // choose launching site on circle of given radius from seed
            double angle = 2.0 * Math.PI * Math.random();
            x = (int) (N/2.0 + radius * Math.cos(angle));
            y = (int) (N/2.0 + radius * Math.sin(angle));

            // particle takes a 2d random walk
            while (true) {
                double r = Math.random();
                if      (r < 0.25) x--;
                else if (r < 0.50) x++;
                else if (r < 0.75) y++;
                else               y--;
 
                // check if entered kill zone
                dist = Math.sqrt((N/2-x)*(N/2-x) + (N/2-y)*(N/2-y));
                if (dist >= Math.min((N-2)/2.0, radius + 25)) break;

                // check if neighboring site is occupied
                if (dla[x-1][y]   || dla[x+1][y]   || dla[x][y-1]   || dla[x][y+1]   ||
                    dla[x-1][y-1] || dla[x+1][y+1] || dla[x-1][y+1] || dla[x+1][y-1] ) {
                    dla[x][y] = true;
                    if (dist > radius) radius = dist;
                    break;
                }
            }

            if (dla[x][y]) {
                particles++;
                pic.set(x, y, colors[(particles / 76) % 256]);
                pic.show();
            }
        }
    }
}