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Task

Implement the Drossel and Schwabl definition of the forest-fire model.



It is basically a 2D   cellular automaton   where each cell can be in three distinct states (empty, tree and burning) and evolves according to the following rules (as given by Wikipedia)

 A burning cell turns into an empty cell
 A tree will burn if at least one neighbor is burning
 A tree ignites with probability   f    even if no neighbor is burning
 An empty space fills with a tree with probability    p 

Neighborhood is the   Moore neighborhood;   boundary conditions are so that on the boundary the cells are always empty ("fixed" boundary condition).

At the beginning, populate the lattice with empty and tree cells according to a specific probability (e.g. a cell has the probability 0.5 to be a tree). Then, let the system evolve.

Task's requirements do not include graphical display or the ability to change parameters (probabilities    p    and    f )   through a graphical or command line interface.



Related tasks

   See   Conway's Game of Life 
   See   Wireworld.


import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
 
public class Fire {
	private static final char BURNING = 'w'; //w looks like fire, right?
	private static final char TREE = 'T';
	private static final char EMPTY = '.';
	private static final double F = 0.2;
	private static final double P = 0.4;
	private static final double TREE_PROB = 0.5;
 
	private static List<String> process(List<String> land){
		List<String> newLand = new LinkedList<String>();
		for(int i = 0; i < land.size(); i++){
			String rowAbove, thisRow = land.get(i), rowBelow;
			if(i == 0){//first row
				rowAbove = null;
				rowBelow = land.get(i + 1);
			}else if(i == land.size() - 1){//last row
				rowBelow = null;
				rowAbove = land.get(i - 1);
			}else{//middle
				rowBelow = land.get(i + 1);
				rowAbove = land.get(i - 1);
			}
			newLand.add(processRows(rowAbove, thisRow, rowBelow));
		}
		return newLand;
	}
 
	private static String processRows(String rowAbove, String thisRow,
			String rowBelow){
		String newRow = "";
		for(int i = 0; i < thisRow.length();i++){
			switch(thisRow.charAt(i)){
			case BURNING:
				newRow+= EMPTY;
				break;
			case EMPTY:
				newRow+= Math.random() < P ? TREE : EMPTY;
				break;
			case TREE:
				String neighbors = "";
				if(i == 0){//first char
					neighbors+= rowAbove == null ? "" : rowAbove.substring(i, i + 2);
					neighbors+= thisRow.charAt(i + 1);
					neighbors+= rowBelow == null ? "" : rowBelow.substring(i, i + 2);
					if(neighbors.contains(Character.toString(BURNING))){
						newRow+= BURNING;
						break;
					}
				}else if(i == thisRow.length() - 1){//last char
					neighbors+= rowAbove == null ? "" : rowAbove.substring(i - 1, i + 1);
					neighbors+= thisRow.charAt(i - 1);
					neighbors+= rowBelow == null ? "" : rowBelow.substring(i - 1, i + 1);
					if(neighbors.contains(Character.toString(BURNING))){
						newRow+= BURNING;
						break;
					}
				}else{//middle
					neighbors+= rowAbove == null ? "" : rowAbove.substring(i - 1, i + 2);
					neighbors+= thisRow.charAt(i + 1);
					neighbors+= thisRow.charAt(i - 1);
					neighbors+= rowBelow == null ? "" : rowBelow.substring(i - 1, i + 2);
					if(neighbors.contains(Character.toString(BURNING))){
						newRow+= BURNING;
						break;
					}
				}
				newRow+= Math.random() < F ? BURNING : TREE;
			}
		}
		return newRow;
	}
 
	public static List<String> populate(int width, int height){
		List<String> land = new LinkedList<String>();
		for(;height > 0; height--){//height is just a copy anyway
			StringBuilder line = new StringBuilder(width);
			for(int i = width; i > 0; i--){
				line.append((Math.random() < TREE_PROB) ? TREE : EMPTY);
			}
			land.add(line.toString());
		}
		return land;
	}
 
	//process the land n times
	public static void processN(List<String> land, int n){
		for(int i = 0;i < n; i++){
			land = process(land);
		}
	}
 
	//process the land n times and print each step along the way
	public static void processNPrint(List<String> land, int n){
		for(int i = 0;i < n; i++){
			land = process(land);
			print(land);
		}
	}
 
	//print the land
	public static void print(List<String> land){
		for(String row: land){
			System.out.println(row);
		}
		System.out.println();
	}
 
	public static void main(String[] args){
		List<String> land = Arrays.asList(".TTT.T.T.TTTT.T",
				"T.T.T.TT..T.T..",
				"TT.TTTT...T.TT.",
				"TTT..TTTTT.T..T",
				".T.TTT....TT.TT",
				"...T..TTT.TT.T.",
				".TT.TT...TT..TT",
				".TT.T.T..T.T.T.",
				"..TTT.TT.T..T..",
				".T....T.....TTT",
				"T..TTT..T..T...",
				"TTT....TTTTTT.T",
				"......TwTTT...T",
				"..T....TTTTTTTT",
				".T.T.T....TT...");
		print(land);
		processNPrint(land, 10);
 
		System.out.println("Random land test:");
 
		land = populate(10, 10);
		print(land);
		processNPrint(land, 10);
	}
}