Filling a map with a Spiral algorithm (description here http://www.commentcamarche.net/forum/affich-1824691-algorithme-niveau-signore)

Spiral.java

import java.awt.Point;
import java.util.Scanner;

public class Spiral {

	private final int n;
	private final int center;
	private final int[][] matrix;
	
	public Spiral(int n)
	{
		this.n = n;
		this.center = n/2;
		this.matrix = new int[n][n];
	}
	/**
	 * Move to the left square
	 */
	public Point left(Point p)
	{
		int x = p.x - 1;
		int y = p.y;
		return new Point(x, y);
	}
	/**
	 * Move to the right square
	 */
	public Point right(Point p)
	{
		int x = p.x + 1;
		int y = p.y;
		return new Point(x, y);
	}
	/**
	 * Move to the upper square
	 */
	public Point up(Point p)
	{
		int x = p.x;
		int y = p.y - 1;
		return new Point(x, y);
	}
	/**
	 * Move to the down square
	 */
	public Point down(Point p)
	{
		int x = p.x;
		int y = p.y + 1;
		return new Point(x, y);
	}
	/**
	 * Move to the next square
	 */
	public Point next(Point p)
	{
		if(p.y == center && p.x == center)
		{
			return right(p);
		}
		Point next = null;
		// find the corner of current inner matrix
		int diff = Math.max(Math.abs(p.x - center), Math.abs(p.y - center));
		int x1 = center - diff;
		int x2 = center + diff;
		int y1 = center - diff;
		int y2 = center + diff;
		System.out.println("diff: "+diff);
		System.out.println("(x1, y1): "+x1+","+y1);
		System.out.println("(x1, y2): "+x1+","+y2);
		System.out.println("(x2, y1): "+x2+","+y1);
		System.out.println("(x2, y2): "+x2+","+y2);
		// decide the next move
		if(p.x == x1)
		{
			if(p.y == y1)
			{
				next = down(p);
			}
			else if(p.y == y2)
			{
				next = right(p);
			}
			else 
			{
				next = down(p);
			}
		}
		else if(p.x == x2)
		{
			if(p.y == y1)
			{
				next = left(p);
			}
			else if(p.y == y2)
			{
				next = right(p);
			}
			else 
			{
				next = up(p);
			}
		}
		else if(p.y == y1)
		{
			next = left(p);
		}
		else
		{
			next = right(p);
		}
		System.out.println("next for "+p+" is "+next.x+","+next.y+" and center is "+center);
		return next;
	}
	/**
	 * Set the value of a given square
	 */
	public void set(int row, int column, int v)
	{
		matrix[row][column] = v;
	}
	/**
	 * Print current state of the matrix
	 */
	public void print()
	{		
		StringBuffer buf = null;
		for(int i=0; i<n; i++)
		{
			buf = new StringBuffer();
			for(int j=0; j<n; j++)
			{
				buf.append(matrix[i][j]).append(" ");
			}	
			System.out.println(buf.toString());
		}
	}
	/**
	 * Run the program
	 */
	public static void main(String[] args)
	{
		System.out.print("Type an odd number representing the matrix size: ");
		Scanner in = new Scanner(System.in);
		int n = in.nextInt(), center = n/2, val = 1;
		in.close();
		Spiral spiral = new Spiral(n);
		Point p = new Point(center, center);
		while(p.x < n && p.y < n)
		{
			spiral.set(p.y, p.x, val++);
			p = colimaçon.next(p);
		}
		colimaçon.print();
	}	
}