Spiral.java

private static class Vec {
    int x, y;
    public Vec(int x, int y) { this.x = x; this.y = y; }
    public Vec add(Vec oth) { return new Vec(x+oth.x, y+oth.y); }
    public Vec rot90CW() { return new Vec(y, -x); }
    public int infDist(Vec oth) { return Math.max(x-oth.x, y-oth.y); }
    public boolean equals....
}

// Returns spiral pattern in a (2n+1) * (2n+1) array
/*


.......
...***.
...*.*.
...*.*.
.....*.
.*****.
.......
 */
public boolean[][] makeSpiral(int n) {
    int dim = 2*n + 1;
    boolean[][] board = new boolean[dim][dim]; // initialized to all false

    Vec cent = new Vec(dim / 2, dim / 2);
    Vec pos = cent;

    Vec dir = new Vec(0, -1);

    int curRadius = 2;

    board[pos.y][pos.x] = true;
    while (radius <= n) {
        while (!pos.equals(cent.add(new Vec(curRadius, curRadius)))) {
            pos = pos.add(dir);
            board[pos.y][pos.x] = true;

            if (pos.add(dir).infDist(cent) > radius) {
                dir = dir.rot90CW();
            }
        }
        radius += 2;
    }

    
}