void
ant(integer x, y, d, list map)
{
    while (-1 < x && x < 100 && -1 < y && y < 100) {
        integer e, p, w;
        data b;

        b = map[y];
        w = b[x >> 3];
        p = 1 << (7 - (x & 7));
        b[x >> 3] = w ^ p;

        d += w & p ? 1 : 3;

        e = d & 1;
        set(e, $e + ((d & 2) - 1) * (2 * e - 1));
    }
}

integer
main(void)
{
    file f;
    list l;

    call_n(100, lb_p_data, l, data().run(13, 0));
    ant(50, 50, 2, l);

    f.create("ant.pbm", 00644).text("P4\n100 100\n");
    l.ucall(f_data, 1, f);

    0;
}