define o() {
    auto i, j

    "P1 "
    w
    h
    for (j = 0; j < h; j++) {
        for (i = 0; i < w; i++) {
            a[j * w + i]
        }
    }
}

define l(w, h, x, y) {
    auto a[], d, i, x[], y[]

    /* d represents one of the four possible directions:
     *             0
     *             ⇑
     *           3⇐ ⇒1
     *             ⇓
     *             2
     * The arrays x[] and y[] contain the changes to the x and y direction for
     * each value of d.
     */
    x[1] = 1
    x[3] = -1
    y[0] = -1
    y[2] = 1

    while (1) {
        i = y * w + x
        if (a[i] == 0) d += 1   /* turn right if white */
        if (a[i] == 1) d -= 1   /* turn left if black */
        if (d < 0) d = 3
        if (d > 3) d = 0
        x += x[d]
        y += y[d]
        a[i] = 1 - a[i]         /* toggle cell colour */
        if (x < 0) break
        if (x == w) break
        if (y < 0) break
        if (y == h) break
    }

    o()
}

l(100, 100, 50, 50)
quit