module SP

function rule90(filename, size)
    height = 2 ^ size
    width = height * 2 - 1
    x = zeros(Bool, width + 2)
    x[height+1] = true
    y = copy(x)
    open(filename, "w") do f
        println(f, "P1")
        println(f, "$width $height")
        for i in 1:height
            for j in 1:width
                println(f, y[j+1] ? "1 " : "0 ")
                y[j+1] = x[j] ⊻ x[j+2]
            end
            println(f, "")
            x, y = y, x
        end
    end
end

# julia> SP.rule90("/tmp/test.pbm", 9)

end