fn xorSmall(left: u32, right: u32) -> u32 {
    var a: u32 = left
    var b: u32 = right
    var bit: u32 = 1
    var out: u32 = 0
    while a > 0 || b > 0 {
        if a % 2 != b % 2 {
            out = out + bit
        }
        a = a / 2
        b = b / 2
        bit = bit * 2
    }
    return out
}

fn gray(n: u32) -> u32 {
    return xorSmall(n, n / 2)
}

pub fn main(world: World) -> Void raises {
    var ok: Bool = true
    let expected: [8]u32 = [0, 1, 3, 2, 6, 7, 5, 4]
    var i: usize = 0
    while i < 8 {
        if gray(i as u32) != expected[i] {
            ok = false
        }
        i = i + 1
    }
    if ok {
        check world.out.write("gray code ok\n")
    }
}