fn rot13(c: u8) -> u8 {
    if c >= 97 && c <= 122 {
        return (c - 97 + 13) % 26 + 97
    }
    if c >= 65 && c <= 90 {
        return (c - 65 + 13) % 26 + 65
    }
    return c
}

pub fn main(world: World) -> Void raises {
    let source: Span<u8> = std.mem.span("Hello, zero!")
    var encoded: [12]u8 = [0_u8; 12]
    var decoded: [12]u8 = [0_u8; 12]
    var i: usize = 0
    while i < std.mem.len(source) {
        encoded[i] = rot13(source[i])
        decoded[i] = rot13(encoded[i])
        i = i + 1
    }
    if encoded[0] == 85 && encoded[1] == 114 && (encoded[7] == 109 && encoded[11] == 33 && (decoded[0] == source[0] && decoded[7] == source[7])) {
        check world.out.write("rot13 ok\n")
    }
}