lib.rs

extern "C" {
    // idx goes from 0..size()
    // color is 0xRRGGBB
    fn set_pixel(idx: u32, color: u32);
    fn get_pixel(idx: u32) -> u32;
    fn size() -> u32;
    fn time() -> u64; // unix ms wall clock
}

#[no_mangle]
#[allow(clippy::missing_safety_doc)]
pub unsafe extern "C" fn tick(time: f32) {
    // time is elapsed seconds, starting at 0
    let p = (time * 5.0) as u32;
    for i in 0..size() {
        if i == p {
            set_pixel(i, 0xFFFFFF);
        } else {
            set_pixel(i, 0x000000);
        }
    }
}