Function is_active_validator 

pub fn is_active_validator(
    validator_idx: usize,
    epoch: u64,
    set_size: usize,
    active_size: usize,
) -> bool

Dynamic validator rotation: is validator validator_idx in the active committee for epoch? A window of active_size validators rotates by one position per epoch over a set of set_size (round-robin), so committee membership churns deterministically. Indices and the window wrap modulo set_size.