Function blackman_window 

pub fn blackman_window(
    n: usize,
    p: usize,
    rm: RoundingMode,
    cc: &mut Consts,
) -> Option<ExactNumArray>

Symmetric Blackman: 0.42 − 0.5 cos(θ) + 0.08 cos(2θ), θ = 2πk/(n−1).