Function regions 

pub fn regions(
    npos: usize,
    span: usize,
    ovl: usize,
) -> Vec<(usize, usize, usize)>

구역: start = 0, SPAN, …; [start−OVL, start+SPAN+OVL) ∩ [0, NPOS). 완료 위치 = hi−1.