Function region 

pub fn region(
    anchor: (u32, u32),
    need: Need,
    density: Density,
    fill: f64,
    parent: &[Rect],
) -> Option<Rect>

The square that holds need at fill of density, every site of it in parent, nearest anchor: the least side whose sites hold the need, then of its placements the one nearest the anchor (Chebyshev), then the one whose centre is nearest, then the lowest. None when no placement of that side fits, or the density holds none of what is needed.