Function turn 

fn turn(
    j1: i32,
    j2: i32,
    m1: i32,
    m2: i32,
    [minimum, maximum]: [i32; 2],
    [first, last]: [i32; 2],
) -> i32

The first degree visit recurs downward, as far as first..=last can tell: max(growth_end, min(switch, decay_start)) with each scan cut off where it no longer decides a degree in the range. Whether a j3 >= first lies below the turn is therefore the same for every range.