tree.is_root() method

[hyanwong]

It's tempting to check whether a node is a root by looking for u in tree.roots without realising that tree.roots might be huge. I think a more efficient way to check is

def is_root(self, u)
    return self.num_samples(u) > 0 and self.parent(u) == tskit.NULL

Is that right @jeromekelleher ? This might be useful for @szhan in #2618

[jeromekelleher]

Yes I agree this is useful. Your code is almost right, we just have to change the num_samples > 0 to take the root threshold into account. (I can't remember what the motivation for root threshold is, it's in the git blame presumably!)

[hyanwong]

Oh yes, I forgot about that. So many complex parts!

[hyanwong]

So I guess:

def is_root(self, u)
    return self.num_samples(u) >= self.root_threshold and self.parent(u) == tskit.NULL