Update tree structures to use "virtual root"?

[jeromekelleher]

I'm not sure how much has been done here with the representation of trees (if anything), but wanted to check if any updates were needed to take advantage of the virtual root (and perhaps traversal functions?) added in tskit-dev/tskit#1704

I'm happy to help where I can.

[molpopgen]

Nothing will/can be done until there's a stable C API release with this in place. Once that happens, we will have to deal with it. Because of how tskit is packaged, the C files have to be manually copied into this project (rather than via a submodule update).

[jeromekelleher]

We'll have a (hopefully last!) 0.99 release dropping soon which will include this. Mostly it should just be a case of making the x_array things one node longer.

[molpopgen]

Sounds good. I am optimistic that the RootIterator will be the only thing affected--we'll see!

[molpopgen]

#183 updated the C API and only the root iterator initialization needed changing. I used the "get left root" function for that. Keeping this issue open in case something else pops up...

[molpopgen]

Note: virtual root is a tsk_id_t, so we'll get to re-use our macros to make a newtype out of this.

[molpopgen]

Note: virtual root is a tsk_id_t, so we'll get to re-use our macros to make a newtype out of this.

Actually, it is a "root"-like thing, so NodeId covers it.