Add TreeSequence.max_time, min_time attributes

[jeromekelleher]

There are many situations where it's handy to know what the max and min times are, and it would be nice if we could avoid having to do a full lookup of the np.max(ts.tables.nodes.time) to do it.

I think we can reasonably add these as attributes of the tsk_treeseq_t struct in C, filling them in during startup.

We can make them available to Python then easily, too.

[benjeffery]

I've put this in on the before-paper milestone as it seems an obviously useful thing to do.

[hyanwong]

Do we also want the minimum and maximum time only counting nodes in trees? That might not be the same as np.max(ts.tables.nodes.time) and np.min(ts.tables.nodes.time), if there are non-sample nodes that are not in the edge tables.

it's a slightly more complex calculation, so seems even more worthwhile caching.

[jeromekelleher]

What would you call this? ts.max_time has a pretty clear interpretation. I guess ts.max_tree_node_time?

What are the applications?

[hyanwong]

We have ts.max_root_time already, which I think is the maximum, right? so it's only the minimum that is missing. I can't remember if max_root_time is cached or not though.

A trivial application is to find the best scales for plotting. I'm sure there are lots of others though. My guess is that most of the time you want the min/max time of nodes in the trees, and want to omit the nodes that aren't in the topologies. In fact, most people will assume that max_time means max_root_time, won't they? And of course it will be as long as the ts is simplified.

[hyanwong]

A couple of extra thoughts here before we make this concrete:

1. Should max_time include the max time of mutations too? If not, then should it actually be named max_node_time (and equivalently, min_node_time)?
2. do we count unreferenced non-sample nodes (i.e. isolated everywhere) as "in the tree sequence"? I can't remember if we actually state this anywhere. It could be argued that tree_sequence.max_time is the max time of any mutation or any node that is in the sequence of trees (i.e. only counting referenced or sampled nodes, and mutations above them)

[petrelharp]

My two cents (mostly, for not complicating things):

1. Should max_time include the max time of mutations too?

Sure, why not.

2. do we count unreferenced non-sample nodes (i.e. isolated everywhere) as "in the tree sequence"?

Yes, and they should not be missed out (unless you have a use case where such nodes are in the tree-sequence-data-structure and have very weird times?).

[jeromekelleher]

I think we should keep ts.max_time simple - it is the maximum of the times of the entities in the data model (and conversely for min_time) - np.max([np.max(ts.tables.nodes.time), np.max(ts.tables.mutations.time)])

If there are other things we want, let's open a separate issue for them so that this issue can be closed by a single PR.

[hyanwong]

Great, if it includes the mutation times too, then I can see max_time and min_time being reasonable names. I think it should be documented about the possibility of them not being in the sequence of trees, though.

I assume that the time of the virtual root, since it is not actually stored in a table (I think?) is not considered to be part of the data model.

[jeromekelleher]

I assume that the time of the virtual root, since it is not actually stored in a table (I think?) is not considered to be part of the data model.

That's right - the time of the virtual root is not stored, and defined as positive infinity.