Add tsk_tree_seek_index

[jeromekelleher]

We never added a method in the C API to seek efficiently to the tree at the kth index. The implementation in the Python API is currently quite naive:

tskit/python/tskit/trees.py

Line 811 in ec992da

def seek_index(self, index):

[molpopgen]

This can probably be rolled into a PR adding constant-time seeking to a tree. I think it will make sense to use this function to advance to a tree by position, one we have the indexes, etc., in place.

[jeromekelleher]

We can implement this straightforwardly by using the breakpoints array, i.e.,

int tsk_tree_seek_index(self, index)
{
      double pos = self->tree_sequence->breakpoints[index]; /* or maybe +1? */
      return tsk_tree_seek(self, pos);
}

We can focus our efforts then on making the "seek to position" efficient.

[molpopgen]

We can implement this straightforwardly by using the breakpoints array, i.e.,

int tsk_tree_seek_index(self, index)
{
      double pos = self->tree_sequence->breakpoints[index]; /* or maybe +1? */
      return tsk_tree_seek(self, pos);
}

We can focus our efforts then on making the "seek to position" efficient.

Dammit -- I wish I knew that that existed.