pos.IndexAtDepth() always equals pos.TraceIndex(pos.Depth())

[zhiqiangxu]

Simplify the code a bit by the invariant that pos.IndexAtDepth() == pos.TraceIndex(pos.Depth()) always holds.

[mbaxter]

/ci authorize e02af61

[ajsutton]

That invariant does not hold. pos.PositionAtIndex returns the index at the current depth of the position which may be above the split depth. ie gindex 2 has PositionAtIndex=0 but a much larger TraceIndex(30) since it follows the right child at each depth to reach depth 30.

[ajsutton]

Sorry, misread - at pos.Depth() yes that should hold, but now I'm suspicious why it's using pos.Depth instead of the split depth.

[ajsutton]

Ah I see - we're determining if we're to the left or right of a position from higher up. Given we explicitly want to compare the trace index of the two positions at the same depth, I'd argue the original code is clearer since it's then comparing the same thing (both position's TraceIndex at a specific depth) vs comparing differently named things that happen to be equivalent in this case.

[zhiqiangxu]

I'm OK with close this PR if @ajsutton thinks the original code is clearer. IMO the new code is more explicit in what it's comparing: the IndexAtDepth, but anyway it may be just a personal preference:)