Today is my 74th birthday. This Engineering Notebook post is my birthday
present to myself.
For the last week, I have been trying to make paste-retaining-clones work.
Unit tests show this command improperly computes v.parents arrays in the
pasted outline.
This morning I saw the way forward:
*Aha!* paste-retaining-clones must recompute *all* v.parents arrays in the
entire outline!
I won't attempt a formal proof, but I am sure this Aha is correct. Note
that the paste-node command does *not *require the complications described
next.
*Recomputing v.parents*
But how to recompute v.parents?
- We can't use Position generators because they all use v.parents!
- We can't use VNode generators because they don't yield VNodes in outline
order.
*Aha! *Define a *c.all_vnode_positions* generator. This *hybrid generator* will
yield VNodes in the same order as p.all_positions *using neither v.parents
nor Positions.*
Is it possible to write this generator? Yes, it is. The generator will
maintain an explicit stack just like Position.stack. *Note*:
c.all_vnode_positions must yield the hidden root VNode.
*Recomputing all v.parents*
Supposing the new generator exists, recomputing all v.parents arrays is
straightforward. Something like this (untested):
for v in c.all_vnode_positions():
v.parents = []
for v in c.all_vnode_positions():
for child in v.children:
child.parents.append(v)
Yes, a one-pass solution is possible. It *might* be faster.
*Summary*
*Aha!* The paste-retaining-clones command must recompute *all *v.parents
ivars. This recomputation is sound. The link correction in
c.checkVnodeLinks is not.
Leo can use neither Position nor VNode generators to do the recomputation.
*Aha!* A new generator, *c.all_vnode_positions*, will yield all VNodes in
outline order using neither v.parents nor Positions.
Given this generator, updating all v.parents ivars will be straightforward.
Edward
P.S. Let us define a *hybrid generator *as a generator yielding VNodes in
the same order as some Position generator *without *using Positions.
Could Leo replace Position generators with hybrids? I think not. For
example, please take a look qtree.full_redraw. It would not be fun to
eliminate Positions there.
In any event, Leo's Position class is never going away :-)
EKR
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