Dotted revno "algebra"

[John Arbash Meinel]

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> These things are a bit more like searches than original names, in the
> sense that they may not be guaranteed unique (indeed may be N:N) and
> they're not necessarily cheap to calculate for all revisions when eg
> printing a log.  In that case I think you would normally want
> something that is extremely cheap to calculate just from the revision
> id.  At which point I would probably come back to using the revision
> object hash from the current format, which won't be stable across
> revision representation changes, but will be stable across branches,
> and istm that we could both print that and look it up quite cheaply.
> 

I'm not sure what makes you think that the revision object hash in the
current format is cheap to look up. We can do it with a new index.
Though we could do the same thing with the hash of the revision_id, and
get one that is stable across representation changes...

And both would suffer from non-locality in the index, though it
certainly would be okay if you only look up a few at a time (which is
usually the case, I would expect.)

It would be significantly cheaper to compute (since you don't have to
read the revision text).

You would lose any sort of commit integrity that people may expect to be
getting.

I don't see a huge win over just using revision-ids, other than being
able to use less than say 10 chars. But if you have to copy and paste,
and can't really hold the value in your head, it doesn't make a huge
difference, IMO.

John
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