annotate & stacking

[John Arbash Meinel]

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Robert Collins wrote:
| So,
|     knit.py has an Annotator object that looks inside knits to perform
| annotations. This appears to duplicate a bunch of logic and also looks
| inside the knit at the _index object etc.
|
| To stack cleanly, I need something that can live within the knit, and
| then build on top of the public VersionedFiles interface (or indeed
| something that lives solely outside and using the public interface).
|
| John - I know you have been working on annotate() recently; are we are a
| position to remove annotate() from VersionedFiles, or otherwise tweak
| things to meet the constraints above?
|
| -Rob

There is quite a bit that is knit/pack specific about _KnitAnnotator.
There are parts that could be made more generic, but it certainly
expects knit-deltas (for example).

It would certainly be possible to write an Annotator class that would
just process fulltexts and have it be fairly simple. However, I'm
guessing it would be quite a bit slower. Being able to re-use the
existing deltas for left-hand parents is a pretty big win for a lot of
the test files I was using.

However, if we go with a full "policy" based system, then we may not
even be able to use the left-hand blocks, and if the underlying storage
uses xdelta, it won't have something we can use either. (xdelta is much
more of a gzip across 2 texts, and throw out the first set, so you get
stuff like RLE, etc.)

So... I would probably go with a generic Annotator that just works on
full texts and ancestries, and implement a custom one that can take
advantage of more intimate knowledge of the layout of knits/packs.

Also, some stuff is pulled into the annotator, so that it can be smart
about which pieces need to be cached, and what can be freed, etc.

John
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