packs and knits

[Aaron Bentley]

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Robert Collins wrote:
> Now, we don't want derivable data in the packs *eventually*, but I think
> a good start would be to get to the point of having optional indices.
> Then we can work on making semi derived data cleanly rederivable.

It seems to me that making derivable data *optional* is useful, but some
derivable data is, nonetheless, expensive to derive.  I think that
discarding such calculations in all cases would be a mistake.

> Now, currently there can only be one representation of a knit delta to
> construct a given text

If you mean that records can only be in knit delta format, that is
correct.  If you mean that, in knit delta format, there is only one way
to represent the delta between two texts, that is not true in the
general case.  Different approaches to sequence matching will produce
different deltas most of the time.

> so it makes sense to me to name the record
> ('text', fileid, revisionid). (for a revision knit that would be
> ('revision', revisionid, etc) - just namespacing.

How would you name a fulltext?

> If this makes sense I'll put a patch together to do a Pack version bump
> and API bump to allow tuple based naming.

I think tuple-based naming would be very nice, and I wish I'd had it for
 Bundle format 4.

How will your proposed bumps affect Bundles?

Aaron
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