VersionedFile.walk deprecated?

[Aaron Bentley]

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Martin Pool wrote:
> Just to expand on what I said before - the problem with walk() is  that
> it returns all lines in the history of the file, even those  which are
> irrelevant to the two versions we're trying to merge.   Touching them is
> not just specific to weave storage, but an  undesirable consequence,
> since we spend time walking lines we can't  possibly use.

Okay, but with weaves, we always have to read the weave from
top-to-bottom anyhow, I think.

> We can make a plan_merge for knits which does something similar, but 
> only deals with lines which can possibly be in the output.  It  requires
> annotated knits.  It should work like this:
> 
>   Do a two-way diff between the two versions to be merged.  Within  the
> difference regions, we need to determine whether each line is  unborn,
> live, or killed in the other side.  Taking the lines from the 
> left-hand-side conflict region first, get the line origins.

Depending on the costs involved, it might be worthwile to diff the
annotated version, to reduce false matches.

>  If that 
> origin is in the merged-revisions set of the right hand side, then  the
> line has been deleted (or moved) on the right hand side.   Otherwise,
> it's new on the left hand side.

Yes, that makes sense.  Something interesting about this process is it
doesn't seem to use line identity-- only position, text, and possibly
annotation.  I suppose it's harder to get identity out of knits, anyhow.

> This can still be improved for some criss-cross merge cases by 
> recording when the user explicitly choses a resolution to a conflict, 
> but I think this may give comparable results to current weave merge.

An interesting thought.  Are you thinking of
http://revctrl.org/MarkMerge and the way it distinguishes between choices?

Aaron
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