4
Documentation on how this actually works is pretty scarce to say the
7
I *think* I understand their merge algorithm though, and it's pretty
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clever. Basically we do a two-way merge between annotated forms of
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the two files: that is, with each line marked with the revision in
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which it last changed. (I am simplifying here by speaking of lines
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and changes, but I don't think it causes any essential problem.)
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Now we walk through each file, line by line. If the change that
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introduced the line state in branch A is already merged into branch B,
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then we can just take B.
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Now is this actually better?
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It may be better in several ways:
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* Do not need to choose just a single ancestor, but rather can
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take advantage of all possible previous changes.
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* Can handle OTHER containing changes which have been merged into
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THIS, but have then been overwritten.
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* Can handle cherrypicks(!) by remembering which lines came in from
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that cherrypick; then we don't need to merge them again.
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* Do we actually need to store the annotations, or can we just infer
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it at the time we do the merge?
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* Can this be accomodated in something like an SCCS weave format? I
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think something like a weave may work, in as much as it is basically
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a concatenation of annotations, but I don't know if it represents
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merges well enough to cope.
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Can this handle binaries or type-specific merges, and if so how?
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Unmergeable binaries are easy: just get the user to pick one. Things
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like XML are harder; we probably need to punt out to a type-specific
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three-way merge. Of course this approach does not forbid also
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offering a 3-way merge.
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I suppose this could be accomodated by an annotation cache on top of
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plain history storage, or by using a storage format such as a weave
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that can efficiently produce annotation information.
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That is to say there is nothing inherently necessary about remembering
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the line history at the point when it is committed, except that it
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might be more efficient to do this once and remember it than to
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There is another interesting approach that can be used even in a tool
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that does not inherently remember annotations:
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Given two files to merge, find all regions of difference. For each
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such, try to find a common ancestor having the same content for the
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region. Subdivide the region if necessary.
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This naive approach is probably infeasible, since it would mean
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checking every possible predecessor.
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Rather than storing or calculating annotations, we could try using a
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complex weave, which allows one file version to be represented as a
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weave of multiple disjoint previous versions. It sounds complex but
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Essentially we store each file as a selection of lines that should be
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turned on in that file. These files might come from any of the
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predecessors that were merged into that file. Complex to get right
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This is written in terms of lines, but it might make more sense to
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just use byte ranges: perhaps more efficient when handling long files,
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and makes binaries less of a special case.
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codeville in fact does *not* seem to do this, though to me it seems
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like a fairly natural corollary of their design.
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This seems to imply holding the file text and ancestry of every branch
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that ever merged into this one, rather than just finding them if we
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later want them. Hm. That is nice in terms of doing smart merges.
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That possibly causes trouble in terms of having a name for these
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branches floating around inside our space, and making sure we don't
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clash with them. It may make sense in terms of having a working
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directory be just a view into a shared database, looking at a
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particular line of development.
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Indeed the main difficulty seems to be of naming branches in this
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space. Perhaps we should move back to using repositories and named
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branches within them, but not rely on branch names being unique out of
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the context of a single repository.
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Wow, this seems to open a big can of worms.
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So the conclusion is that this is very cool, but it does not require a
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fundamental change of model and can be implemented later.
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removed
doc/compared-codeville.txt
