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A "pvec" is a changeset property based on the theory of vector clocks
that can be compared to discover relatedness without consulting a
graph. This can be useful for tasks like determining how a
disconnected patch relates to a repository.
Currently a pvec consist of 448 bits, of which 24 are 'depth' and the
remainder are a bit vector. It is represented as a 70-character base85
string.
Construction:
- a root changeset has a depth of 0 and a bit vector based on its hash
- a normal commit has a changeset where depth is increased by one and
one bit vector bit is flipped based on its hash
- a merge changeset pvec is constructed by copying changes from one pvec into
the other to balance its depth
Properties:
- for linear changes, difference in depth is always <= hamming distance
- otherwise, changes are probably divergent
- when hamming distance is < 200, we can reliably detect when pvecs are near
Issues:
- hamming distance ceases to work over distances of ~ 200
- detecting divergence is less accurate when the common ancestor is very close
to either revision or total distance is high
- this could probably be improved by modeling the relation between
delta and hdist
Uses:
- a patch pvec can be used to locate the nearest available common ancestor for
resolving conflicts
- ordering of patches can be established without a DAG
- two head pvecs can be compared to determine whether push/pull/merge is needed
and approximately how many changesets are involved
- can be used to find a heuristic divergence measure between changesets on
different branches
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