VCS comparison table
peff at peff.net
Thu Oct 26 12:13:39 BST 2006
On Thu, Oct 26, 2006 at 12:52:05PM +0200, Vincent Ladeuil wrote:
> Ok, so git make a distinction between the commit (code created by
> someone) and the tree (code only).
Yes (a commit is a tree, zero or more parents, commit message, and
> Commits are defined by their parents.
> Trees are defined by their content only ?
> Calculate a sha1 representing the content (or the content of the
> diff from parent) of all the files and dirs in the tree ? Or
> from the sha1s of the files and dirs themselves recursively based
> on sha1s of the files and dirs they contain ?
Recursively. Each tree is an ordered list of 4-tuples: pathname, type,
sha1, mode. If the type is "blob" then the sha1 is the hash of the file
contents. If the type is "tree" then the sha1 is the id of a sub-tree.
The id of a tree is the sha1 hash of the data structure.
> I ask because the later seems to provide some nice effects
> similar to what makes BDD
> (http://en.wikipedia.org/wiki/Binary_decision_diagram) so
> efficient: you can compare graphs of any complexity or size in
> O(1) by just comparing their signatures.
Yes, if two trees' hashes compare equal, they contain the same data. I
believe we are not currently using this optimization to find merge
differences, but there was some discussion earlier this week about doing
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