Subject: Re: [JGIT RFC] How read versions of a specific object

From: "Shawn O. Pearce" <[email-masked]>
Date: Tue, 6 Jan 2009 20:04:17 -0800

Imran M Yousuf <[email-masked]> wrote:
> I am trying to read all or n-th version of an object. Currently to do
> this I am using the following piece of code, which has to walk to
> every commit is present and from there prepare a set of its object id,
> it is definitely expensive if the commit history is huge, is there a
> faster/better way to achieve it?

Not really. You can more efficiently use JGit and reduce some of
the overheads, but that's about it.

> for (int i = 0; i < App.OBJECT_COUNT;
>             ++i) {
>             ObjectWalk objectWalk = new ObjectWalk(repo);

Don't use ObjectWalk, use a RevWalk.  You don't need it to keep
track of tree or blob identities.  The ObjectWalk code has more
overhead to do that bookkeeping.

>                     Commit revision = repo.mapCommit(revObject.getId());
>                     Tree versionTree = repo.mapTree(revision.getTreeId());
>                     if (versionTree.existsBlob(isbn)) {
>                         revisions.add(versionTree.findBlobMember(isbn).getId());

Use a TreeWalk to do this.  Its quicker because it doesn't
have to parse as much data to come up with the same result.

More specifically there's a static factory method that sets up for
a path limited walk and returns the TreeWalk pointing at that entry.

You can use the fact that RevWalk.next() returns a RevCommit to get
you the RevTree, which is the tree you need to give to the TreeWalk
constructor (its the root level tree of the commit).

But if App.OBJECT_COUNT is quite large and covers most of your
objects, you are probably better off using a loop over the commits
and diff'ing against the ancestor:

	final HashMap<String, Set<ObjectId>> versions = ...;
	final RevWalk rw = new RevWalk(repo);
	final TreeWalk tw = new TreeWalk(repo);
	rw.markStart(rw.parseCommit(repo.parse(HEAD)));
	tw.setFilter(TreeFilter.ANY_DIFF);

	RevCommit c;
	while ((c = rw.next()) != null) {
		final ObjectId[] p = new ObjectId[c.getParentCount() + 1];
		for (int i = 0; i < c.getParentCount(); i++) {
			rw.parse(c.getParent(i));
			p[i] = c.getParent(i).getTree();
		}
		final int me = p.length -1;
		p[me] = c.getTree();
		tw.reset(p);
		while (tw.next()) {
			if (tw.getFileMode(me).getObjectType() == Constants.OBJ_BLOB) {
				// This path was modified relative to the ancestor(s).
				//
				String s = tw.getPathString();
				Set<ObjectId> i = versions.get(s);
				if (i == null)
					versions.put(s, i = new HashSet<ObjectId>());
				i.add(tw.getObjectId(me));
			}

			if (tw.isSubtree()) {
				// make sure we recurse into modified directories
				tw.enterSubtree();
			}
		}
	}

-- 
Shawn.