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

On Wed, Jan 7, 2009 at 10:04 AM, Shawn O. Pearce [off-list ref] wrote:

Imran M Yousuf [off-list ref] wrote:

quoted

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.

Thanks Shawn, for pointing it out and it actually does improve the
performance, for every lookup its like 200ms.

Best regards,

Imran

quoted

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.

quoted

                    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.

-- 
Imran M Yousuf
Entrepreneur & Software Engineer
Smart IT Engineering
Dhaka, Bangladesh
Email: imran@smartitengineering.com
Blog: http://imyousuf-tech.blogs.smartitengineering.com/
Mobile: +880-1711402557