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

> Imran M Yousuf <imyousuf@gmail.com> 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.
>