What's the fastest way to find Tags pointing to Commits?

Question

With libgit2sharp I would like to do the following:

foreach( Commit commit in repo.Commits )
{
    // How to implement assignedTags?
    foreach( Tag tag in commit.assignedTags ) {}
}

I want to get all tags assigned to the current commit. Whats the best way to do that? Iterate through all Tags and see if tag.Target.Sha == commit.Sha? Thats not very performant. Is there another way?

Answer 1

So I want to get all tags assigned to the current commit. Whats the best way to do that? Iterate through all Tags and see if tag.Target.Sha == commit.Sha? Thats not very performant. Is there another way?

There are two things to take into account when it comes to Tags.

• A Tag can point to something else than a Commit (A Tree or a Blob, for instance)
• A Tag can point to another Tag (chained annotated tags)

The code below should fit your need by taking these points above into account.

Note: repo.Commits will only enumerate the commits reachable from the current branch (HEAD). The code below extends this to easily browse all the reachable commits.

...

using (var repo = new Repository("Path/to/your/repo"))
{
    // Build up a cached dictionary of all the tags that point to a commit
    var dic = TagsPerPeeledCommitId(repo);

    // Let's enumerate all the reachable commits (similarly to `git log --all`)
    foreach (Commit commit in repo.Commits.QueryBy(new CommitFilter {Since = repo.Refs}))
    {
        foreach (var tags in AssignedTags(commit, dic))
        {
            Console.WriteLine("Tag {0} points at {1}", tags.Name, commit.Id);
        }
    }
}

....

private static IEnumerable<Tag> AssignedTags(Commit commit, Dictionary<ObjectId, List<Tag>> tags)
{
    if (!tags.ContainsKey(commit.Id))
    {
        return Enumerable.Empty<Tag>();
    }

    return tags[commit.Id];
}

private static Dictionary<ObjectId, List<Tag>> TagsPerPeeledCommitId(Repository repo)
{
    var tagsPerPeeledCommitId = new Dictionary<ObjectId, List<Tag>>();

    foreach (Tag tag in repo.Tags)
    {
        GitObject peeledTarget = tag.PeeledTarget;

        if (!(peeledTarget is Commit))
        {
            // We're not interested by Tags pointing at Blobs or Trees
            continue;
        }

        ObjectId commitId = peeledTarget.Id;

        if (!tagsPerPeeledCommitId.ContainsKey(commitId))
        {
            // A Commit may be pointed at by more than one Tag
            tagsPerPeeledCommitId.Add(commitId, new List<Tag>());
        }

        tagsPerPeeledCommitId[commitId].Add(tag);
    }

    return tagsPerPeeledCommitId;
}

Answer 2

Here is another version of nulltoken's answer but with using ILookupclass instead of dictionary. A bit nicer IMO:

private static ILookup<ObjectId, Tag> CreateCommitIdToTagLookup(Repository repo)
{
    var commitIdToTagLookup =
        repo.Tags
        .Select(tag => new { Commit = tag.PeeledTarget as Commit, Tag = tag })
        .Where(x => x.Commit != null)
        .ToLookup(x => x.Commit.Id, x => x.Tag);

    return commitIdToTagLookup;
}

and simple usage example:

using (var repo = new Repository("Path/to/your/repo"))
{
    var commitIdToTagLookup = CreateCommitIdToTagLookup(repo);

    foreach (var commit in repo.Commits)
    {
        foreach (var tag in commitIdToTagLookup[commit.Id])
        {
            Console.WriteLine($"Tag {tag.FriendlyName} points at {commit.Id}");
        }
    }
}