What's the fastest way to find Tags pointing to Commits? - libgit2

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}");
}
}
}