Improve nbgv get-commits <version> perf

This improves scalability (cuts memory required by half for large repos) and streaming performance as we can print valid answers before we have found them all.

Author: AArnott

src/NerdBank.GitVersioning/GitExtensions.cs

@@ -336,7 +336,7 @@ public static IEnumerable<Commit> GetCommitsFromVersion(this Repository repo, Ve
             Requires.NotNull(version, nameof(version));
 
             var tracker = new GitWalkTracker(repoRelativeProjectDirectory);
-            var possibleCommits = from commit in GetCommitsReachableFromRefs(repo).Distinct()
+            var possibleCommits = from commit in GetCommitsReachableFromRefs(repo)
                                   let commitVersionOptions = tracker.GetVersion(commit)
                                   where commitVersionOptions != null
                                   where !IsCommitIdMismatch(version, commitVersionOptions, commit)
@@ -830,45 +830,29 @@ private static IEnumerable<Commit> GetCommitsReachableFromRefs(Repository repo)
         {
             Requires.NotNull(repo, nameof(repo));
 
-            var commits = new HashSet<Commit>();
+            var visitedCommitIds = new HashSet<ObjectId>();
+            var breadthFirstQueue = new Queue<Commit>();
+
+            // Start the discovery with HEAD, and all commits that have refs pointing to them.
+            breadthFirstQueue.Enqueue(repo.Head.Tip);
             foreach (var reference in repo.Refs)
             {
                 var commit = reference.ResolveToDirectReference()?.Target as Commit;
-                if (commit != null)
+                if (commit is object)
                 {
-                    AddReachableCommitsFrom(commit, commits);
+                    breadthFirstQueue.Enqueue(commit);
                 }
             }
 
-            return commits;
-        }
-
-        /// <summary>
-        /// Adds a commit and all its ancestors to a set.
-        /// </summary>
-        /// <param name="startingCommit">The starting commit to add.</param>
-        /// <param name="set">
-        /// The set into which the <paramref name="startingCommit"/>
-        /// and all its ancestors are to be added.
-        /// </param>
-        private static void AddReachableCommitsFrom(Commit startingCommit, HashSet<Commit> set)
-        {
-            Requires.NotNull(startingCommit, nameof(startingCommit));
-            Requires.NotNull(set, nameof(set));
-
-            var stack = new Stack<Commit>();
-            stack.Push(startingCommit);
-            while (stack.Count > 0)
+            while (breadthFirstQueue.Count > 0)
             {
-                var currentCommit = stack.Pop();
-                if (set.Add(currentCommit))
+                Commit head = breadthFirstQueue.Dequeue();
+                if (visitedCommitIds.Add(head.Id))
                 {
-                    foreach (var parent in currentCommit.Parents)
+                    yield return head;
+                    foreach (Commit parent in head.Parents)
                     {
-                        if (!set.Contains(parent))
-                        {
-                            stack.Push(parent);
-                        }
+                        breadthFirstQueue.Enqueue(parent);
                     }
                 }
             }

src/nbgv/Program.cs

@@ -530,7 +530,7 @@ private static ExitCodes OnGetCommitsCommand(string projectPath, string version,
             }
 
             string repoRelativeProjectDir = GetRepoRelativePath(searchPath, repository);
-            var candidateCommits = GitExtensions.GetCommitsFromVersion(repository, parsedVersion, repoRelativeProjectDir).ToList();
+            var candidateCommits = GitExtensions.GetCommitsFromVersion(repository, parsedVersion, repoRelativeProjectDir);
             PrintCommits(quiet, searchPath, repository, candidateCommits);
 
             return ExitCodes.OK;
@@ -772,7 +772,7 @@ private static string ShouldHaveTrailingDirectorySeparator(string path)
             return path + Path.DirectorySeparatorChar;
         }
 
-        private static void PrintCommits(bool quiet, string projectDirectory, LibGit2Sharp.Repository repository, List<LibGit2Sharp.Commit> candidateCommits, bool includeOptions = false)
+        private static void PrintCommits(bool quiet, string projectDirectory, LibGit2Sharp.Repository repository, IEnumerable<LibGit2Sharp.Commit> candidateCommits, bool includeOptions = false)
         {
             int index = 1;
             foreach (var commit in candidateCommits)