1 files changed, 72 insertions, 0 deletions
diff --git a/app/models/projects/forks/divergence_counts.rb b/app/models/projects/forks/divergence_counts.rb
new file mode 100644
index 00000000000..7d630b00083
--- /dev/null
+++ b/app/models/projects/forks/divergence_counts.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+module Projects
+  module Forks
+    # Class for calculating the divergence of a fork with the source project
+    class DivergenceCounts
+      LATEST_COMMITS_COUNT = 10
+      EXPIRATION_TIME = 8.hours
+
+      def initialize(project, ref)
+        @project = project
+        @fork_repo = project.repository
+        @source_repo = project.fork_source.repository
+        @ref = ref
+      end
+
+      def counts
+        ahead, behind = divergence_counts
+
+        { ahead: ahead, behind: behind }
+      end
+
+      private
+
+      attr_reader :project, :fork_repo, :source_repo, :ref
+
+      def cache_key
+        @cache_key ||= ['project_forks', project.id, ref, 'divergence_counts']
+      end
+
+      def divergence_counts
+        fork_sha = fork_repo.commit(ref).sha
+        source_sha = source_repo.commit.sha
+
+        cached_source_sha, cached_fork_sha, counts = Rails.cache.read(cache_key)
+        return counts if cached_source_sha == source_sha && cached_fork_sha == fork_sha
+
+        counts = calculate_divergence_counts(fork_sha, source_sha)
+
+        Rails.cache.write(cache_key, [source_sha, fork_sha, counts], expires_in: EXPIRATION_TIME)
+
+        counts
+      end
+
+      def calculate_divergence_counts(fork_sha, source_sha)
+        # If the upstream latest commit exists in the fork repo, then
+        # it's possible to calculate divergence counts within the fork repository.
+        return fork_repo.diverging_commit_count(fork_sha, source_sha) if fork_repo.commit(source_sha)
+
+        # Otherwise, we need to find a commit that exists both in the fork and upstream
+        # in order to use this commit as a base for calculating divergence counts.
+        # Considering the fact that a user usually creates a fork to contribute to the upstream,
+        # it is expected that they have a limited number of commits ahead of upstream.
+        # Let's take the latest N commits and check their existence upstream.
+        last_commits_shas = fork_repo.commits(ref, limit: LATEST_COMMITS_COUNT).map(&:sha)
+        existence_hash = source_repo.check_objects_exist(last_commits_shas)
+        first_matched_commit_sha = last_commits_shas.find { |sha| existence_hash[sha] }
+
+        # If we can't find such a commit, we return early and tell the user that the branches
+        # have diverged and action is required.
+        return unless first_matched_commit_sha
+
+        # Otherwise, we use upstream to calculate divergence counts from the matched commit
+        ahead, behind = source_repo.diverging_commit_count(first_matched_commit_sha, source_sha)
+        # And add the number of commits a fork is ahead of the first matched commit
+        ahead += last_commits_shas.index(first_matched_commit_sha)
+
+        [ahead, behind]
+      end
+    end
+  end
+end