1 files changed, 82 insertions, 0 deletions
diff --git a/app/finders/repositories/changelog_tag_finder.rb b/app/finders/repositories/changelog_tag_finder.rb
new file mode 100644
index 00000000000..3c110e6c65d
--- /dev/null
+++ b/app/finders/repositories/changelog_tag_finder.rb
@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+module Repositories
+  # A finder class for getting the tag of the last release before a given
+  # version, used when generating changelogs.
+  #
+  # Imagine a project with the following tags:
+  #
+  # * v1.0.0
+  # * v1.1.0
+  # * v2.0.0
+  #
+  # If the version supplied is 2.1.0, the tag returned will be v2.0.0. And when
+  # the version is 1.1.1, or 1.2.0, the returned tag will be v1.1.0.
+  #
+  # To obtain the tags, this finder requires a regular expression (using the re2
+  # syntax) to be provided. This regex must produce the following named
+  # captures:
+  #
+  # - major (required)
+  # - minor (required)
+  # - patch (required)
+  # - pre
+  # - meta
+  #
+  # If the `pre` group has a value, the tag is ignored. If any of the required
+  # capture groups don't have a value, the tag is also ignored.
+  class ChangelogTagFinder
+    def initialize(project, regex: Gitlab::Changelog::Config::DEFAULT_TAG_REGEX)
+      @project = project
+      @regex = regex
+    end
+
+    def execute(new_version)
+      tags = {}
+      versions = [new_version]
+
+      begin
+        regex = Gitlab::UntrustedRegexp.new(@regex)
+      rescue RegexpError => ex
+        # The error messages produced by default are not very helpful, so we
+        # raise a better one here. We raise the specific error here so its
+        # message is displayed in the API (where we catch this specific
+        # error).
+        raise(
+          Gitlab::Changelog::Error,
+          "The regular expression to use for finding the previous tag for a version is invalid: #{ex.message}"
+        )
+      end
+
+      @project.repository.tags.each do |tag|
+        matches = regex.match(tag.name)
+
+        next unless matches
+
+        # When using this class for generating changelog data for a range of
+        # commits, we want to compare against the tag of the last _stable_
+        # release; not some random RC that came after that.
+        next if matches[:pre]
+
+        major = matches[:major]
+        minor = matches[:minor]
+        patch = matches[:patch]
+        build = matches[:meta]
+
+        next unless major && minor && patch
+
+        version = "#{major}.#{minor}.#{patch}"
+        version += "+#{build}" if build
+
+        tags[version] = tag
+        versions << version
+      end
+
+      VersionSorter.sort!(versions)
+
+      index = versions.index(new_version)
+
+      tags[versions[index - 1]] if index&.positive?
+    end
+  end
+end