1 files changed, 94 insertions, 0 deletions
diff --git a/lib/gitlab/quick_actions/users_extractor.rb b/lib/gitlab/quick_actions/users_extractor.rb
new file mode 100644
index 00000000000..06e04c74312
--- /dev/null
+++ b/lib/gitlab/quick_actions/users_extractor.rb
@@ -0,0 +1,94 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module QuickActions
+    class UsersExtractor
+      MAX_QUICK_ACTION_USERS = 100
+
+      Error = Class.new(ArgumentError)
+      TooManyError = Class.new(Error) do
+        def limit
+          MAX_QUICK_ACTION_USERS
+        end
+      end
+
+      MissingError = Class.new(Error)
+      TooManyFoundError = Class.new(TooManyError)
+      TooManyRefsError = Class.new(TooManyError)
+
+      attr_reader :text, :current_user, :project, :group, :target
+
+      def initialize(current_user, project:, group:, target:, text:)
+        @current_user = current_user
+        @project = project
+        @group = group
+        @target = target
+        @text = text
+      end
+
+      def execute
+        return [] unless text.present?
+
+        users = collect_users
+
+        check_users!(users)
+
+        users
+      end
+
+      private
+
+      def collect_users
+        users = []
+        users << current_user if me?
+        users += find_referenced_users if references.any?
+
+        users
+      end
+
+      def check_users!(users)
+        raise TooManyFoundError if users.size > MAX_QUICK_ACTION_USERS
+
+        found = found_names(users)
+        missing = references.filter_map do
+          "'#{_1}'" unless found.include?(_1.downcase.delete_prefix('@'))
+        end
+
+        raise MissingError, missing.to_sentence if missing.present?
+      end
+
+      def found_names(users)
+        users.map(&:username).map(&:downcase).to_set
+      end
+
+      def find_referenced_users
+        raise TooManyRefsError if references.size > MAX_QUICK_ACTION_USERS
+
+        User.by_username(usernames).limit(MAX_QUICK_ACTION_USERS)
+      end
+
+      def usernames
+        references.map { _1.delete_prefix('@') }
+      end
+
+      def references
+        @references ||= begin
+          refs = args - ['me']
+          # nb: underscores may be passed in escaped to protect them from markdown rendering
+          refs.map! { _1.gsub(/\\_/, '_') }
+          refs
+        end
+      end
+
+      def args
+        @args ||= text.split(/\s|,/).map(&:strip).select(&:present?).uniq - ['and']
+      end
+
+      def me?
+        args&.include?('me')
+      end
+    end
+  end
+end
+
+Gitlab::QuickActions::UsersExtractor.prepend_mod_with('Gitlab::QuickActions::UsersExtractor')