1 files changed, 97 insertions, 0 deletions
diff --git a/app/models/label_note.rb b/app/models/label_note.rb
new file mode 100644
index 00000000000..680952cf421
--- /dev/null
+++ b/app/models/label_note.rb
@@ -0,0 +1,97 @@
+# frozen_string_literal: true
+
+class LabelNote < Note
+  attr_accessor :resource_parent
+  attr_reader :events
+
+  def self.from_events(events, resource: nil, resource_parent: nil)
+    resource ||= events.first.issuable
+
+    attrs = {
+      system: true,
+      author: events.first.user,
+      created_at: events.first.created_at,
+      discussion_id: events.first.discussion_id,
+      noteable: resource,
+      system_note_metadata: SystemNoteMetadata.new(action: 'label'),
+      events: events,
+      resource_parent: resource_parent
+    }
+
+    if resource_parent.is_a?(Project)
+      attrs[:project_id] = resource_parent.id
+    end
+
+    LabelNote.new(attrs)
+  end
+
+  def events=(events)
+    @events = events
+
+    update_outdated_markdown
+  end
+
+  def cached_html_up_to_date?(markdown_field)
+    true
+  end
+
+  def note
+    @note ||= note_text
+  end
+
+  def note_html
+    @note_html ||= "<p dir=\"auto\">#{note_text(html: true)}</p>"
+  end
+
+  def project
+    resource_parent if resource_parent.is_a?(Project)
+  end
+
+  def group
+    resource_parent if resource_parent.is_a?(Group)
+  end
+
+  private
+
+  def update_outdated_markdown
+    events.each do |event|
+      if event.outdated_markdown?
+        event.refresh_invalid_reference
+      end
+    end
+  end
+
+  def note_text(html: false)
+    added = labels_str('added', label_refs_by_action('add', html))
+    removed = labels_str('removed', label_refs_by_action('remove', html))
+
+    [added, removed].compact.join(' and ')
+  end
+
+  # returns string containing added/removed labels including
+  # count of deleted labels:
+  #
+  # added ~1 ~2 + 1 deleted label
+  # added 3 deleted labels
+  # added ~1 ~2 labels
+  def labels_str(prefix, label_refs)
+    existing_refs = label_refs.select { |ref| ref.present? }.sort
+    refs_str = existing_refs.empty? ? nil : existing_refs.join(' ')
+
+    deleted = label_refs.count - existing_refs.count
+    deleted_str = deleted == 0 ? nil : "#{deleted} deleted"
+
+    return nil unless refs_str || deleted_str
+
+    label_list_str = [refs_str, deleted_str].compact.join(' + ')
+    suffix = 'label'.pluralize(deleted > 0 ? deleted : existing_refs.count)
+
+    "#{prefix} #{label_list_str} #{suffix}"
+  end
+
+  def label_refs_by_action(action, html)
+    field = html ? :reference_html : :reference
+
+    events.select { |e| e.action == action }.map(&field)
+  end
+end