New 'unseenStatus' field for certain secondary message types

ts/MessageSeenStatus.ts

@@ -0,0 +1,25 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+/**
+ * `SeenStatus` represents either the idea that a message doesn't need to track its seen
+ * status, or the standard unseen/seen status pair.
+ *
+ * Unseen is a lot like unread - except that unseen messages only affect the placement
+ * of the last seen indicator and the count it shows. Unread messages will affect the
+ * left pane badging for conversations, as well as the overall badge count on the app.
+ */
+export enum SeenStatus {
+  NotApplicable = 0,
+  Unseen = 1,
+  Seen = 2,
+}
+
+const STATUS_NUMBERS: Record<SeenStatus, number> = {
+  [SeenStatus.NotApplicable]: 0,
+  [SeenStatus.Unseen]: 1,
+  [SeenStatus.Seen]: 2,
+};
+
+export const maxSeenStatus = (a: SeenStatus, b: SeenStatus): SeenStatus =>
+  STATUS_NUMBERS[a] > STATUS_NUMBERS[b] ? a : b;