Storage Service: Fetch updates on any group record merge

ts/util/diffArraysAsSets.ts

@@ -0,0 +1,26 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function diffArraysAsSets<T>(
+  starting: Array<T>,
+  current: Array<T>
+): { added: Array<T>; removed: Array<T> } {
+  const startingSet = new Set(starting);
+  const currentSet = new Set(current);
+
+  const removed = [];
+  for (const item of startingSet) {
+    if (!currentSet.has(item)) {
+      removed.push(item);
+    }
+  }
+
+  const added = [];
+  for (const item of currentSet) {
+    if (!startingSet.has(item)) {
+      added.push(item);
+    }
+  }
+
+  return { added, removed };
+}