feat(friends): pure category filter + count helpers

src/api/friends/friendCategory.helpers.ts

@@ -0,0 +1,32 @@
+import { MessengerFriend } from './MessengerFriend';
+
+/**
+ * Filter a friend list to a single category. categoryId 0 means
+ * "All" (no filtering) and returns the list unchanged.
+ */
+export const filterFriendsByCategory = (friends: MessengerFriend[], categoryId: number): MessengerFriend[] =>
+{
+    if(!friends) return [];
+
+    if(!categoryId) return friends;
+
+    return friends.filter(friend => (friend.categoryId === categoryId));
+};
+
+/**
+ * Count how many friends belong to each category id. Used to render
+ * member counts on the group chips.
+ */
+export const countFriendsByCategory = (friends: MessengerFriend[]): Map<number, number> =>
+{
+    const counts = new Map<number, number>();
+
+    if(!friends) return counts;
+
+    for(const friend of friends)
+    {
+        counts.set(friend.categoryId, (counts.get(friend.categoryId) ?? 0) + 1);
+    }
+
+    return counts;
+};