Revert "Revert "Permission phase 2""

This reverts commit 8b9244b079.

backend/src/lib/fn/array.ts

@@ -81,3 +81,25 @@ export const chunkArray = <T>(array: T[], chunkSize: number): T[][] => {
   }
   return chunks;
 };
+
+/*
+ * Returns all items from the first list that
+ * do not exist in the second list.
+ */
+export const diff = <T>(
+  root: readonly T[],
+  other: readonly T[],
+  identity: (item: T) => string | number | symbol = (t: T) => t as unknown as string | number | symbol
+): T[] => {
+  if (!root?.length && !other?.length) return [];
+  if (root?.length === undefined) return [...other];
+  if (!other?.length) return [...root];
+  const bKeys = other.reduce(
+    (acc, item) => {
+      acc[identity(item)] = true;
+      return acc;
+    },
+    {} as Record<string | number | symbol, boolean>
+  );
+  return root.filter((a) => !bKeys[identity(a)]);
+};