More work on permission utils and eager permission checks

backend/src/utils/getMissingPermissions.ts

@@ -0,0 +1,17 @@
+import { Constants, Permission } from "eris";
+
+/**
+ * @param resolvedPermissions A Permission object from e.g. GuildChannel#permissionsOf() or Member#permission
+ * @param requiredPermissions Bitmask of required permissions
+ * @return Bitmask of missing permissions
+ */
+export function getMissingPermissions(resolvedPermissions: Permission, requiredPermissions: number | bigint): bigint {
+  const allowedPermissions = BigInt(resolvedPermissions.allow);
+  const nRequiredPermissions = BigInt(requiredPermissions);
+
+  if (Boolean(allowedPermissions & BigInt(Constants.Permissions.administrator))) {
+    return BigInt(0);
+  }
+
+  return nRequiredPermissions & ~allowedPermissions;
+}