feat(lib): add domain manipulation functions

lib/default.nix

@@ -1,5 +1,20 @@
 { lib, ... }:
-{
+let
+  inherit (lib) singleton;
+  inherit (lib.strings) splitString;
+  inherit (builtins) length concatStringsSep tail;
+in
+rec {
+  # Helpers
+  splitDomain = domain: splitString "." domain;
+  shortenList =
+    count: list:
+    let
+      len = length list;
+    in
+    if len <= count then list else (shortenList count (tail list));
+
+  # Modules
   mkStrOption = lib.mkOption {
     type = lib.types.str;
     default = "";
@@ -16,4 +31,10 @@
       type = lib.types.path;
       default = path;
     };
+
+  # Domains
+  isRootDomain = domain: length (splitDomain domain) <= 2;
+  mkRootDomain = domain: concatStringsSep "." (shortenList 2 (splitDomain domain));
+  mkWildcardDomain = rootDomain: concatStringsSep "." ((singleton "*") ++ (splitDomain rootDomain));
+  mkHost = domain: if isRootDomain domain then domain else mkWildcardDomain (mkRootDomain domain);
 }