Minor cleanup

zones/reverse-ipv6.nix

@@ -5,14 +5,6 @@ let
   pvvv6Prefix = builtins.substring 0 14 (normalizev6 "2001:700:300:1900::");
   domainSuffix = ".pvv.ntnu.no.";
 
-  # Pad a number or string with zeroes to length 4 (32 -> "0032")
-  padTo4 =
-    n:
-    let
-      padded = "0000${toString n}";
-    in
-    builtins.substring (builtins.stringLength padded - 4) 4 padded;
-
   # Takes an IPv6 address on the format nnnnXXXXnnnn (literal XXXX) and replaces XXXX with enough zeroes to make a full 128 bit address
   padDoubleColon =
     addr:
@@ -20,8 +12,9 @@ let
       parts = lib.splitString "XXXX" addr;
       nibbleCount = builtins.foldl' builtins.add 0 (map builtins.stringLength parts);
       zeros = lib.concatStrings (builtins.genList (i: "0") (32 - nibbleCount));
+      padding = lib.fixedWidthString (32 - nibbleCount) "0" "";
     in
-    lib.replaceString "XXXX" zeros addr;
+    lib.replaceString "XXXX" padding addr;
 
   # Translate an IPv6 address (e.g. 2001:700:300:1900::1) into the fully expanded 20010700030019000000000000000001
   normalizev6 =
@@ -29,7 +22,7 @@ let
     (lib.pipe addr [
       (lib.replaceString "::" ":XXXX:")
       (lib.splitString ":")
-      (map padTo4)
+      (map (lib.fixedWidthString 4 "0"))
       (map (lib.splitString ""))
       (lib.flatten)
       (lib.concatStrings)