Starting out simple. Small.

lib/functions/buildLocationList.nix

@@ -0,0 +1 @@
+locations: builtins.attrNames locations

lib/metanix.nix

@@ -1,45 +1,12 @@
-# lib/metanix.nix
 { data, lib }:
 
 let
-  buildSystemIndex =
+  indexOf = name: names:
     let
-      flattenSystems = locationName: subnetName: systems:
-        lib.mapAttrsToList
-          (systemName: systemCfg: {
-            inherit systemName locationName subnetName;
-            primary = systemCfg.primary or false;
-            macs = systemCfg.macAddresses or [ ];
-          })
-          systems;
-
-      allSystems =
-        builtins.concatLists (
-          lib.mapAttrsToList
-            (locationName: location:
-              builtins.concatLists (
-                lib.mapAttrsToList
-                  (subnetName: subnet:
-                    flattenSystems locationName subnetName (subnet.systems or { })
-                  )
-                  (location.subnets or { })
-              )
-            )
-            data.locations
-        );
+      indexed = builtins.imap0 (i: v: { inherit i v; }) names;
+      matches = builtins.filter (x: x.v == name) indexed;
     in
-    builtins.foldl'
-      (acc: entry:
-        let
-          existing = acc.${entry.systemName} or [ ];
-        in
-        acc // {
-          ${entry.systemName} = existing ++ [ builtins.removeAttrs entry [ "systemName" ] ];
-        }
-      )
-      { }
-      allSystems;
+    if matches == [ ] then null else (builtins.head matches).i;
+
 in
-{
-  inherit buildSystemIndex;
-}
+{ indexOf = indexOf; }