lib: ip: add 'nthInRange4'

And use it to refactor 'rangeIp4'

lib/ip.nix

@@ -86,27 +86,29 @@ rec {
   # Pretty print a parsed subnet into a human readable form
   prettySubnet4 = { baseIp, cidr, ... }: "${prettyIp4 baseIp}/${toString cidr}";
 
+  # Get the nth address from an IPv4 range, without checking if it is in range
+  nthInRange4 = { from, to }: n:
+    let
+      carry = lhs: { carry, acc }:
+        let
+          totVal = lhs + carry;
+        in
+        {
+          carry = totVal / 256;
+          acc = [ (mod totVal 256) ] ++ acc;
+        };
+      carried = foldr carry { carry = n; acc = [ ]; } from;
+    in
+    carried.acc;
+
   # Convert an IPv4 range into a list of all its constituent addresses
   rangeIp4 =
-    { from, to }:
+    { from, to } @ arg:
     let
       numAddresses =
         builtins.foldl' (acc: rhs: acc * 256 + rhs)
           0
           (zipListsWith (lhs: rhs: lhs - rhs) to from);
-      addToBase = n:
-        let
-          carry = lhs: { carry, acc }:
-            let
-              totVal = lhs + carry;
-            in
-            {
-              carry = totVal / 256;
-              acc = [ (mod totVal 256) ] ++ acc;
-            };
-          carried = foldr carry { carry = n; acc = [ ]; } from;
-        in
-        carried.acc;
     in
-    map addToBase (range 0 numAddresses);
+    map (nthInRange4 arg) (range 0 numAddresses);
 }