adding modularity

utils/binary-cache.nix

@@ -0,0 +1,22 @@
+{
+  lib,
+  ...
+}: {
+
+  # ...
+  nix.settings = {
+    # allow substituters to be used
+
+    substituters = [
+        # also add mirrors?
+        #   "https://mirrors.ustc.edu.cn/nix-channels/store"
+        "https://cache.nixos.org"
+    ];
+
+    trusted-public-keys = [
+      # the default public key of cache.nixos.org, it's built-in, no need to add it here
+      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+    ];
+  };
+
+}

utils/garbage-collection.nix

@@ -0,0 +1,23 @@
+{ lib, pkgs, ... }:
+
+{
+  # ...
+
+  # Limit the number of generations to keep
+  boot.loader.systemd-boot.configurationLimit = 10;
+  # boot.loader.grub.configurationLimit = 10;
+
+  # Perform garbage collection weekly to maintain low disk usage
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 1w";
+  };
+
+  # Optimize storage
+  # You can also manually optimize the store via:
+  #    nix-store --optimise
+  # Refer to the following link for more details:
+  # https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-auto-optimise-store
+  nix.settings.auto-optimise-store = true;
+}