nix-config/modules/nixos/core/nix.nix

{
  flake.modules.nixos.core =
    { userName, ... }:
    {
      nix = {
        channel.enable = false;

        gc = {
          automatic = true;
          options = "--delete-older-than 7d";
          dates = "weekly";
          persistent = true;
        };

        optimise.automatic = true;

        settings = {
          experimental-features = [
            "nix-command"
            "flakes"
          ];

          max-jobs = "auto";

          # Make legacy nix commands use the XDG base directories instead of creating directories in $HOME.
          use-xdg-base-directories = true;

          # The maximum number of parallel TCP connections used to fetch files from binary caches and by other downloads.
          # It defaults to 25. 0 means no limit.
          http-connections = 128;

          # This option defines the maximum number of substitution jobs that Nix will try to run in
          # parallel. The default is 16. The minimum value one can choose is 1 and lower values will be
          # interpreted as 1.
          max-substitution-jobs = 128;

          # The number of lines of the tail of the log to show if a build fails.
          log-lines = 25;

          # When free disk space in /nix/store drops below min-free during a build, Nix performs a
          # garbage-collection until max-free bytes are available or there is no more garbage.
          # A value of 0 (the default) disables this feature.
          min-free = 128000000; # 128 MB
          max-free = 1000000000; # 1 GB

          # Prevent garbage collection from altering nix-shells managed by nix-direnv
          # https://github.com/nix-community/nix-direnv#installation
          keep-outputs = true;
          keep-derivations = true;

          # If set to true, Nix will keep building derivations even if some fail. The default is false.
          keep-going = true;

          # Automatically detect files in the store that have identical contents, and replaces
          # them with hard links to a single copy. This saves disk space.
          auto-optimise-store = true;

          # Whether to warn about dirty Git/Mercurial trees.
          warn-dirty = false;

          # The timeout (in seconds) for establishing connections in the binary cache substituter.
          # It corresponds to curl’s –connect-timeout option. A value of 0 means no limit.
          connect-timeout = 5;

          # Allow the use of cachix
          trusted-users = [
            "root"
            "${userName}"
          ];
          allowed-users = [
            "root"
            "${userName}"
          ];

          builders-use-substitutes = true;

          # If set to true, Nix will fall back to building from source if a binary substitute
          # fails. This is equivalent to the –fallback flag. The default is false.
          fallback = true;
        };
      };
    };
}