{
  config,
  lib,
  pkgs,
  ...
}:
let
  cfg = config.system.autoUpdate;
in
{

  options = {

    system.autoUpdate = {

      enable = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = ''
          Whether to periodically update the NixOS
	  configuration to the latest version.
        '';
      };

      flake = lib.mkOption {
        type = lib.types.nullOr lib.types.str;
        default = null;
        example = "github:kloenk/nix";
        description = ''
          The Flake URI of the NixOS configuration to build.
        '';
      };

      flags = lib.mkOption {
        type = lib.types.listOf lib.types.str;
        default = [ ];
        example = [
          "-I"
          "stuff=/home/alice/nixos-stuff"
          "--option"
          "extra-binary-caches"
          "http://my-cache.example.org/"
        ];
        description = ''
          Any additional flags passed to {command}`nixos-rebuild`.

          If you are using flakes and use a local repo you can add
          {command}`[ "--update-input" "nixpkgs" "--commit-lock-file" ]`
          to update nixpkgs.
        '';
      };

      dates = lib.mkOption {
        type = lib.types.str;
        default = "04:40";
        example = "daily";
        description = ''
          How often or when upgrade occurs. For most desktop and server systems
          a sufficient upgrade frequency is once a day.

          The format is described in
          {manpage}`systemd.time(7)`.
        '';
      };

      randomizedDelaySec = lib.mkOption {
        default = "0";
        type = lib.types.str;
        example = "45min";
        description = ''
          Add a randomized delay before each automatic upgrade.
          The delay will be chosen between zero and this value.
          This value must be a time span in the format specified by
          {manpage}`systemd.time(7)`
        '';
      };

      fixedRandomDelay = lib.mkOption {
        default = false;
        type = lib.types.bool;
        example = true;
        description = ''
          Make the randomized delay consistent between runs.
          This reduces the jitter between automatic upgrades.
          See {option}`randomizedDelaySec` for configuring the randomized delay.
        '';
      };

      rebootWindow = lib.mkOption {
        description = ''
          Define a lower and upper time value (in HH:MM format) which
          constitute a time window during which reboots are allowed after an upgrade.
          This option only has an effect when {option}`allowReboot` is enabled.
          The default value of `null` means that reboots are allowed at any time.
        '';
        default = null;
        example = {
          lower = "01:00";
          upper = "05:00";
        };
        type =
          with lib.types;
          nullOr (submodule {
            options = {
              lower = lib.mkOption {
                description = "Lower limit of the reboot window";
                type = lib.types.strMatching "[[:digit:]]{2}:[[:digit:]]{2}";
                example = "01:00";
              };

              upper = lib.mkOption {
                description = "Upper limit of the reboot window";
                type = lib.types.strMatching "[[:digit:]]{2}:[[:digit:]]{2}";
                example = "05:00";
              };
            };
          });
      };

      persistent = lib.mkOption {
        default = true;
        type = lib.types.bool;
        example = false;
        description = ''
          Takes a boolean argument. If true, the time when the service
          unit was last triggered is stored on disk. When the timer is
          activated, the service unit is triggered immediately if it
          would have been triggered at least once during the time when
          the timer was inactive. Such triggering is nonetheless
          subject to the delay imposed by RandomizedDelaySec=. This is
          useful to catch up on missed runs of the service when the
          system was powered down.
        '';
      };

    };

  };

  config = lib.mkIf cfg.enable {

    system.autoUpgrade.flags = (
        [
          "--refresh"
          "--flake ${cfg.flake}"
        ]
    );

    systemd.services.nixos-update = {
      description = "NixOS Upgrade";

      restartIfChanged = false;
      unitConfig.X-StopOnRemoval = false;

      serviceConfig.Type = "oneshot";

      environment =
        config.nix.envVars
        // {
          inherit (config.environment.sessionVariables) NIX_PATH;
          HOME = "/root";
        }
        // config.networking.proxy.envVars;

      path = with pkgs; [
        coreutils
        gnutar
        xz.bin
        gzip
        gitMinimal
        config.nix.package.out
        config.programs.ssh.package
      ];

      script = import ./script.nix {
	  inherit cfg;
          nixos-rebuild = "${config.system.build.nixos-rebuild}/bin/nixos-rebuild";
          date = "${pkgs.coreutils}/bin/date";
          realpath = "${pkgs.coreutils}/bin/realpath";
          stat = "${pkgs.coreutils}/bin/stat";
          cut = "${pkgs.coreutils}/bin/cut";
          head = "${pkgs.coreutils}/bin/head";
          grep = "${pkgs.gnugrep}/bin/grep";
          grub-reboot = "${pkgs.grub2}/bin/grub-reboot";
          shutdown = "${config.systemd.package}/bin/shutdown";
          bootctl = "${config.systemd.package}/bin/bootctl";
          systemd-analyze = "${config.systemd.package}/bin/systemd-analyze";
          upgradeFlag = "--upgrade";
          units = "${pkgs.units}/bin/units";
          jq = "${pkgs.jq}/bin/jq";
          hostname = "${pkgs.nettools}/bin/hostname";
	};

      startAt = cfg.dates;

      after = [ "network-online.target" ];
      wants = [ "network-online.target" ];
    };

    systemd.timers.nixos-update = {
      timerConfig = {
        RandomizedDelaySec = cfg.randomizedDelaySec;
        FixedRandomDelay = cfg.fixedRandomDelay;
        Persistent = cfg.persistent;
      };
    };
  };

}