{ lib, systemdUtils }:

let
  inherit (systemdUtils.lib)
    assertValueOneOf
    automountConfig
    checkUnitConfig
    makeJobScript
    mountConfig
    serviceConfig
    unitConfig
    unitNameType
    ;

  inherit (lib)
    any
    concatMap
    filterOverrides
    isList
    literalExpression
    mergeEqualOption
    mkIf
    mkMerge
    mkOption
    mkOptionType
    singleton
    toList
    types
    ;

  checkService = checkUnitConfig "Service" [
    (assertValueOneOf "Type" [
      "exec" "simple" "forking" "oneshot" "dbus" "notify" "notify-reload" "idle"
    ])
    (assertValueOneOf "Restart" [
      "no" "on-success" "on-failure" "on-abnormal" "on-abort" "always"
    ])
  ];

in rec {

  unitOption = mkOptionType {
    name = "systemd option";
    merge = loc: defs:
      let
        defs' = filterOverrides defs;
      in
        if any (def: isList def.value) defs'
        then concatMap (def: toList def.value) defs'
        else mergeEqualOption loc defs';
  };

  sharedOptions = {

    enable = mkOption {
      default = true;
      type = types.bool;
      description = ''
        If set to false, this unit will be a symlink to
        /dev/null. This is primarily useful to prevent specific
        template instances
        (e.g. `serial-getty@ttyS0`) from being
        started. Note that `enable=true` does not
        make a unit start by default at boot; if you want that, see
        `wantedBy`.
      '';
    };

    name = lib.mkOption {
      type = lib.types.str;
      description = ''
        The name of this systemd unit, including its extension.
        This can be used to refer to this unit from other systemd units.
      '';
    };

    overrideStrategy = mkOption {
      default = "asDropinIfExists";
      type = types.enum [ "asDropinIfExists" "asDropin" ];
      description = ''
        Defines how unit configuration is provided for systemd:

        `asDropinIfExists` creates a unit file when no unit file is provided by the package
        otherwise a drop-in file name `overrides.conf`.

        `asDropin` creates a drop-in file named `overrides.conf`.
        Mainly needed to define instances for systemd template units (e.g. `systemd-nspawn@mycontainer.service`).

        See also {manpage}`systemd.unit(5)`.
      '';
    };

    requiredBy = mkOption {
      default = [];
      type = types.listOf unitNameType;
      description = ''
        Units that require (i.e. depend on and need to go down with) this unit.
        As discussed in the `wantedBy` option description this also creates
        `.requires` symlinks automatically.
      '';
    };

    upheldBy = mkOption {
      default = [];
      type = types.listOf unitNameType;
      description = ''
        Keep this unit running as long as the listed units are running. This is a continuously
        enforced version of wantedBy.
      '';
    };

    wantedBy = mkOption {
      default = [];
      type = types.listOf unitNameType;
      description = ''
        Units that want (i.e. depend on) this unit. The default method for
        starting a unit by default at boot time is to set this option to
        `["multi-user.target"]` for system services. Likewise for user units
        (`systemd.user.<name>.*`) set it to `["default.target"]` to make a unit
        start by default when the user `<name>` logs on.

        This option creates a `.wants` symlink in the given target that exists
        statelessly without the need for running `systemctl enable`.
        The `[Install]` section described in {manpage}`systemd.unit(5)` however is
        not supported because it is a stateful process that does not fit well
        into the NixOS design.
      '';
    };

    aliases = mkOption {
      default = [];
      type = types.listOf unitNameType;
      description = "Aliases of that unit.";
    };

  };

  concreteUnitOptions = sharedOptions // {

    text = mkOption {
      type = types.nullOr types.str;
      default = null;
      description = "Text of this systemd unit.";
    };

    unit = mkOption {
      internal = true;
      description = "The generated unit.";
    };

  };

  commonUnitOptions = {
    options = sharedOptions // {

      description = mkOption {
        default = "";
        type = types.singleLineStr;
        description = "Description of this unit used in systemd messages and progress indicators.";
      };

      documentation = mkOption {
        default = [];
        type = types.listOf types.str;
        description = "A list of URIs referencing documentation for this unit or its configuration.";
      };

      requires = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          Start the specified units when this unit is started, and stop
          this unit when the specified units are stopped or fail.
        '';
      };

      wants = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          Start the specified units when this unit is started.
        '';
      };

      upholds = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          Keeps the specified running while this unit is running. A continuous version of `wants`.
        '';
      };

      after = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          If the specified units are started at the same time as
          this unit, delay this unit until they have started.
        '';
      };

      before = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          If the specified units are started at the same time as
          this unit, delay them until this unit has started.
        '';
      };

      bindsTo = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          Like ‘requires’, but in addition, if the specified units
          unexpectedly disappear, this unit will be stopped as well.
        '';
      };

      partOf = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          If the specified units are stopped or restarted, then this
          unit is stopped or restarted as well.
        '';
      };

      conflicts = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          If the specified units are started, then this unit is stopped
          and vice versa.
        '';
      };

      requisite = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          Similar to requires. However if the units listed are not started,
          they will not be started and the transaction will fail.
        '';
      };

      unitConfig = mkOption {
        default = {};
        example = { RequiresMountsFor = "/data"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Unit]` section of the unit.  See
          {manpage}`systemd.unit(5)` for details.
        '';
      };

      onFailure = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          A list of one or more units that are activated when
          this unit enters the "failed" state.
        '';
      };

      onSuccess = mkOption {
        default = [];
        type = types.listOf unitNameType;
        description = ''
          A list of one or more units that are activated when
          this unit enters the "inactive" state.
        '';
      };

      startLimitBurst = mkOption {
         type = types.int;
         description = ''
           Configure unit start rate limiting. Units which are started
           more than startLimitBurst times within an interval time
           interval are not permitted to start any more.
         '';
      };

      startLimitIntervalSec = mkOption {
         type = types.int;
         description = ''
           Configure unit start rate limiting. Units which are started
           more than startLimitBurst times within an interval time
           interval are not permitted to start any more.
         '';
      };

    };
  };

  stage2CommonUnitOptions = {
    imports = [
      commonUnitOptions
    ];

    options = {
      restartTriggers = mkOption {
        default = [];
        type = types.listOf types.unspecified;
        description = ''
          An arbitrary list of items such as derivations.  If any item
          in the list changes between reconfigurations, the service will
          be restarted.
        '';
      };

      reloadTriggers = mkOption {
        default = [];
        type = types.listOf unitOption;
        description = ''
          An arbitrary list of items such as derivations.  If any item
          in the list changes between reconfigurations, the service will
          be reloaded.  If anything but a reload trigger changes in the
          unit file, the unit will be restarted instead.
        '';
      };
    };
  };
  stage1CommonUnitOptions = commonUnitOptions;

  serviceOptions = { name, config, ... }: {
    options = {

      environment = mkOption {
        default = {};
        type = with types; attrsOf (nullOr (oneOf [ str path package ]));
        example = { PATH = "/foo/bar/bin"; LANG = "nl_NL.UTF-8"; };
        description = "Environment variables passed to the service's processes.";
      };

      path = mkOption {
        default = [];
        type = with types; listOf (oneOf [ package str ]);
        description = ''
          Packages added to the service's {env}`PATH`
          environment variable.  Both the {file}`bin`
          and {file}`sbin` subdirectories of each
          package are added.
        '';
      };

      serviceConfig = mkOption {
        default = {};
        example =
          { RestartSec = 5;
          };
        type = types.addCheck (types.attrsOf unitOption) checkService;
        description = ''
          Each attribute in this set specifies an option in the
          `[Service]` section of the unit.  See
          {manpage}`systemd.service(5)` for details.
        '';
      };

      enableStrictShellChecks = mkOption {
        type = types.bool;
        description = "Enable running shellcheck on the generated scripts for this unit.";
        # The default gets set in systemd-lib.nix because we don't have access to
        # the full NixOS config here.
        defaultText = literalExpression "config.systemd.enableStrictShellChecks";
      };

      script = mkOption {
        type = types.lines;
        default = "";
        description = "Shell commands executed as the service's main process.";
      };

      scriptArgs = mkOption {
        type = types.str;
        default = "";
        example = "%i";
        description = ''
          Arguments passed to the main process script.
          Can contain specifiers (`%` placeholders expanded by systemd, see {manpage}`systemd.unit(5)`).
        '';
      };

      preStart = mkOption {
        type = types.lines;
        default = "";
        description = ''
          Shell commands executed before the service's main process
          is started.
        '';
      };

      postStart = mkOption {
        type = types.lines;
        default = "";
        description = ''
          Shell commands executed after the service's main process
          is started.
        '';
      };

      reload = mkOption {
        type = types.lines;
        default = "";
        description = ''
          Shell commands executed when the service's main process
          is reloaded.
        '';
      };

      preStop = mkOption {
        type = types.lines;
        default = "";
        description = ''
          Shell commands executed to stop the service.
        '';
      };

      postStop = mkOption {
        type = types.lines;
        default = "";
        description = ''
          Shell commands executed after the service's main process
          has exited.
        '';
      };

      jobScripts = mkOption {
        type = with types; coercedTo path singleton (listOf path);
        internal = true;
        description = "A list of all job script derivations of this unit.";
        default = [];
      };

    };

    config = mkMerge [
      (mkIf (config.preStart != "") rec {
        jobScripts = makeJobScript {
          name = "${name}-pre-start";
          text = config.preStart;
          inherit (config) enableStrictShellChecks;
        };
        serviceConfig.ExecStartPre = [ jobScripts ];
      })
      (mkIf (config.script != "") rec {
        jobScripts = makeJobScript {
          name = "${name}-start";
          text = config.script;
          inherit (config) enableStrictShellChecks;
        };
        serviceConfig.ExecStart = jobScripts + " " + config.scriptArgs;
      })
      (mkIf (config.postStart != "") rec {
        jobScripts = makeJobScript {
          name = "${name}-post-start";
          text = config.postStart;
          inherit (config) enableStrictShellChecks;
        };
        serviceConfig.ExecStartPost = [ jobScripts ];
      })
      (mkIf (config.reload != "") rec {
        jobScripts = makeJobScript {
          name = "${name}-reload";
          text = config.reload;
          inherit (config) enableStrictShellChecks;
        };
        serviceConfig.ExecReload = jobScripts;
      })
      (mkIf (config.preStop != "") rec {
        jobScripts = makeJobScript {
          name = "${name}-pre-stop";
          text = config.preStop;
          inherit (config) enableStrictShellChecks;
        };
        serviceConfig.ExecStop = jobScripts;
      })
      (mkIf (config.postStop != "") rec {
        jobScripts = makeJobScript {
          name = "${name}-post-stop";
          text = config.postStop;
          inherit (config) enableStrictShellChecks;
        };
        serviceConfig.ExecStopPost = jobScripts;
      })
    ];

  };

  stage2ServiceOptions = {
    imports = [
      stage2CommonUnitOptions
      serviceOptions
    ];

    options = {
      restartIfChanged = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether the service should be restarted during a NixOS
          configuration switch if its definition has changed.
        '';
      };

      reloadIfChanged = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether the service should be reloaded during a NixOS
          configuration switch if its definition has changed.  If
          enabled, the value of {option}`restartIfChanged` is
          ignored.

          This option should not be used anymore in favor of
          {option}`reloadTriggers` which allows more granular
          control of when a service is reloaded and when a service
          is restarted.
        '';
      };

      stopIfChanged = mkOption {
        type = types.bool;
        default = true;
        description = ''
          If set, a changed unit is restarted by calling
          {command}`systemctl stop` in the old configuration,
          then {command}`systemctl start` in the new one.
          Otherwise, it is restarted in a single step using
          {command}`systemctl restart` in the new configuration.
          The latter is less correct because it runs the
          `ExecStop` commands from the new
          configuration.
        '';
      };

      startAt = mkOption {
        type = with types; either str (listOf str);
        default = [];
        example = "Sun 14:00:00";
        description = ''
          Automatically start this unit at the given date/time, which
          must be in the format described in
          {manpage}`systemd.time(7)`.  This is equivalent
          to adding a corresponding timer unit with
          {option}`OnCalendar` set to the value given here.
        '';
        apply = v: if isList v then v else [ v ];
      };
    };
  };

  stage1ServiceOptions = {
    imports = [
      stage1CommonUnitOptions
      serviceOptions
    ];
  };


  socketOptions = {
    options = {

      listenStreams = mkOption {
        default = [];
        type = types.listOf types.str;
        example = [ "0.0.0.0:993" "/run/my-socket" ];
        description = ''
          For each item in this list, a `ListenStream`
          option in the `[Socket]` section will be created.
        '';
      };

      listenDatagrams = mkOption {
        default = [];
        type = types.listOf types.str;
        example = [ "0.0.0.0:993" "/run/my-socket" ];
        description = ''
          For each item in this list, a `ListenDatagram`
          option in the `[Socket]` section will be created.
        '';
      };

      socketConfig = mkOption {
        default = {};
        example = { ListenStream = "/run/my-socket"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Socket]` section of the unit.  See
          {manpage}`systemd.socket(5)` for details.
        '';
      };
    };

  };

  stage2SocketOptions = {
    imports = [
      stage2CommonUnitOptions
      socketOptions
    ];
  };

  stage1SocketOptions = {
    imports = [
      stage1CommonUnitOptions
      socketOptions
    ];
  };


  timerOptions = {
    options = {

      timerConfig = mkOption {
        default = {};
        example = { OnCalendar = "Sun 14:00:00"; Unit = "foo.service"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Timer]` section of the unit.  See
          {manpage}`systemd.timer(5)` and
          {manpage}`systemd.time(7)` for details.
        '';
      };

    };
  };

  stage2TimerOptions = {
    imports = [
      stage2CommonUnitOptions
      timerOptions
    ];
  };

  stage1TimerOptions = {
    imports = [
      stage1CommonUnitOptions
      timerOptions
    ];
  };


  pathOptions = {
    options = {

      pathConfig = mkOption {
        default = {};
        example = { PathChanged = "/some/path"; Unit = "changedpath.service"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Path]` section of the unit.  See
          {manpage}`systemd.path(5)` for details.
        '';
      };

    };
  };

  stage2PathOptions = {
    imports = [
      stage2CommonUnitOptions
      pathOptions
    ];
  };

  stage1PathOptions = {
    imports = [
      stage1CommonUnitOptions
      pathOptions
    ];
  };


  mountOptions = {
    options = {

      what = mkOption {
        example = "/dev/sda1";
        type = types.str;
        description = "Absolute path of device node, file or other resource. (Mandatory)";
      };

      where = mkOption {
        example = "/mnt";
        type = types.str;
        description = ''
          Absolute path of a directory of the mount point.
          Will be created if it doesn't exist. (Mandatory)
        '';
      };

      type = mkOption {
        default = "";
        example = "ext4";
        type = types.str;
        description = "File system type.";
      };

      options = mkOption {
        default = "";
        example = "noatime";
        type = types.commas;
        description = "Options used to mount the file system.";
      };

      mountConfig = mkOption {
        default = {};
        example = { DirectoryMode = "0775"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Mount]` section of the unit.  See
          {manpage}`systemd.mount(5)` for details.
        '';
      };

    };
  };

  stage2MountOptions = {
    imports = [
      stage2CommonUnitOptions
      mountOptions
    ];
  };

  stage1MountOptions = {
    imports = [
      stage1CommonUnitOptions
      mountOptions
    ];
  };

  automountOptions = {
    options = {

      where = mkOption {
        example = "/mnt";
        type = types.str;
        description = ''
          Absolute path of a directory of the mount point.
          Will be created if it doesn't exist. (Mandatory)
        '';
      };

      automountConfig = mkOption {
        default = {};
        example = { DirectoryMode = "0775"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Automount]` section of the unit.  See
          {manpage}`systemd.automount(5)` for details.
        '';
      };

    };
  };

  stage2AutomountOptions = {
    imports = [
      stage2CommonUnitOptions
      automountOptions
    ];
  };

  stage1AutomountOptions = {
    imports = [
      stage1CommonUnitOptions
      automountOptions
    ];
  };

  sliceOptions = {
    options = {

      sliceConfig = mkOption {
        default = {};
        example = { MemoryMax = "2G"; };
        type = types.attrsOf unitOption;
        description = ''
          Each attribute in this set specifies an option in the
          `[Slice]` section of the unit.  See
          {manpage}`systemd.slice(5)` for details.
        '';
      };

    };
  };

  stage2SliceOptions = {
    imports = [
      stage2CommonUnitOptions
      sliceOptions
    ];
  };

  stage1SliceOptions = {
    imports = [
      stage1CommonUnitOptions
      sliceOptions
    ];
  };

}