{ config, lib, pkgs, ... }:

let
  cfg = config.systant;
  settingsFormat = pkgs.formats.toml { };
  configFile =
    if cfg.configFile != null
    then cfg.configFile
    else if cfg.settings != { }
    then settingsFormat.generate "systant-config.toml" cfg.settings
    else null;
in
{
  options.systant = {
    enable = lib.mkEnableOption "systant system monitoring agent";

    package = lib.mkOption {
      type = lib.types.package;
      default = pkgs.systant;
      defaultText = lib.literalExpression "pkgs.systant";
      description = "The systant package to use.";
    };

    configFile = lib.mkOption {
      type = lib.types.nullOr lib.types.path;
      default = null;
      description = ''
        Path to the systant configuration file (TOML).
        If set, this takes precedence over the settings option.
      '';
    };

    settings = lib.mkOption {
      type = settingsFormat.type;
      default = { };
      description = ''
        Configuration for systant in Nix attribute set form.
        Will be converted to TOML. Ignored if configFile is set.
      '';
      example = lib.literalExpression ''
        {
          mqtt = {
            broker = "mqtt://localhost:1883";
            topicPrefix = "systant";
          };
          entities = {
            cpu_usage = {
              type = "sensor";
              state_command = "awk '/^cpu / {u=$2+$4; t=$2+$4+$5; print int(u*100/t)}' /proc/stat";
              unit = "%";
              icon = "mdi:cpu-64-bit";
              name = "CPU Usage";
            };
          };
          homeassistant = {
            discovery = true;
            discoveryPrefix = "homeassistant";
          };
        }
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    # Make the package available system-wide
    environment.systemPackages = [ cfg.package ];

    # Systemd user service - runs in user session with user's environment
    systemd.user.services.systant = {
      description = "Systant system monitoring agent";
      wantedBy = [ "default.target" ];
      after = [ "network-online.target" ];
      wants = [ "network-online.target" ];

      serviceConfig = {
        Type = "simple";
        ExecStart =
          if configFile != null
          then "${cfg.package}/bin/systant run --config ${configFile}"
          else "${cfg.package}/bin/systant run";
        Restart = "on-failure";
        RestartSec = "5s";
      };
    };
  };
}