{ config, lib, pkgs, ... }:
let
  cfg = config.services.uptime;

  serviceSubmodule = lib.types.submodule {
    options = {
      name = lib.mkOption {
        type = lib.types.str;
        description = "Service name. Used as the log filename and the row label.";
      };
      url = lib.mkOption {
        type = lib.types.str;
        description = "URL to probe.";
      };
    };
  };

  categorySubmodule = lib.types.submodule {
    options = {
      description = lib.mkOption {
        type = lib.types.str;
        default = "";
        description = "Free-form text shown above this category's table.";
      };
      intervalSeconds = lib.mkOption {
        type = lib.types.int;
        default = 60;
        description = "Minimum interval between probes for services in this category.";
      };
      hideUrls = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = "If true, omit the URL column when rendering services in this category.";
      };
      services = lib.mkOption {
        type = lib.types.listOf serviceSubmodule;
        default = [];
        example = [{ name = "git"; url = "https://git.ily.rs"; }];
        description = "Ordered list of services to probe. Render order matches list order.";
      };
    };
  };

  configFile = pkgs.writeText "uptime-config" (lib.concatStringsSep "\n" (
    (lib.imap0 (i: cat:
      "CAT\t${toString i}\t${toString cat.intervalSeconds}\t${if cat.hideUrls then "1" else "0"}\t${cat.description}"
    ) cfg.categories)
    ++
    (lib.concatLists (lib.imap0 (i: cat:
      map (svc: "SVC\t${toString i}\t${svc.name}\t${svc.url}") cat.services
    ) cfg.categories))
  ));

  introFile = pkgs.writeText "uptime-intro" cfg.intro;

  catIntervals = map (c: c.intervalSeconds) cfg.categories;
  timerSeconds =
    if catIntervals == []
    then 60
    else lib.foldl' (a: b: if a < b then a else b) 86400 catIntervals;

  runScript = pkgs.writeShellApplication {
    name = "uptime-run";
    runtimeInputs = with pkgs; [ curl gawk coreutils ];
    text = builtins.readFile ./run.sh;
  };
in
{
  options.services.uptime = {
    enable = lib.mkEnableOption "minimal text-only uptime status page";

    outputPath = lib.mkOption {
      type = lib.types.str;
      default = "/var/lib/uptime/status.txt";
      description = ''
        Path where the rendered status page is written.
        Must be writable by the uptime user. The default lives inside the
        unit's StateDirectory, which is created and owned automatically.
      '';
    };

    intro = lib.mkOption {
      type = lib.types.str;
      default = "";
      description = "Free-form text shown at the top of the status page (before any categories).";
    };

    categories = lib.mkOption {
      type = lib.types.listOf categorySubmodule;
      default = [];
      description = "Ordered list of service categories. Each is rendered as its own table.";
    };

    retentionDays = lib.mkOption {
      type = lib.types.int;
      default = 90;
      description = "How many days of log entries to keep on disk.";
    };

    displayDays = lib.mkOption {
      type = lib.types.int;
      default = 30;
      description = "How many days of history to render in the long-term bar.";
    };

    displayHours = lib.mkOption {
      type = lib.types.int;
      default = 24;
      description = "How many hours of recent history to render in the short-term bar (1 cell = 1 hour).";
    };
  };

  config = lib.mkIf cfg.enable {
    assertions = [{
      assertion = cfg.displayDays <= cfg.retentionDays;
      message = "services.uptime.displayDays (${toString cfg.displayDays}) must be <= retentionDays (${toString cfg.retentionDays}).";
    }];

    users.users.uptime = {
      isSystemUser = true;
      group = "uptime";
      home = "/var/lib/uptime";
    };
    users.groups.uptime = {};

    systemd.services.uptime = {
      description = "Probe configured services and render the status page";
      after = [ "network-online.target" ];
      wants = [ "network-online.target" ];
      environment = {
        CONFIG_PATH = "${configFile}";
        INTRO_PATH = "${introFile}";
        OUTPUT_PATH = cfg.outputPath;
        RETENTION_DAYS = toString cfg.retentionDays;
        DISPLAY_DAYS = toString cfg.displayDays;
        DISPLAY_HOURS = toString cfg.displayHours;
      };
      serviceConfig = {
        Type = "oneshot";
        ExecStart = lib.getExe runScript;
        User = "uptime";
        Group = "uptime";
        StateDirectory = "uptime";
        StateDirectoryMode = "0755";
        UMask = "0022";
      };
    };

    systemd.timers.uptime = {
      description = "Periodic uptime probe";
      wantedBy = [ "timers.target" ];
      timerConfig = {
        OnBootSec = "1min";
        OnUnitActiveSec = "${toString timerSeconds}s";
        Unit = "uptime.service";
      };
    };
  };
}