{ config, pkgs, ... }:

with pkgs.lib;

let

  uid = config.ids.uids.gpsd;
  gid = config.ids.gids.gpsd;
  cfg = config.services.gpsd;
  
in

{

  ###### interface

  options = {
  
    services.gpsd = {

      enable = mkOption {
        default = false;
        description = ''
          Whether to enable `gpsd', a GPS service daemon.
        '';
      };

      device = mkOption {
        default = "/dev/ttyUSB0";
        description = ''
          A device may be a local serial device for GPS input, or a URL of the form:
               <literal>[{dgpsip|ntrip}://][user:passwd@]host[:port][/stream]</literal>
          in which case it specifies an input source for DGPS or ntrip data.
        '';
      };

      readonly = mkOption {
        default = true;
        description = ''
          Whether to enable the broken-device-safety, otherwise
          known as read-only mode.  Some popular bluetooth and USB
          receivers lock up or become totally inaccessible when
          probed or reconfigured.  This switch prevents gpsd from
          writing to a receiver.  This means that gpsd cannot
          configure the receiver for optimal performance, but it
          also means that gpsd cannot break the receiver.  A better
          solution would be for Bluetooth to not be so fragile.  A
          platform independent method to identify
          serial-over-Bluetooth devices would also be nice.
        '';
      };

      port = mkOption {
        default = 2947;
        description = ''
          The port where to listen for TCP connections.
        '';
      };

      debugLevel = mkOption {
        default = 0;
        description = ''
          The debugging level.
        '';
      };

    };

  };


  ###### implementation
  
  config = mkIf cfg.enable {
  
    users.extraUsers = singleton
      { name = "gpsd";
        inherit uid;
        description = "gpsd daemon user";
        home = "/var/empty";
      };

    users.extraGroups = singleton
      { name = "gpsd";
        inherit gid;
      };

    jobAttrs.gpsd =
      { description = "GPSD daemon";

        startOn = "network-interfaces/started";
        stopOn = "network-interfaces/stop";

        exec =
          ''
            respawn ${pkgs.gpsd}/sbin/gpsd -D "${toString cfg.debugLevel}"   \
                       -S "${toString cfg.port}"                             \
                       ${if cfg.readonly then "-b" else ""}                  \
                       "${cfg.device}"
          '';
      };

  };
  
}