{ config, lib, pkgs, ... }:
with lib;
let
  cfg = config.services.syncthing.autoRegister;
  localpkgs = import ../. { inherit pkgs; };
  register-script = pkgs.writeShellScript "syncthing-autoregister-script" ''
    ${localpkgs.syncthing-set-id}/bin/syncthing-set-id ${
      escapeShellArg cfg.path
    }
  '';
in {
  options = {
    services.syncthing.autoRegister = {
      enable = mkEnableOption ''
        Automatically write the local syncthing device id to the nix module
        file <option>services.syncthing.autoRegister.path</option>.
      '';
      path = mkOption {
        type = types.path;
        description = ''
          Path of nix module file to write our syncthing
          device id into.  It will be written into
          services.syncthing.declarative.devices.<hostname>.id.  This file
          can be imported as a NixOS module.

          We recommend using a separate file just for this purpose.  I.e.,
          don't point this at your main NixOS config.  Instead, import this
          file from your main NixOS config.  (The current nix-configuration
          read-modify-write implementation only supports attrsets of strings;
          it does not support functions, which you probably use in your
          main configuration.)
        '';
      };
    };
  };
  config = mkIf cfg.enable {
    systemd.services.syncthing-autoregister = {
      after = [ "syncthing.service" "syncthing-init.service" ];
      wantedBy = [ "multi-user.target" ];
      environment.NIX_PATH = config.environment.variables.NIX_PATH;
      serviceConfig = {
        User = config.services.syncthing.user;
        RemainAfterExit = true;
        Type = "oneshot";
        ExecStart = register-script;
      };
    };
  };
}