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

with lib;

let
  cfg = config.services.syncthing;
  defaultUser = "syncthing";

  devices = mapAttrsToList (name: device: {
    deviceID = device.id;
    inherit (device) name addresses introducer;
  }) cfg.declarative.devices;

  folders = mapAttrsToList ( _: folder: {
    inherit (folder) path id label type;
    devices = map (device: { deviceId = cfg.declarative.devices.${device}.id; }) folder.devices;
    rescanIntervalS = folder.rescanInterval;
    fsWatcherEnabled = folder.watch;
    fsWatcherDelayS = folder.watchDelay;
    ignorePerms = folder.ignorePerms;
    ignoreDelete = folder.ignoreDelete;
    versioning = folder.versioning;
  }) (filterAttrs (
    _: folder:
    folder.enable
  ) cfg.declarative.folders);

  # get the api key by parsing the config.xml
  getApiKey = pkgs.writers.writeDash "getAPIKey" ''
    ${pkgs.libxml2}/bin/xmllint \
      --xpath 'string(configuration/gui/apikey)'\
      ${cfg.configDir}/config.xml
  '';

  updateConfig = pkgs.writers.writeDash "merge-syncthing-config" ''
    set -efu
    # wait for syncthing port to open
    until ${pkgs.curl}/bin/curl -Ss ${cfg.guiAddress} -o /dev/null; do
      sleep 1
    done

    API_KEY=$(${getApiKey})
    OLD_CFG=$(${pkgs.curl}/bin/curl -Ss \
      -H "X-API-Key: $API_KEY" \
      ${cfg.guiAddress}/rest/system/config)

    # generate the new config by merging with the nixos config options
    NEW_CFG=$(echo "$OLD_CFG" | ${pkgs.jq}/bin/jq -s '.[] as $in | $in * {
      "devices": (${builtins.toJSON devices}${optionalString (! cfg.declarative.overrideDevices) " + $in.devices"}),
      "folders": (${builtins.toJSON folders}${optionalString (! cfg.declarative.overrideFolders) " + $in.folders"})
    }')

    # POST the new config to syncthing
    echo "$NEW_CFG" | ${pkgs.curl}/bin/curl -Ss \
      -H "X-API-Key: $API_KEY" \
      ${cfg.guiAddress}/rest/system/config -d @-

    # restart syncthing after sending the new config
    ${pkgs.curl}/bin/curl -Ss \
      -H "X-API-Key: $API_KEY" \
      -X POST \
      ${cfg.guiAddress}/rest/system/restart
  '';
in {
  ###### interface
  options = {
    services.syncthing = {

      enable = mkEnableOption ''
        Syncthing - the self-hosted open-source alternative
        to Dropbox and Bittorrent Sync. Initial interface will be
        available on http://127.0.0.1:8384/.
      '';

      declarative = {
        cert = mkOption {
          type = types.nullOr types.str;
          default = null;
          description = ''
            Path to users cert.pem file, will be copied into the syncthing's
            <literal>configDir</literal>
          '';
        };

        key = mkOption {
          type = types.nullOr types.str;
          default = null;
          description = ''
            Path to users key.pem file, will be copied into the syncthing's
            <literal>configDir</literal>
          '';
        };

        overrideDevices = mkOption {
          type = types.bool;
          default = true;
          description = ''
            Whether to delete the devices which are not configured via the
            <literal>declarative.devices</literal> option.
            If set to false, devices added via the webinterface will
            persist but will have to be deleted manually.
          '';
        };

        devices = mkOption {
          default = {};
          description = ''
            Peers/devices which syncthing should communicate with.
          '';
          example = {
            bigbox = {
              id = "7CFNTQM-IMTJBHJ-3UWRDIU-ZGQJFR6-VCXZ3NB-XUH3KZO-N52ITXR-LAIYUAU";
              addresses = [ "tcp://192.168.0.10:51820" ];
            };
          };
          type = types.attrsOf (types.submodule ({ name, ... }: {
            options = {

              name = mkOption {
                type = types.str;
                default = name;
                description = ''
                  Name of the device
                '';
              };

              addresses = mkOption {
                type = types.listOf types.str;
                default = [];
                description = ''
                  The addresses used to connect to the device.
                  If this is let empty, dynamic configuration is attempted
                '';
              };

              id = mkOption {
                type = types.str;
                description = ''
                  The id of the other peer, this is mandatory. It's documented at
                  https://docs.syncthing.net/dev/device-ids.html
                '';
              };

              introducer = mkOption {
                type = types.bool;
                default = false;
                description = ''
                  If the device should act as an introducer and be allowed
                  to add folders on this computer.
                '';
              };

            };
          }));
        };

        overrideFolders = mkOption {
          type = types.bool;
          default = true;
          description = ''
            Whether to delete the folders which are not configured via the
            <literal>declarative.folders</literal> option.
            If set to false, folders added via the webinterface will persist
            but will have to be deleted manually.
          '';
        };

        folders = mkOption {
          default = {};
          description = ''
            folders which should be shared by syncthing.
          '';
          example = literalExample ''
            {
              "/home/user/sync" = {
                id = "syncme";
                devices = [ "bigbox" ];
              };
            }
          '';
          type = types.attrsOf (types.submodule ({ name, ... }: {
            options = {

              enable = mkOption {
                type = types.bool;
                default = true;
                description = ''
                  share this folder.
                  This option is useful when you want to define all folders
                  in one place, but not every machine should share all folders.
                '';
              };

              path = mkOption {
                type = types.str;
                default = name;
                description = ''
                  The path to the folder which should be shared.
                '';
              };

              id = mkOption {
                type = types.str;
                default = name;
                description = ''
                  The id of the folder. Must be the same on all devices.
                '';
              };

              label = mkOption {
                type = types.str;
                default = name;
                description = ''
                  The label of the folder.
                '';
              };

              devices = mkOption {
                type = types.listOf types.str;
                default = [];
                description = ''
                  The devices this folder should be shared with. Must be defined
                  in the <literal>declarative.devices</literal> attribute.
                '';
              };

              versioning = mkOption {
                default = null;
                description = ''
                  How to keep changed/deleted files with syncthing.
                  There are 4 different types of versioning with different parameters.
                  See https://docs.syncthing.net/users/versioning.html
                '';
                example = [
                  {
                    versioning = {
                      type = "simple";
                      params.keep = "10";
                    };
                  }
                  {
                    versioning = {
                      type = "trashcan";
                      params.cleanoutDays = "1000";
                    };
                  }
                  {
                    versioning = {
                      type = "staggered";
                      params = {
                        cleanInterval = "3600";
                        maxAge = "31536000";
                        versionsPath = "/syncthing/backup";
                      };
                    };
                  }
                  {
                    versioning = {
                      type = "external";
                      params.versionsPath = pkgs.writers.writeBash "backup" ''
                        folderpath="$1"
                        filepath="$2"
                        rm -rf "$folderpath/$filepath"
                      '';
                    };
                  }
                ];
                type = with types; nullOr (submodule {
                  options = {
                    type = mkOption {
                      type = enum [ "external" "simple" "staggered" "trashcan" ];
                      description = ''
                        Type of versioning.
                        See https://docs.syncthing.net/users/versioning.html
                      '';
                    };
                    params = mkOption {
                      type = attrsOf (either str path);
                      description = ''
                        Parameters for versioning. Structure depends on versioning.type.
                        See https://docs.syncthing.net/users/versioning.html
                      '';
                    };
                  };
                });
              };

              rescanInterval = mkOption {
                type = types.int;
                default = 3600;
                description = ''
                  How often the folders should be rescaned for changes.
                '';
              };

              type = mkOption {
                type = types.enum [ "sendreceive" "sendonly" "receiveonly" ];
                default = "sendreceive";
                description = ''
                  Whether to send only changes from this folder, only receive them
                  or propagate both.
                '';
              };

              watch = mkOption {
                type = types.bool;
                default = true;
                description = ''
                  Whether the folder should be watched for changes by inotify.
                '';
              };

              watchDelay = mkOption {
                type = types.int;
                default = 10;
                description = ''
                  The delay after an inotify event is triggered.
                '';
              };

              ignorePerms = mkOption {
                type = types.bool;
                default = true;
                description = ''
                  Whether to propagate permission changes.
                '';
              };

              ignoreDelete = mkOption {
                type = types.bool;
                default = false;
                description = ''
                  Whether to delete files in destination. See <link
                  xlink:href="https://docs.syncthing.net/advanced/folder-ignoredelete.html">
                  upstream's docs</link>.
                '';
              };

            };
          }));
        };
      };

      guiAddress = mkOption {
        type = types.str;
        default = "127.0.0.1:8384";
        description = ''
          Address to serve the GUI.
        '';
      };

      systemService = mkOption {
        type = types.bool;
        default = true;
        description = "Auto launch Syncthing as a system service.";
      };

      user = mkOption {
        type = types.str;
        default = defaultUser;
        description = ''
          Syncthing will be run under this user (user will be created if it doesn't exist.
          This can be your user name).
        '';
      };

      group = mkOption {
        type = types.str;
        default = defaultUser;
        description = ''
          Syncthing will be run under this group (group will not be created if it doesn't exist.
          This can be your user name).
        '';
      };

      all_proxy = mkOption {
        type = with types; nullOr str;
        default = null;
        example = "socks5://address.com:1234";
        description = ''
          Overwrites all_proxy environment variable for the syncthing process to
          the given value. This is normaly used to let relay client connect
          through SOCKS5 proxy server.
        '';
      };

      dataDir = mkOption {
        type = types.path;
        default = "/var/lib/syncthing";
        description = ''
          Path where synced directories will exist.
        '';
      };

      configDir = mkOption {
        type = types.path;
        description = ''
          Path where the settings and keys will exist.
        '';
        default =
          let
            nixos = config.system.stateVersion;
            cond  = versionAtLeast nixos "19.03";
          in cfg.dataDir + (optionalString cond "/.config/syncthing");
      };

      openDefaultPorts = mkOption {
        type = types.bool;
        default = false;
        example = literalExample "true";
        description = ''
          Open the default ports in the firewall:
            - TCP 22000 for transfers
            - UDP 21027 for discovery
          If multiple users are running syncthing on this machine, you will need to manually open a set of ports for each instance and leave this disabled.
          Alternatively, if are running only a single instance on this machine using the default ports, enable this.
        '';
      };

      package = mkOption {
        type = types.package;
        default = pkgs.syncthing;
        defaultText = "pkgs.syncthing";
        example = literalExample "pkgs.syncthing";
        description = ''
          Syncthing package to use.
        '';
      };
    };
  };

  imports = [
    (mkRemovedOptionModule ["services" "syncthing" "useInotify"] ''
      This option was removed because syncthing now has the inotify functionality included under the name "fswatcher".
      It can be enabled on a per-folder basis through the webinterface.
    '')
  ];

  ###### implementation

  config = mkIf cfg.enable {

    networking.firewall = mkIf cfg.openDefaultPorts {
      allowedTCPPorts = [ 22000 ];
      allowedUDPPorts = [ 21027 ];
    };

    systemd.packages = [ pkgs.syncthing ];

    users.users = mkIf (cfg.systemService && cfg.user == defaultUser) {
      ${defaultUser} =
        { group = cfg.group;
          home  = cfg.dataDir;
          createHome = true;
          uid = config.ids.uids.syncthing;
          description = "Syncthing daemon user";
        };
    };

    users.groups = mkIf (cfg.systemService && cfg.group == defaultUser) {
      ${defaultUser}.gid =
        config.ids.gids.syncthing;
    };

    systemd.services = {
      syncthing = mkIf cfg.systemService {
        description = "Syncthing service";
        after = [ "network.target" ];
        environment = {
          STNORESTART = "yes";
          STNOUPGRADE = "yes";
          inherit (cfg) all_proxy;
        } // config.networking.proxy.envVars;
        wantedBy = [ "multi-user.target" ];
        serviceConfig = {
          Restart = "on-failure";
          SuccessExitStatus = "2 3 4";
          RestartForceExitStatus="3 4";
          User = cfg.user;
          Group = cfg.group;
          ExecStartPre = mkIf (cfg.declarative.cert != null || cfg.declarative.key != null)
            "+${pkgs.writers.writeBash "syncthing-copy-keys" ''
              install -dm700 -o ${cfg.user} -g ${cfg.group} ${cfg.configDir}
              ${optionalString (cfg.declarative.cert != null) ''
                install -Dm400 -o ${cfg.user} -g ${cfg.group} ${toString cfg.declarative.cert} ${cfg.configDir}/cert.pem
              ''}
              ${optionalString (cfg.declarative.key != null) ''
                install -Dm400 -o ${cfg.user} -g ${cfg.group} ${toString cfg.declarative.key} ${cfg.configDir}/key.pem
              ''}
            ''}"
          ;
          ExecStart = ''
            ${cfg.package}/bin/syncthing \
              -no-browser \
              -gui-address=${cfg.guiAddress} \
              -home=${cfg.configDir}
          '';
          MemoryDenyWriteExecute = true;
          NoNewPrivileges = true;
          PrivateDevices = true;
          PrivateMounts = true;
          PrivateTmp = true;
          PrivateUsers = true;
          ProtectControlGroups = true;
          ProtectHostname = true;
          ProtectKernelModules = true;
          ProtectKernelTunables = true;
          RestrictNamespaces = true;
          RestrictRealtime = true;
          RestrictSUIDSGID = true;
          CapabilityBoundingSet = [
            "~CAP_SYS_PTRACE" "~CAP_SYS_ADMIN"
            "~CAP_SETGID" "~CAP_SETUID" "~CAP_SETPCAP"
            "~CAP_SYS_TIME" "~CAP_KILL"
          ];
        };
      };
      syncthing-init = mkIf (
        cfg.declarative.devices != {} || cfg.declarative.folders != {}
      ) {
        after = [ "syncthing.service" ];
        wantedBy = [ "multi-user.target" ];

        serviceConfig = {
          User = cfg.user;
          RemainAfterExit = true;
          Type = "oneshot";
          ExecStart = updateConfig;
        };
      };

      syncthing-resume = {
        wantedBy = [ "suspend.target" ];
      };
    };
  };
}