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

with lib;

let
  cfg = config.clerie.gre-tunnel;

  generateInterfaceUnit = isIPv6: (name: tunnel:
    nameValuePair "gre-tunnel-${name}" {
        description = "GRE Tunnel - ${name}";
        requires = [ "network-online.target" ];
        after = [ "network.target" "network-online.target" ];
        wantedBy = [ "multi-user.target" ];
        environment.DEVICE = name;
        path = with pkgs; [ iproute ];

        serviceConfig = {
          Type = "oneshot";
          RemainAfterExit = true;
        };

        script = ''
          ${tunnel.preSetup}
          ip${optionalString isIPv6 " -6"} tunnel add ${name} mode ${optionalString isIPv6 "ip6"}gre remote ${tunnel.remote} local ${tunnel.local}
          ip link set ${name} up
          ip${optionalString isIPv6 " -6"} a add ${tunnel.address} dev ${name}
          ${tunnel.postSetup}
        '';

        postStop = ''
          ip link set ${name} down
          ip tunnel del ${name}
          ${tunnel.postShutdown}
        '';
      });

  checkOpts = { config, ... }@moduleAttrs: {
    options = {
      remote = mkOption {
        type = types.str;
        description = "Address of reciever.";
      };
      local = mkOption {
        type = types.str;
        description = "Address our packets originate from.";
      };
      address = mkOption {
        type = types.str;
        description = "Our address in this tunnel.";
      };
      preSetup = mkOption {
        type = types.str;
        default = "";
        description = "Commands called at the start of the interface setup.";
      };
      postSetup = mkOption {
        type = types.str;
        default = "";
        description = "Commands called at the end of the interface setup.";
      };
      postShutdown = mkOption {
        type = types.str;
        default = "";
        description = "Commands called after shutting down the interface.";
      };
    };
  };

in {
  options = {
    clerie.gre-tunnel = {
      enable = mkEnableOption "Declarative Policy-Routing";
      ipv6 = mkOption {
        type = with types; attrsOf (submodule checkOpts);
        default = {};
      };
      ipv4 = mkOption {
        type = with types; attrsOf (submodule checkOpts);
        default = {};
      };
    };
  };

  config = mkIf cfg.enable {
    systemd.services =
    (mapAttrs' (generateInterfaceUnit false) cfg.ipv4)
    // (mapAttrs' (generateInterfaceUnit true) cfg.ipv6);
  };
}