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

with lib;

let
  cfg = config.services.prometheus.exporters;

  # each attribute in `exporterOpts` is expected to have specified:
  #   - port        (types.int):   port on which the exporter listens
  #   - serviceOpts (types.attrs): config that is merged with the
  #                                default definition of the exporter's
  #                                systemd service
  #   - extraOpts   (types.attrs): extra configuration options to
  #                                configure the exporter with, which
  #                                are appended to the default options
  #
  #  Note that `extraOpts` is optional, but a script for the exporter's
  #  systemd service must be provided by specifying either
  #  `serviceOpts.script` or `serviceOpts.serviceConfig.ExecStart`
  exporterOpts = {
    blackbox = import ./exporters/blackbox.nix { inherit config lib pkgs; };
    collectd = import ./exporters/collectd.nix { inherit config lib pkgs; };
    fritzbox = import ./exporters/fritzbox.nix { inherit config lib pkgs; };
    json     = import ./exporters/json.nix     { inherit config lib pkgs; };
    minio    = import ./exporters/minio.nix    { inherit config lib pkgs; };
    nginx    = import ./exporters/nginx.nix    { inherit config lib pkgs; };
    node     = import ./exporters/node.nix     { inherit config lib pkgs; };
    snmp     = import ./exporters/snmp.nix     { inherit config lib pkgs; };
    unifi    = import ./exporters/unifi.nix    { inherit config lib pkgs; };
    varnish  = import ./exporters/varnish.nix  { inherit config lib pkgs; };
  };

  mkExporterOpts = ({ name, port }: {
    enable = mkEnableOption "the prometheus ${name} exporter";
    port = mkOption {
      type = types.int;
      default = port;
      description = ''
        Port to listen on.
      '';
    };
    listenAddress = mkOption {
      type = types.str;
      default = "0.0.0.0";
      description = ''
        Address to listen on.
      '';
    };
    extraFlags = mkOption {
      type = types.listOf types.str;
      default = [];
      description = ''
        Extra commandline options to pass to the ${name} exporter.
      '';
    };
    openFirewall = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Open port in firewall for incoming connections.
      '';
    };
    firewallFilter = mkOption {
      type = types.str;
      default = "-p tcp -m tcp --dport ${toString port}";
      example = literalExample ''
        "-i eth0 -p tcp -m tcp --dport ${toString port}"
      '';
      description = ''
        Specify a filter for iptables to use when
        <option>services.prometheus.exporters.${name}.openFirewall</option>
        is true. It is used as `ip46tables -I INPUT <option>firewallFilter</option> -j ACCEPT`.
      '';
    };
    user = mkOption {
      type = types.str;
      default = "nobody";
      description = ''
        User name under which the ${name} exporter shall be run.
        Has no effect when <option>systemd.services.prometheus-${name}-exporter.serviceConfig.DynamicUser</option> is true.
      '';
    };
    group = mkOption {
      type = types.str;
      default = "nobody";
      description = ''
        Group under which the ${name} exporter shall be run.
        Has no effect when <option>systemd.services.prometheus-${name}-exporter.serviceConfig.DynamicUser</option> is true.
      '';
    };
  });

  mkSubModule = { name, port, extraOpts, serviceOpts }: {
    ${name} = mkOption {
      type = types.submodule {
        options = (mkExporterOpts {
          inherit name port;
        } // extraOpts);
      };
      internal = true;
      default = {};
    };
  };

  mkSubModules = (foldl' (a: b: a//b) {}
    (mapAttrsToList (name: opts: mkSubModule {
      inherit name;
      inherit (opts) port serviceOpts;
      extraOpts = opts.extraOpts or {};
    }) exporterOpts)
  );

  mkExporterConf = { name, conf, serviceOpts }:
    mkIf conf.enable {
      networking.firewall.extraCommands = mkIf conf.openFirewall ''
        ip46tables -I INPUT ${conf.firewallFilter} -j ACCEPT
      '';
      systemd.services."prometheus-${name}-exporter" = mkMerge ([{
        wantedBy = [ "multi-user.target" ];
        after = [ "network.target" ];
        serviceConfig = {
          Restart = mkDefault "always";
          PrivateTmp = mkDefault true;
          WorkingDirectory = mkDefault /tmp;
        } // mkIf (!(serviceOpts.serviceConfig.DynamicUser or false)) {
          User = conf.user;
          Group = conf.group;
        };
      } serviceOpts ]);
  };
in
{
  options.services.prometheus.exporters = mkOption {
    type = types.submodule {
      options = (mkSubModules);
    };
    description = "Prometheus exporter configuration";
    default = {};
    example = literalExample ''
      {
        node = {
          enable = true;
          enabledCollectors = [ "systemd" ];
        };
        varnish.enable = true;
      }
    '';
  };

  config = mkMerge ([{
    assertions = [{
      assertion = (cfg.snmp.configurationPath == null) != (cfg.snmp.configuration == null);
      message = ''
        Please ensure you have either `services.prometheus.exporters.snmp.configuration'
          or `services.prometheus.exporters.snmp.configurationPath' set!
      '';
    }];
  }] ++ [(mkIf config.services.minio.enable {
    services.prometheus.exporters.minio.minioAddress  = mkDefault "http://localhost:9000";
    services.prometheus.exporters.minio.minioAccessKey = mkDefault config.services.minio.accessKey;
    services.prometheus.exporters.minio.minioAccessSecret = mkDefault config.services.minio.secretKey;
  })] ++ (mapAttrsToList (name: conf:
    mkExporterConf {
      inherit name;
      inherit (conf) serviceOpts;
      conf = cfg.${name};
    }) exporterOpts)
  );
}