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

with lib;

let

  cfg4 = config.services.dhcpd4;
  cfg6 = config.services.dhcpd6;

  writeConfig = cfg: pkgs.writeText "dhcpd.conf"
    ''
      default-lease-time 600;
      max-lease-time 7200;
      authoritative;
      ddns-update-style interim;
      log-facility local1; # see dhcpd.nix

      ${cfg.extraConfig}

      ${lib.concatMapStrings
          (machine: ''
            host ${machine.hostName} {
              hardware ethernet ${machine.ethernetAddress};
              fixed-address ${machine.ipAddress};
            }
          '')
          cfg.machines
      }
    '';

  dhcpdService = postfix: cfg: optionalAttrs cfg.enable {
    "dhcpd${postfix}" = {
      description = "DHCPv${postfix} server";
      wantedBy = [ "multi-user.target" ];
      after = [ "network.target" ];

      preStart = ''
        mkdir -m 755 -p ${cfg.stateDir}
        touch ${cfg.stateDir}/dhcpd.leases
      '';

      serviceConfig =
        let
          configFile = if cfg.configFile != null then cfg.configFile else writeConfig cfg;
          args = [ "@${pkgs.dhcp}/sbin/dhcpd" "dhcpd${postfix}" "-${postfix}"
                   "-pf" "/run/dhcpd${postfix}/dhcpd.pid"
                   "-cf" "${configFile}"
                   "-lf" "${cfg.stateDir}/dhcpd.leases"
                   "-user" "dhcpd" "-group" "nogroup"
                 ] ++ cfg.extraFlags
                   ++ cfg.interfaces;

        in {
          ExecStart = concatMapStringsSep " " escapeShellArg args;
          Type = "forking";
          Restart = "always";
          RuntimeDirectory = [ "dhcpd${postfix}" ];
          PIDFile = "/run/dhcpd${postfix}/dhcpd.pid";
        };
    };
  };

  machineOpts = {...}: {
    config = {

      hostName = mkOption {
        type = types.str;
        example = "foo";
        description = ''
          Hostname which is assigned statically to the machine.
        '';
      };

      ethernetAddress = mkOption {
        type = types.str;
        example = "00:16:76:9a:32:1d";
        description = ''
          MAC address of the machine.
        '';
      };

      ipAddress = mkOption {
        type = types.str;
        example = "192.168.1.10";
        description = ''
          IP address of the machine.
        '';
      };

    };
  };

  dhcpConfig = postfix: {

    enable = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Whether to enable the DHCPv${postfix} server.
      '';
    };

    stateDir = mkOption {
      type = types.path;
      # We use /var/lib/dhcp for DHCPv4 to save backwards compatibility.
      default = "/var/lib/dhcp${if postfix == "4" then "" else postfix}";
      description = ''
        State directory for the DHCP server.
      '';
    };

    extraConfig = mkOption {
      type = types.lines;
      default = "";
      example = ''
        option subnet-mask 255.255.255.0;
        option broadcast-address 192.168.1.255;
        option routers 192.168.1.5;
        option domain-name-servers 130.161.158.4, 130.161.33.17, 130.161.180.1;
        option domain-name "example.org";
        subnet 192.168.1.0 netmask 255.255.255.0 {
          range 192.168.1.100 192.168.1.200;
        }
      '';
      description = ''
        Extra text to be appended to the DHCP server configuration
        file. Currently, you almost certainly need to specify something
        there, such as the options specifying the subnet mask, DNS servers,
        etc.
      '';
    };

    extraFlags = mkOption {
      type = types.listOf types.str;
      default = [];
      description = ''
        Additional command line flags to be passed to the dhcpd daemon.
      '';
    };

    configFile = mkOption {
      type = types.nullOr types.path;
      default = null;
      description = ''
        The path of the DHCP server configuration file.  If no file
        is specified, a file is generated using the other options.
      '';
    };

    interfaces = mkOption {
      type = types.listOf types.str;
      default = ["eth0"];
      description = ''
        The interfaces on which the DHCP server should listen.
      '';
    };

    machines = mkOption {
      type = types.listOf (types.submodule machineOpts);
      default = [];
      example = [
        { hostName = "foo";
          ethernetAddress = "00:16:76:9a:32:1d";
          ipAddress = "192.168.1.10";
        }
        { hostName = "bar";
          ethernetAddress = "00:19:d1:1d:c4:9a";
          ipAddress = "192.168.1.11";
        }
      ];
      description = ''
        A list mapping Ethernet addresses to IPv${postfix} addresses for the
        DHCP server.
      '';
    };

  };

in

{

  ###### interface

  options = {

    services.dhcpd4 = dhcpConfig "4";
    services.dhcpd6 = dhcpConfig "6";

  };


  ###### implementation

  config = mkIf (cfg4.enable || cfg6.enable) {

    users = {
      extraUsers.dhcpd = {
        uid = config.ids.uids.dhcpd;
        description = "DHCP daemon user";
      };
    };

    systemd.services = dhcpdService "4" cfg4 // dhcpdService "6" cfg6;

  };

}