{ lib, pkgs, config, ...}:
let
  inherit (lib)
    concatStrings concatStringsSep mapAttrsToList mkOption types;
  inherit (builtins) toString;
  inherit (pkgs.pseudofile) dir symlink;
  passwd-file  =
    let lines =  mapAttrsToList (name: u: "${name}:${if u ? passwd  then u.passwd else "!!"}:${toString u.uid}:${toString u.gid}:${u.gecos}:${u.dir}:${u.shell}\n" )
      config.users;
    in concatStrings lines;
  group-file =
    let lines = mapAttrsToList
      (name: {gid, usernames ? []}:
        "${name}:x:${toString gid}:${concatStringsSep "," usernames}\n" )
      config.groups;
    in concatStrings lines;
in {
  options = {
    users =  mkOption {
      type = types.attrsOf (types.submodule {
        options = {
          passwd = mkOption {
            type = types.str;
            default = "!!";
          };
          uid = mkOption {
            type = types.int;
          };
          gid = mkOption {
            type = types.int;
          };
          gecos = mkOption {
            type = types.str;
            default = "";
            example = "Jo Q User";
          };
          dir = mkOption {
            type = types.str;
            default = "/run";
          };
          shell = mkOption {
            type = types.str;
            default = "/bin/sh";
          };
        };
      });
    };
    groups =  mkOption {
      type = types.attrsOf (types.submodule {
        options = {
          gid = mkOption {
            type = types.int;
          };
          usernames = mkOption {
            type = types.listOf types.str;
            default = [];
          };
        };
      });
    };
  };
  config = {
    filesystem = dir {
      etc = dir {
        passwd = { file = passwd-file; };
        group = { file = group-file; };
      };
    };
  };
}