{pkgs, config, ...}:

with pkgs.lib;

let

  ids = config.ids;

  
  # User accounts to be created/updated by NixOS.
  users =
    let
      defaultUsers =
        [ { name = "root";
            uid = ids.uids.root;
            description = "System administrator";
            home = "/root";
            shell = config.users.defaultUserShell;
            group = "root";
          }
          { name = "nobody";
            uid = ids.uids.nobody;
            description = "Unprivileged account (don't use!)";
          }
        ];

      addAttrs =
        { name
        , description
        , uid ? ""
        , group ? "nogroup"
        , extraGroups ? []
        , home ? "/var/empty"
        , shell ? (if useDefaultShell then config.users.defaultUserShell else "/noshell")
        , createHome ? false
        , useDefaultShell ? false
        , password ? null
        }:
        { inherit name description uid group extraGroups home shell createHome password; };

    in map addAttrs (defaultUsers ++ config.users.extraUsers);


  # Groups to be created/updated by NixOS.
  groups =
    let
      defaultGroups = 
        [ { name = "root";
            gid = ids.gids.root;
          }
          { name = "wheel";
            gid = ids.gids.wheel;
          }
          { name = "disk";
            gid = ids.gids.disk;
          }
          { name = "kmem";
            gid = ids.gids.kmem;
          }
          { name = "tty";
            gid = ids.gids.tty;
          }
          { name = "floppy";
            gid = ids.gids.floppy;
          }
          { name = "uucp";
            gid = ids.gids.uucp;
          }
          { name = "lp";
            gid = ids.gids.lp;
          }
          { name = "cdrom";
            gid = ids.gids.cdrom;
          }
          { name = "tape";
            gid = ids.gids.tape;
          }
          { name = "video";
            gid = ids.gids.video;
          }
          { name = "dialout";
            gid = ids.gids.dialout;
          }
          { name = "nogroup";
            gid = ids.gids.nogroup;
          }
          { name = "users";
            gid = ids.gids.users;
          }
          { name = "nixbld";
            gid = ids.gids.nixbld;
          }
          { name = "utmp";
            gid = ids.gids.utmp;
          }
        ];

      addAttrs =
        { name, gid ? "" }:
        { inherit name gid; };

    in map addAttrs (defaultGroups ++ config.users.extraGroups);


  # Note: the 'X' in front of the password is to distinguish between
  # having an empty password, and not having a password.
  serializedUser = u: "${u.name}\n${u.description}\n${toString u.uid}\n${u.group}\n${toString (concatStringsSep "," u.extraGroups)}\n${u.home}\n${u.shell}\n${toString u.createHome}\n${if u.password != null then "X" + u.password else ""}\n";
  serializedGroup = g: "${g.name}\n${toString g.gid}";
  
  # keep this extra file so that cat can be used to pass special chars such as "`" which is used in the avahi daemon
  usersFile = pkgs.writeText "users" (concatStrings (map serializedUser users));
  
in

{

  ###### interface

  options = {
  
    users.extraUsers = mkOption {
      default = [];
      example =
        [ { name = "alice";
            uid = 1234;
            description = "Alice";
            home = "/home/alice";
            createHome = true;
            group = "users";
            extraGroups = ["wheel"];
            shell = "/bin/sh";
            password = "foobar";
          }
        ];
      description = ''
        Additional user accounts to be created automatically by the system.
      '';
    };

    users.extraGroups = mkOption {
      default = [];
      example =
        [ { name = "students";
            gid = 1001;
          }
        ];
      description = ''
        Additional groups to be created automatically by the system.
      '';
    };

  };
  

  ###### implementation

  config = {

    system.activationScripts.users = fullDepEntry
      ''
        echo "updating users..."

        cat ${usersFile} | while true; do
            read name || break
            read description
            read uid
            read group
            read extraGroups
            read home
            read shell
            read createHome
            read password

            if ! curEnt=$(getent passwd "$name"); then
                useradd --system \
                    --comment "$description" \
                    ''${uid:+--uid $uid} \
                    --gid "$group" \
                    --groups "$extraGroups" \
                    --home "$home" \
                    --shell "$shell" \
                    ''${createHome:+--create-home} \
                    "$name"
                if test "''${password:0:1}" = 'X'; then
                    echo "''${password:1}" | ${pkgs.shadow}/bin/passwd --stdin "$name"
                fi
            else
                #echo "updating user $name..."
                oldIFS="$IFS"; IFS=:; set -- $curEnt; IFS="$oldIFS"
                prevUid=$3
                prevHome=$6
                # Don't change the UID if it's the same, otherwise usermod
                # will complain.
                if test "$prevUid" = "$uid"; then unset uid; fi
                # Don't change the home directory if it's the same to prevent
                # unnecessary warnings about logged in users.
                if test "$prevHome" = "$home"; then unset home; fi
                usermod \
                    --comment "$description" \
                    ''${uid:+--uid $uid} \
                    --gid "$group" \
                    --groups "$extraGroups" \
                    ''${home:+--home "$home"} \
                    --shell "$shell" \
                    "$name"
            fi

        done
      '' [ "groups" ];

    system.activationScripts.groups = fullDepEntry
      ''
        echo "updating groups..."
        
        while true; do
            read name || break
            read gid

            if ! curEnt=$(getent group "$name"); then
                groupadd --system \
                    ''${gid:+--gid $gid} \
                    "$name"
            else
                #echo "updating group $name..."
                oldIFS="$IFS"; IFS=:; set -- $curEnt; IFS="$oldIFS"
                prevGid=$3
                if test -n "$gid" -a "$prevGid" != "$gid"; then
                    groupmod --gid $gid "$name"
                fi
            fi
        done <<EndOfGroupList
        ${concatStringsSep "\n" (map serializedGroup groups)}
        EndOfGroupList
      '' [ "rootPasswd" "binsh" "etc" "var" ];

  };

}