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

let

  inherit (lib.attrsets)
    attrNames
    attrValues
    mapAttrsToList
    removeAttrs
    ;
  inherit (lib.lists)
    all
    allUnique
    concatLists
    concatMap
    elem
    isList
    map
    ;
  inherit (lib.modules) mkDefault mkIf;
  inherit (lib.options) mkEnableOption mkOption mkPackageOption;
  inherit (lib.strings)
    concatLines
    match
    optionalString
    toLower
    ;
  inherit (lib.trivial) isInt;
  inherit (lib.types)
    addCheck
    attrsOf
    coercedTo
    either
    enum
    int
    lines
    listOf
    nonEmptyStr
    nullOr
    oneOf
    path
    port
    singleLineStr
    strMatching
    submodule
    ;

  scalarType =
    # see the option's description below for the
    # handling/transformation of each possible type
    oneOf [
      (enum [
        true
        null
      ])
      int
      path
      singleLineStr
    ];

  # TSM rejects servername strings longer than 64 chars.
  servernameType = strMatching "[^[:space:]]{1,64}";

  serverOptions =
    { name, config, ... }:
    {
      freeformType = attrsOf (either scalarType (listOf scalarType));
      # Client system-options file directives are explained here:
      # https://www.ibm.com/docs/en/storage-protect/8.2.1?topic=utilities-processing-options
      options.servername = mkOption {
        type = servernameType;
        default = name;
        example = "mainTsmServer";
        description = ''
          Local name of the IBM TSM server,
          must not contain space or more than 64 chars.
        '';
      };
      options.tcpserveraddress = mkOption {
        type = nonEmptyStr;
        example = "tsmserver.company.com";
        description = ''
          Host/domain name or IP address of the IBM TSM server.
        '';
      };
      options.tcpport = mkOption {
        type = addCheck port (p: p <= 32767);
        default = 1500; # official default
        description = ''
          TCP port of the IBM TSM server.
          TSM does not support ports above 32767.
        '';
      };
      options.nodename = mkOption {
        type = nonEmptyStr;
        example = "MY-TSM-NODE";
        description = ''
          Target node name on the IBM TSM server.
        '';
      };
      options.genPasswd = mkEnableOption ''
        automatic client password generation.
        This option does *not* cause a line in
        {file}`dsm.sys` by itself, but generates a
        corresponding `passwordaccess` directive.
        The password will be stored in the directory
        given by the option {option}`passworddir`.
        *Caution*:
        If this option is enabled and the server forces
        to renew the password (e.g. on first connection),
        a random password will be generated and stored
      '';
      options.passwordaccess = mkOption {
        type = enum [
          "generate"
          "prompt"
        ];
        visible = false;
      };
      options.passworddir = mkOption {
        type = nullOr path;
        default = null;
        example = "/home/alice/tsm-password";
        description = ''
          Directory that holds the TSM
          node's password information.
        '';
      };
      options.inclexcl = mkOption {
        type = coercedTo lines (pkgs.writeText "inclexcl.dsm.sys") (nullOr path);
        default = null;
        example = ''
          exclude.dir     /nix/store
          include.encrypt /home/.../*
        '';
        description = ''
          Text lines with `include.*` and `exclude.*` directives
          to be used when sending files to the IBM TSM server,
          or an absolute path pointing to a file with such lines.
        '';
      };
      config.commmethod = mkDefault "v6tcpip"; # uses v4 or v6, based on dns lookup result
      config.passwordaccess = if config.genPasswd then "generate" else "prompt";
    };

  options.programs.tsmClient = {
    enable = mkEnableOption ''
      IBM Storage Protect (Tivoli Storage Manager, TSM)
      client command line applications with a
      client system-options file "dsm.sys"
    '';
    servers = mkOption {
      type = attrsOf (submodule serverOptions);
      default = { };
      example.mainTsmServer = {
        tcpserveraddress = "tsmserver.company.com";
        nodename = "MY-TSM-NODE";
        compression = "yes";
      };
      description = ''
        Server definitions ("stanzas")
        for the client system-options file.
        The name of each entry will be used for
        the internal `servername` by default.
        Each attribute will be transformed into a line
        with a key-value pair within the server's stanza.
        Integers as values will be
        canonically turned into strings.
        The boolean value `true` will be turned
        into a line with just the attribute's name.
        The value `null` will not generate a line.
        A list as values generates an entry for
        each value, according to the rules above.
      '';
    };
    defaultServername = mkOption {
      type = nullOr servernameType;
      default = null;
      example = "mainTsmServer";
      description = ''
        If multiple server stanzas are declared with
        {option}`programs.tsmClient.servers`,
        this option may be used to name a default
        server stanza that IBM TSM uses in the absence of
        a user-defined {file}`dsm.opt` file.
        This option translates to a
        `defaultserver` configuration line.
      '';
    };
    dsmSysText = mkOption {
      type = lines;
      readOnly = true;
      description = ''
        This configuration key contains the effective text
        of the client system-options file "dsm.sys".
        It should not be changed, but may be
        used to feed the configuration into other
        TSM-depending packages used on the system.
      '';
    };
    package = mkPackageOption pkgs "tsm-client" {
      example = "tsm-client-withGui";
      extraDescription = ''
        It will be used with `.override`
        to add paths to the client system-options file.
      '';
    };
    wrappedPackage =
      mkPackageOption pkgs "tsm-client" {
        default = null;
        extraDescription = ''
          This option is to provide the effective derivation,
          wrapped with the path to the
          client system-options file "dsm.sys".
          It should not be changed, but exists
          for other modules that want to call TSM executables.
        '';
      }
      // {
        readOnly = true;
      };
  };

  cfg = config.programs.tsmClient;
  servernames = map (s: s.servername) (attrValues cfg.servers);

  assertions = [
    {
      assertion = allUnique (map toLower servernames);
      message = ''
        TSM server names
        (option `programs.tsmClient.servers`)
        contain duplicate name
        (note that server names are case insensitive).
      '';
    }
    {
      assertion = (cfg.defaultServername != null) -> (elem cfg.defaultServername servernames);
      message = ''
        TSM default server name
        `programs.tsmClient.defaultServername="${cfg.defaultServername}"`
        not found in server names in
        `programs.tsmClient.servers`.
      '';
    }
  ]
  ++ (mapAttrsToList (name: serverCfg: {
    assertion = all (key: null != match "[^[:space:]]+" key) (attrNames serverCfg);
    message = ''
      TSM server setting names in
      `programs.tsmClient.servers.${name}.*`
      contain spaces, but that's not allowed.
    '';
  }) cfg.servers)
  ++ (mapAttrsToList (name: serverCfg: {
    assertion = allUnique (map toLower (attrNames serverCfg));
    message = ''
      TSM server setting names in
      `programs.tsmClient.servers.${name}.*`
      contain duplicate names
      (note that setting names are case insensitive).
    '';
  }) cfg.servers);

  makeDsmSysLines =
    key: value:
    # Turn a key-value pair from the server options attrset
    # into zero (value==null), one (scalar value) or
    # more (value is list) configuration stanza lines.
    if isList value then
      concatMap (makeDsmSysLines key) value
    # recurse into list
    else if value == null then
      [ ]
    # skip `null` value
    else
      [

        "  ${key}${
            if value == true then
              ""
            # just output key if value is `true`
            else if isInt value then
              "  ${toString value}"
            else if path.check value then
              "  \"${value}\""
            # enclose path in ".."
            else if singleLineStr.check value then
              "  ${value}"
            else
              throw "assertion failed: cannot convert type" # should never happen
          }"

      ];

  makeDsmSysStanza =
    { servername, ... }@serverCfg:
    let
      # drop special values that should not go into server config block
      attrs = removeAttrs serverCfg [
        "servername"
        "genPasswd"
      ];
    in
    ''
      servername  ${servername}
      ${concatLines (concatLists (mapAttrsToList makeDsmSysLines attrs))}
    '';

  dsmSysText = ''
    ****  IBM Storage Protect (Tivoli Storage Manager)
    ****  client system-options file "dsm.sys".
    ****  Do not edit!
    ****  This file is generated by NixOS configuration.

    ${optionalString (cfg.defaultServername != null) "defaultserver  ${cfg.defaultServername}"}

    ${concatLines (map makeDsmSysStanza (attrValues cfg.servers))}
  '';

in

{

  inherit options;

  config = mkIf cfg.enable {
    inherit assertions;
    programs.tsmClient.dsmSysText = dsmSysText;
    programs.tsmClient.wrappedPackage = cfg.package.override rec {
      dsmSysCli = pkgs.writeText "dsm.sys" cfg.dsmSysText;
      dsmSysApi = dsmSysCli;
    };
    environment.systemPackages = [ cfg.wrappedPackage ];
  };

  meta.maintainers = [ lib.maintainers.yarny ];

}
