lib :

with lib;

rec {
  paramsToConf = cfg : ps : mkConf 0 (paramsToRenderedStrings cfg ps);

  # mkConf takes an indentation level (which usually starts at 0) and a nested
  # attribute set of strings and will render that set to a strongswan.conf style
  # configuration format. For example:
  #
  #   mkConf 0 {a = "1"; b = { c = { "foo" = "2"; "bar" = "3"; }; d = "4";};}   =>   ''
  #   a = 1
  #   b {
  #     c {
  #       foo = 2
  #       bar = 3
  #     }
  #     d = 4
  #   }''
  mkConf = indent : ps :
    concatMapStringsSep "\n"
      (name:
        let value = ps.${name};
            indentation = replicate indent " ";
        in
        indentation + (
          if isAttrs value
          then "${name} {\n" +
                 mkConf (indent + 2) value + "\n" +
               indentation + "}"
          else "${name} = ${value}"
        )
      )
      (attrNames ps);

  replicate = n : c : concatStrings (builtins.genList (_x : c) n);

  # `paramsToRenderedStrings cfg ps` converts the NixOS configuration `cfg`
  # (typically the "config" argument of a NixOS module) and the set of
  # parameters `ps` (an attribute set where the values are constructed using the
  # parameter constructors in ./param-constructors.nix) to a nested attribute
  # set of strings (rendered parameters).
  paramsToRenderedStrings = cfg : ps :
    filterEmptySets (
      (mapParamsRecursive (path: name: param:
        let value = attrByPath path null cfg;
        in optionalAttrs (value != null) (param.render name value)
      ) ps));

  filterEmptySets = set : filterAttrs (n: v: (v != null)) (mapAttrs (name: value:
    if isAttrs value
    then let value' = filterEmptySets value;
         in if value' == {}
            then null
            else value'
    else value
  ) set);

  # Recursively map over every parameter in the given attribute set.
  mapParamsRecursive = mapAttrsRecursiveCond' (as: (!(as ? _type && as._type == "param")));

  mapAttrsRecursiveCond' = cond: f: set:
    let
      recurse = path: set:
        let
          g =
            name: value:
            if isAttrs value && cond value
              then { ${name} = recurse (path ++ [name]) value; }
              else f (path ++ [name]) name value;
        in mapAttrs'' g set;
    in recurse [] set;

  mapAttrs'' = f: set:
    foldl' (a: b: a // b) {} (map (attr: f attr set.${attr}) (attrNames set));

  # Extract the options from the given set of parameters.
  paramsToOptions = ps :
    mapParamsRecursive (_path: name: param: { ${name} = param.option; }) ps;

}
