{
  config,
  options,
  kubenix,
  pkgs,
  lib,
  ...
}:
with lib; let
  cfg = config.submodules;
  parentConfig = config;

  matchesVersion = requiredVersion: version:
    if requiredVersion != null
    then
      if hasPrefix "~" requiredVersion
      then (builtins.match (removePrefix "~" requiredVersion) version) != null
      else requiredVersion == version
    else true;

  getDefaults = {
    name,
    version,
    tags,
    features,
  }:
    catAttrs "default" (filter
      (
        submoduleDefault:
          (submoduleDefault.name == null || submoduleDefault.name == name)
          && (matchesVersion submoduleDefault.version version)
          && (
            (length submoduleDefault.tags == 0)
            || (length (intersectLists submoduleDefault.tags tags)) > 0
          )
          && (
            (length submoduleDefault.features == 0)
            || (length (intersectLists submoduleDefault.features features)) > 0
          )
      )
      config.submodules.defaults);

  specialArgs =
    cfg.specialArgs
    // {
      parentConfig = config;
    };

  findSubmodule = {
    name,
    version ? null,
    latest ? true,
  }: let
    matchingSubmodules =
      filter
      (
        el:
          el.definition.name
          == name
          && (matchesVersion version el.definition.version)
      )
      cfg.imports;

    versionSortedSubmodules =
      sort
      (
        s1: s2:
          if builtins.compareVersions s1.definition.version s2.definition.version > 0
          then true
          else false
      )
      matchingSubmodules;

    matchingModule =
      if length versionSortedSubmodules == 0
      then
        throw "No module found ${name}/${
          if version == null
          then "latest"
          else version
        }"
      else head versionSortedSubmodules;
  in
    matchingModule;

  passthruConfig =
    mapAttrsToList
    (name: opt: {
      ${name} = mkMerge (mapAttrsToList
        (
          _: inst:
            if inst.passthru.enable
            then inst.config.submodule.passthru.${name} or {}
            else {}
        )
        config.submodules.instances);

      _module.args = mkMerge (mapAttrsToList
        (
          _: inst:
            if inst.passthru.enable
            then inst.config.submodule.passthru._module.args or {}
            else {}
        )
        config.submodules.instances);
    })
    (removeAttrs options ["_definedNames" "_module" "_m" "submodules"]);

  submoduleWithSpecialArgs = opts: specialArgs: let
    opts' = toList opts;
    inherit (lib.modules) evalModules;
  in
    mkOptionType rec {
      name = "submodule";
      check = x: isAttrs x || isFunction x;
      merge = loc: defs: let
        coerce = def:
          if isFunction def
          then def
          else {config = def;};
        modules =
          opts'
          ++ map (def: {
            _file = def.file;
            imports = [(coerce def.value)];
          })
          defs;
      in
        (evalModules {
          inherit modules specialArgs;
          prefix = loc;
        })
        .config;
      getSubOptions = prefix:
        (evalModules
          {
            modules = opts';
            inherit prefix specialArgs;
            # This is a work-around due to the fact that some sub-modules,
            # such as the one included in an attribute set, expects a "args"
            # attribute to be given to the sub-module. As the option
            # evaluation does not have any specific attribute name, we
            # provide a default one for the documentation.
            #
            # This is mandatory as some option declaration might use the
            # "name" attribute given as argument of the submodule and use it
            # as the default of option declarations.
            #
            # Using lookalike unicode single angle quotation marks because
            # of the docbook transformation the options receive. In all uses
            # > and < wouldn't be encoded correctly so the encoded values
            # would be used, and use of `<` and `>` would break the XML document.
            # It shouldn't cause an issue since this is cosmetic for the manual.
            args.name = "‹name›";
          })
        .options;
      getSubModules = opts';
      substSubModules = m: submoduleWithSpecialArgs m specialArgs;
      functor =
        (defaultFunctor name)
        // {
          # Merging of submodules is done as part of mergeOptionDecls, as we have to annotate
          # each submodule with its location.
          payload = [];
          binOp = lhs: rhs: [];
        };
    };
in {
  imports = [./base.nix];

  options = {
    submodules.specialArgs = mkOption {
      description = "Special args to pass to submodules. These arguments can be used for imports";
      type = types.attrs;
      default = {};
    };

    submodules.defaults = mkOption {
      description = "List of defaults to apply to submodule instances";
      type = types.listOf (types.submodule ({config, ...}: {
        options = {
          name = mkOption {
            description = "Name of the submodule to apply defaults for";
            type = types.nullOr types.str;
            default = null;
          };

          version = mkOption {
            description = ''
              Version of submodule to apply defaults for. If version starts with
              "~" it is threated as regex pattern for example "~1.0.*
            '';
            type = types.nullOr types.str;
            default = null;
          };

          tags = mkOption {
            description = "List of tags to apply defaults for";
            type = types.listOf types.str;
            default = [];
          };

          features = mkOption {
            description = "List of features that submodule has to have to apply defaults";
            type = types.listOf types.str;
            default = [];
          };

          default = mkOption {
            description = "Default to apply to submodule instance";
            type = types.unspecified;
            default = {};
          };
        };
      }));
      default = [];
    };

    submodules.propagate.enable = mkOption {
      description = "Whether to propagate defaults and imports from parent to child";
      type = types.bool;
      default = true;
    };

    submodules.imports = mkOption {
      description = "List of submodule imports";
      type = types.listOf (
        types.coercedTo
        types.path
        (module: {inherit module;})
        (
          types.submodule ({
            name,
            config,
            ...
          }: let
            evaledSubmodule' = evalModules {
              inherit specialArgs;
              modules = config.modules ++ [./base.nix];
              check = false;
            };

            evaledSubmodule =
              if (!(elem "submodule" evaledSubmodule'.config._m.features))
              then throw "no submodule defined"
              else evaledSubmodule';
          in {
            options = {
              module = mkOption {
                description = "Module defining submodule";
                type = types.unspecified;
              };

              modules = mkOption {
                description = "List of modules defining submodule";
                type = types.listOf types.unspecified;
                default = [config.module];
              };

              features = mkOption {
                description = "List of features exposed by submodule";
                type = types.listOf types.str;
              };

              definition = mkOption {
                description = "Submodule definition";
                type = types.attrs;
              };

              exportAs = mkOption {
                description = "Name under which to register exports";
                type = types.nullOr types.str;
                default = null;
              };
            };

            config = {
              definition = {
                inherit (evaledSubmodule.config.submodule) name description version tags exports;
              };

              features = evaledSubmodule.config._m.features;
            };
          })
        )
      );
      default = [];
    };

    submodules.instances = mkOption {
      description = "Attribute set of submodule instances";
      default = {};
      type = types.attrsOf (types.submodule ({
        name,
        config,
        options,
        ...
      }: let
        # submodule associated with
        submodule = findSubmodule {
          name = config.submodule;
          version = config.version;
        };

        # definition of a submodule
        submoduleDefinition = submodule.definition;

        # submodule defaults
        defaults = getDefaults {
          name = submoduleDefinition.name;
          version = submoduleDefinition.version;
          tags = submoduleDefinition.tags;
          features = submodule.features;
        };
      in {
        options = {
          name = mkOption {
            description = "Submodule instance name";
            type = types.str;
            default = name;
          };

          submodule = mkOption {
            description = "Name of the submodule to use";
            type = types.str;
            default = name;
          };

          version = mkOption {
            description = ''
              Version of submodule to use, if version starts with "~" it is
              threated as regex pattern for example "~1.0.*"
            '';
            type = types.nullOr types.str;
            default = null;
          };

          passthru.enable = mkOption {
            description = "Whether to passthru submodule resources";
            type = types.bool;
            default = true;
          };

          config = mkOption {
            description = "Submodule instance ${config.name} for ${submoduleDefinition.name}:${submoduleDefinition.version} config";
            type =
              submoduleWithSpecialArgs
              ({...}: {
                imports = submodule.modules ++ defaults ++ [./base.nix];
                _module.args.pkgs = pkgs;
                _module.args.name = config.name;
                _module.args.submodule = config;
                submodule.args = mkAliasDefinitions options.args;
              })
              specialArgs;
            default = {};
          };

          args = mkOption {
            description = "Submodule arguments (alias of config.submodule.args)";
          };
        };
      }));
    };
    default = {};
  };

  config = mkMerge ([
      {
        # register exported functions as args
        _module.args = mkMerge (map
          (submodule: {
            ${submodule.exportAs} = submodule.definition.exports;
          })
          (filter (submodule: submodule.exportAs != null) cfg.imports));

        _m.features = ["submodules"];

        submodules.specialArgs.kubenix = kubenix;

        # passthru kubenix.project to submodules
        submodules.defaults = mkMerge [
          [
            {
              default = {
                kubenix.project = parentConfig.kubenix.project;
              };
            }
          ]

          (map
            (propagate: {
              features = propagate.features;
              default = propagate.module;
            })
            config._m.propagate)
        ];
      }

      (mkIf cfg.propagate.enable {
        # if propagate is enabled and submodule has submodules included propagage defaults and imports
        submodules.defaults = [
          {
            features = ["submodules"];
            default = {
              submodules = {
                defaults = cfg.defaults;
                imports = cfg.imports;
              };
            };
          }
        ];
      })
    ]
    ++ passthruConfig);
}