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

with lib;

let


  # This attribute is responsible for creating boot entries for
  # child configuration. They are only (directly) accessible
  # when the parent configuration is boot default. For example,
  # you can provide an easy way to boot the same configuration
  # as you use, but with another kernel
  # !!! fix this
  children =
    mapAttrs
      (childName: childConfig: childConfig.configuration.system.build.toplevel)
      config.specialisation;

  systemBuilder =
    let
      kernelPath = "${config.boot.kernelPackages.kernel}/" +
        "${config.system.boot.loader.kernelFile}";
      initrdPath = "${config.system.build.initialRamdisk}/" +
        "${config.system.boot.loader.initrdFile}";
    in ''
      mkdir $out

      # Containers don't have their own kernel or initrd.  They boot
      # directly into stage 2.
      ${optionalString (!config.boot.isContainer) ''
        if [ ! -f ${kernelPath} ]; then
          echo "The bootloader cannot find the proper kernel image."
          echo "(Expecting ${kernelPath})"
          false
        fi

        ln -s ${kernelPath} $out/kernel
        ln -s ${config.system.modulesTree} $out/kernel-modules
        ${optionalString (config.hardware.deviceTree.package != null) ''
          ln -s ${config.hardware.deviceTree.package} $out/dtbs
        ''}

        echo -n "$kernelParams" > $out/kernel-params

        ln -s ${initrdPath} $out/initrd

        ln -s ${config.system.build.initialRamdiskSecretAppender}/bin/append-initrd-secrets $out

        ln -s ${config.hardware.firmware}/lib/firmware $out/firmware
      ''}

      echo "$activationScript" > $out/activate
      echo "$dryActivationScript" > $out/dry-activate
      substituteInPlace $out/activate --subst-var out
      substituteInPlace $out/dry-activate --subst-var out
      chmod u+x $out/activate $out/dry-activate
      unset activationScript dryActivationScript
      ${pkgs.stdenv.shell} -n $out/activate
      ${pkgs.stdenv.shell} -n $out/dry-activate

      cp ${config.system.build.bootStage2} $out/init
      substituteInPlace $out/init --subst-var-by systemConfig $out

      ln -s ${config.system.build.etc}/etc $out/etc
      ln -s ${config.system.path} $out/sw
      ln -s "$systemd" $out/systemd

      echo -n "$configurationName" > $out/configuration-name
      echo -n "systemd ${toString config.systemd.package.interfaceVersion}" > $out/init-interface-version
      echo -n "$nixosLabel" > $out/nixos-version
      echo -n "${config.boot.kernelPackages.stdenv.hostPlatform.system}" > $out/system

      mkdir $out/specialisation
      ${concatStringsSep "\n"
      (mapAttrsToList (name: path: "ln -s ${path} $out/specialisation/${name}") children)}

      mkdir $out/bin
      export localeArchive="${config.i18n.glibcLocales}/lib/locale/locale-archive"
      substituteAll ${./switch-to-configuration.pl} $out/bin/switch-to-configuration
      chmod +x $out/bin/switch-to-configuration
      ${optionalString (pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) ''
        if ! output=$($perl/bin/perl -c $out/bin/switch-to-configuration 2>&1); then
          echo "switch-to-configuration syntax is not valid:"
          echo "$output"
          exit 1
        fi
      ''}

      echo -n "${toString config.system.extraDependencies}" > $out/extra-dependencies

      ${config.system.extraSystemBuilderCmds}
    '';

  # Putting it all together.  This builds a store path containing
  # symlinks to the various parts of the built configuration (the
  # kernel, systemd units, init scripts, etc.) as well as a script
  # `switch-to-configuration' that activates the configuration and
  # makes it bootable.
  baseSystem = pkgs.stdenvNoCC.mkDerivation {
    name = "nixos-system-${config.system.name}-${config.system.nixos.label}";
    preferLocalBuild = true;
    allowSubstitutes = false;
    buildCommand = systemBuilder;

    inherit (pkgs) coreutils;
    systemd = config.systemd.package;
    shell = "${pkgs.bash}/bin/sh";
    su = "${pkgs.shadow.su}/bin/su";
    utillinux = pkgs.util-linux;

    kernelParams = config.boot.kernelParams;
    installBootLoader =
      config.system.build.installBootLoader
      or "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
    activationScript = config.system.activationScripts.script;
    dryActivationScript = config.system.dryActivationScript;
    nixosLabel = config.system.nixos.label;

    configurationName = config.boot.loader.grub.configurationName;

    # Needed by switch-to-configuration.
    perl = pkgs.perl.withPackages (p: with p; [ FileSlurp NetDBus XMLParser XMLTwig ]);
  };

  # Handle assertions and warnings

  failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);

  baseSystemAssertWarn = if failedAssertions != []
    then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
    else showWarnings config.warnings baseSystem;

  # Replace runtime dependencies
  system = foldr ({ oldDependency, newDependency }: drv:
      pkgs.replaceDependency { inherit oldDependency newDependency drv; }
    ) baseSystemAssertWarn config.system.replaceRuntimeDependencies;

in

{
  imports = [
    (mkRemovedOptionModule [ "nesting" "clone" ] "Use `specialisation.«name» = { inheritParentConfig = true; configuration = { ... }; }` instead.")
    (mkRemovedOptionModule [ "nesting" "children" ] "Use `specialisation.«name».configuration = { ... }` instead.")
  ];

  options = {

    system.build = mkOption {
      internal = true;
      default = {};
      type = types.attrs;
      description = ''
        Attribute set of derivations used to setup the system.
      '';
    };

    specialisation = mkOption {
      default = {};
      example = lib.literalExpression "{ fewJobsManyCores.configuration = { nix.buildCores = 0; nix.maxJobs = 1; }; }";
      description = ''
        Additional configurations to build. If
        <literal>inheritParentConfig</literal> is true, the system
        will be based on the overall system configuration.

        To switch to a specialised configuration
        (e.g. <literal>fewJobsManyCores</literal>) at runtime, run:

        <screen>
        <prompt># </prompt>sudo /run/current-system/specialisation/fewJobsManyCores/bin/switch-to-configuration test
        </screen>
      '';
      type = types.attrsOf (types.submodule (
        local@{ ... }: let
          extend = if local.config.inheritParentConfig
            then extendModules
            else noUserModules.extendModules;
        in {
          options.inheritParentConfig = mkOption {
            type = types.bool;
            default = true;
            description = "Include the entire system's configuration. Set to false to make a completely differently configured system.";
          };

          options.configuration = mkOption {
            default = {};
            description = ''
              Arbitrary NixOS configuration.

              Anything you can add to a normal NixOS configuration, you can add
              here, including imports and config values, although nested
              specialisations will be ignored.
            '';
            visible = "shallow";
            inherit (extend { modules = [ ./no-clone.nix ]; }) type;
          };
        })
      );
    };

    system.boot.loader.id = mkOption {
      internal = true;
      default = "";
      description = ''
        Id string of the used bootloader.
      '';
    };

    system.boot.loader.kernelFile = mkOption {
      internal = true;
      default = pkgs.stdenv.hostPlatform.linux-kernel.target;
      defaultText = literalExpression "pkgs.stdenv.hostPlatform.linux-kernel.target";
      type = types.str;
      description = ''
        Name of the kernel file to be passed to the bootloader.
      '';
    };

    system.boot.loader.initrdFile = mkOption {
      internal = true;
      default = "initrd";
      type = types.str;
      description = ''
        Name of the initrd file to be passed to the bootloader.
      '';
    };

    system.copySystemConfiguration = mkOption {
      type = types.bool;
      default = false;
      description = ''
        If enabled, copies the NixOS configuration file
        (usually <filename>/etc/nixos/configuration.nix</filename>)
        and links it from the resulting system
        (getting to <filename>/run/current-system/configuration.nix</filename>).
        Note that only this single file is copied, even if it imports others.
      '';
    };

    system.extraSystemBuilderCmds = mkOption {
      type = types.lines;
      internal = true;
      default = "";
      description = ''
        This code will be added to the builder creating the system store path.
      '';
    };

    system.extraDependencies = mkOption {
      type = types.listOf types.package;
      default = [];
      description = ''
        A list of packages that should be included in the system
        closure but not otherwise made available to users. This is
        primarily used by the installation tests.
      '';
    };

    system.replaceRuntimeDependencies = mkOption {
      default = [];
      example = lib.literalExpression "[ ({ original = pkgs.openssl; replacement = pkgs.callPackage /path/to/openssl { }; }) ]";
      type = types.listOf (types.submodule (
        { ... }: {
          options.original = mkOption {
            type = types.package;
            description = "The original package to override.";
          };

          options.replacement = mkOption {
            type = types.package;
            description = "The replacement package.";
          };
        })
      );
      apply = map ({ original, replacement, ... }: {
        oldDependency = original;
        newDependency = replacement;
      });
      description = ''
        List of packages to override without doing a full rebuild.
        The original derivation and replacement derivation must have the same
        name length, and ideally should have close-to-identical directory layout.
      '';
    };

    system.name = mkOption {
      type = types.str;
      default =
        if config.networking.hostName == ""
        then "unnamed"
        else config.networking.hostName;
      defaultText = literalExpression ''
        if config.networking.hostName == ""
        then "unnamed"
        else config.networking.hostName;
      '';
      description = ''
        The name of the system used in the <option>system.build.toplevel</option> derivation.
        </para><para>
        That derivation has the following name:
        <literal>"nixos-system-''${config.system.name}-''${config.system.nixos.label}"</literal>
      '';
    };

  };


  config = {

    system.extraSystemBuilderCmds =
      optionalString
        config.system.copySystemConfiguration
        ''ln -s '${import ../../../lib/from-env.nix "NIXOS_CONFIG" <nixos-config>}' \
            "$out/configuration.nix"
        '';

    system.build.toplevel = system;

  };

  # uses extendModules to generate a type
  meta.buildDocsInSandbox = false;
}