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

with lib;

let
  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.kernel.enable ''
        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

      ${if config.boot.initrd.systemd.enable then ''
        cp ${config.system.build.bootStage2} $out/prepare-root
        substituteInPlace $out/prepare-root --subst-var-by systemConfig $out
        # This must not be a symlink or the abs_path of the grub builder for the tests
        # will resolve the symlink and we end up with a path that doesn't point to a
        # system closure.
        cp "$systemd/lib/systemd/systemd" $out/init
      '' else ''
        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 "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 $out/bin
      export localeArchive="${config.i18n.glibcLocales}/lib/locale/locale-archive"
      export distroId=${config.system.nixos.distroId};
      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
      ''}

      ${config.system.systemBuilderCommands}

      echo -n "$extraDependencies" > $out/extra-dependencies

      ${optionalString (!config.boot.isContainer && config.boot.bootspec.enable) ''
        ${config.boot.bootspec.writer}
        ${config.boot.bootspec.validator} "$out/${config.boot.bootspec.filename}"
      ''}

      ${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;
    activationScript = config.system.activationScripts.script;
    dryActivationScript = config.system.dryActivationScript;
    nixosLabel = config.system.nixos.label;

    inherit (config.system) extraDependencies;

    # Needed by switch-to-configuration.
    perl = pkgs.perl.withPackages (p: with p; [ ConfigIniFiles FileSlurp ]);
  } // config.system.systemBuilderArgs);

  # 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;

  systemWithBuildDeps = system.overrideAttrs (o: {
    systemBuildClosure = pkgs.closureInfo { rootPaths = [ system.drvPath ]; };
    buildCommand = o.buildCommand + ''
      ln -sn $systemBuildClosure $out/build-closure
    '';
  });

in

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

  options = {

    system.boot.loader.id = mkOption {
      internal = true;
      default = "";
      description = lib.mdDoc ''
        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 = lib.mdDoc ''
        Name of the kernel file to be passed to the bootloader.
      '';
    };

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

    system.build = {
      installBootLoader = mkOption {
        internal = true;
        # "; true" => make the `$out` argument from switch-to-configuration.pl
        #             go to `true` instead of `echo`, hiding the useless path
        #             from the log.
        default = "echo 'Warning: do not know how to make this configuration bootable; please enable a boot loader.' 1>&2; true";
        description = lib.mdDoc ''
          A program that writes a bootloader installation script to the path passed in the first command line argument.

          See `nixos/modules/system/activation/switch-to-configuration.pl`.
        '';
        type = types.unique {
          message = ''
            Only one bootloader can be enabled at a time. This requirement has not
            been checked until NixOS 22.05. Earlier versions defaulted to the last
            definition. Change your configuration to enable only one bootloader.
          '';
        } (types.either types.str types.package);
      };

      toplevel = mkOption {
        type = types.package;
        readOnly = true;
        description = lib.mdDoc ''
          This option contains the store path that typically represents a NixOS system.

          You can read this path in a custom deployment tool for example.
        '';
      };
    };


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

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

    system.systemBuilderArgs = mkOption {
      type = types.attrsOf types.unspecified;
      internal = true;
      default = {};
      description = lib.mdDoc ''
        `lib.mkDerivation` attributes that will be passed to the top level system builder.
      '';
    };

    system.forbiddenDependenciesRegex = mkOption {
      default = "";
      example = "-dev$";
      type = types.str;
      description = lib.mdDoc ''
        A POSIX Extended Regular Expression that matches store paths that
        should not appear in the system closure, with the exception of {option}`system.extraDependencies`, which is not checked.
      '';
    };

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

    system.extraDependencies = mkOption {
      type = types.listOf types.package;
      default = [];
      description = lib.mdDoc ''
        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 = lib.mdDoc "The original package to override.";
          };

          options.replacement = mkOption {
            type = types.package;
            description = lib.mdDoc "The replacement package.";
          };
        })
      );
      apply = map ({ original, replacement, ... }: {
        oldDependency = original;
        newDependency = replacement;
      });
      description = lib.mdDoc ''
        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 = lib.mdDoc ''
        The name of the system used in the {option}`system.build.toplevel` derivation.

        That derivation has the following name:
        `"nixos-system-''${config.system.name}-''${config.system.nixos.label}"`
      '';
    };

    system.includeBuildDependencies = mkOption {
      type = types.bool;
      default = false;
      description = lib.mdDoc ''
        Whether to include the build closure of the whole system in
        its runtime closure.  This can be useful for making changes
        fully offline, as it includes all sources, patches, and
        intermediate outputs required to build all the derivations
        that the system depends on.

        Note that this includes _all_ the derivations, down from the
        included applications to their sources, the compilers used to
        build them, and even the bootstrap compiler used to compile
        the compilers. This increases the size of the system and the
        time needed to download its dependencies drastically: a
        minimal configuration with no extra services enabled grows
        from ~670MiB in size to 13.5GiB, and takes proportionally
        longer to download.
      '';
    };

  };


  config = {

    system.extraSystemBuilderCmds =
      optionalString
        config.system.copySystemConfiguration
        ''ln -s '${import ../../../lib/from-env.nix "NIXOS_CONFIG" <nixos-config>}' \
            "$out/configuration.nix"
        '' +
      optionalString
        (config.system.forbiddenDependenciesRegex != "")
        ''
          if [[ $forbiddenDependenciesRegex != "" && -n $closureInfo ]]; then
            if forbiddenPaths="$(grep -E -- "$forbiddenDependenciesRegex" $closureInfo/store-paths)"; then
              echo -e "System closure $out contains the following disallowed paths:\n$forbiddenPaths"
              exit 1
            fi
          fi
        '';

    system.systemBuilderArgs = lib.optionalAttrs (config.system.forbiddenDependenciesRegex != "") {
      inherit (config.system) forbiddenDependenciesRegex;
      closureInfo = pkgs.closureInfo { rootPaths = [
        # override to avoid  infinite recursion (and to allow using extraDependencies to add forbidden dependencies)
        (config.system.build.toplevel.overrideAttrs (_: { extraDependencies = []; closureInfo = null; }))
      ]; };
    };

    system.build.toplevel = if config.system.includeBuildDependencies then systemWithBuildDeps else system;

  };

}