{ config, lib, pkgs, ... }:
let
  # Taken from https://nixos.wiki/wiki/Overlays
  overlaysCompat = pkgs.writeTextFile {
    name = "overlays-compat";
    destination = "/overlays.nix";
    text = ''
      self: super:
      with super.lib;
      let
        # Load the system config and get the `nixpkgs.overlays` option
        overlays = (import <nixpkgs/nixos> { }).config.nixpkgs.overlays;
      in
        # Apply all overlays to the input of the current "main" overlay
        foldl' (flip extends) (_: super) overlays self
    '';
  };
in
{
  # Options that affect multiple modules
  options.sbruder = {
    full = lib.mkOption {
      type = lib.types.bool;
      description = ''
        Whether to build the full system. If disabled, the system closure will
        be smaller, but some features will not be available.
      '';
      default = true;
    };
    gui.enable = lib.mkEnableOption "gui";
    games.enable = lib.mkEnableOption "games";
  };

  # All modules are imported but non-essential modules are activated by
  # configuration options
  imports = [
    ../pkgs/modules.nix
    ./cups.nix
    ./docker.nix
    ./fonts.nix
    ./grub.nix
    ./gui.nix
    ./initrd-ssh.nix
    ./libvirt.nix
    ./locales.nix
    ./media-proxy.nix
    ./network-manager.nix
    ./office.nix
    ./prometheus/node_exporter.nix
    ./pubkeys.nix
    ./pulseaudio.nix
    ./restic.nix
    ./secrets.nix
    ./ssh.nix
    ./tools.nix
    ./udev.nix
    ./unfree.nix
    ./wireguard
  ];

  config = lib.mkMerge [
    {
      # Essential system tools
      environment.systemPackages = with pkgs; [
        git
        git-crypt # used to store secrets in configuration
        git-lfs # not so essential, but required to clone config
        htop
        tmux
        vim
      ];

      # Clean temporary files on boot
      boot.cleanTmpDir = true;

      # Set zsh as default shell
      programs.zsh.enable = true;
      users.defaultUserShell = pkgs.zsh;

      # command-not-found does not work without channels
      programs.command-not-found.enable = false;

      # Sane swapping
      boot.kernel.sysctl."vm.swapiness" = 10;

      # Store logs persistently
      services.journald.extraConfig = "Storage = persistent";

      # Hard drive monitoring
      services.smartd.enable = lib.mkDefault true;
      # Network monitoring
      services.vnstat.enable = true;

      # Authentication/Encryption agents
      programs.gnupg.agent.enable = true;
      programs.ssh.startAgent = true;

      # When this is set to true (default), routing everything through a
      # wireguard tunnel does not work.
      networking.firewall.checkReversePath = false;

      nix = {
        nixPath = [
          "/var/src" # pinned nixpkgs and configuration
          "nixpkgs=/var/src/nixpkgs" # for nix run
          "nixpkgs-overlays=${overlaysCompat}"
        ];
        # Make sudoers trusted nix users
        trustedUsers = [ "@wheel" ];

        # On-the-fly optimisation of nix store
        autoOptimiseStore = true;
        # Keep output of derivations with gc root
        extraOptions = ''
          keep-outputs = true
          keep-derivations = true
        '';

        # Make nix build in background less noticeable
        daemonIONiceLevel = 5; # 0-7
      };
      systemd.services.nix-daemon.serviceConfig.CPUSchedulingPolicy = "batch";

      nixpkgs.config = {
        # Add unstable channel
        packageOverrides = pkgs: {
          unstable = import (import ../nix/sources.nix).nixpkgs-unstable {
            config = config.nixpkgs.config;
            overlays = config.nixpkgs.overlays;
          };
        };
      };

      nixpkgs.overlays = [
        (import ../pkgs)
      ];

      # Globally set Let’s Encrypt requirements
      security.acme = {
        acceptTerms = true;
        email = "security@sbruder.de";
      };
    }
    (lib.mkIf (!config.sbruder.full) {
      # Adapted from nixpkgs/nixos/modules/profiles/minimal.nix
      i18n.supportedLocales = map
        (locale: locale + "/UTF-8")
        ((lib.singleton config.i18n.defaultLocale)
          ++ (lib.attrValues config.i18n.extraLocaleSettings));

      documentation.enable = lib.mkDefault false;
    })
  ];
}