{
  description = "NixOS system configuration";

  inputs = {
    flake-utils.url = "github:numtide/flake-utils";

    nixpkgs.url = "github:nixos/nixpkgs/nixos-21.05";
    nixpkgs-unstable.url = "github:nixos/nixpkgs/nixos-unstable";

    home-manager.url = "github:nix-community/home-manager/release-21.05";
    home-manager.inputs.nixpkgs.follows = "nixpkgs";

    krops.url = "github:Mic92/krops";
    krops.inputs.flake-utils.follows = "flake-utils";
    krops.inputs.nixpkgs.follows = "nixpkgs";

    nixos-hardware.url = "github:nixos/nixos-hardware/master";

    nix-pre-commit-hooks.url = "github:cachix/pre-commit-hooks.nix/master";
    nix-pre-commit-hooks.inputs.flake-utils.follows = "flake-utils";
    nix-pre-commit-hooks.inputs.nixpkgs.follows = "nixpkgs";

    sops-nix.url = "github:Mic92/sops-nix";
    sops-nix.inputs.nixpkgs.follows = "nixpkgs";

    infinisilSystem.url = "github:Infinisil/system/91c5df20db68a995155218c5334db0e394185ca8";
    infinisilSystem.flake = false;

    nixpkgs-overlay.url = "git+https://git.sbruder.de/simon/nixpkgs-overlay";
    nixpkgs-overlay.inputs.flake-utils.follows = "flake-utils";
    nixpkgs-overlay.inputs.nixpkgs.follows = "nixpkgs";
    nixpkgs-overlay.inputs.nix-pre-commit-hooks.follows = "nix-pre-commit-hooks";

    bang-evaluator.url = "git+https://git.sbruder.de/simon/bangs";
    bang-evaluator.inputs.flake-utils.follows = "flake-utils";
    bang-evaluator.inputs.nixpkgs.follows = "nixpkgs";

    aria2_exporter.url = "github:sbruder/aria2_exporter";
    aria2_exporter.inputs.flake-utils.follows = "flake-utils";
    aria2_exporter.inputs.nixpkgs.follows = "nixpkgs";

    AriaNg.url = "git+https://git.sbruder.de/simon/AriaNg";
    AriaNg.inputs.flake-utils.follows = "flake-utils";
    AriaNg.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs =
    { self
    , flake-utils
    , krops
    , nix-pre-commit-hooks
    , nixpkgs
    , nixpkgs-overlay
    , ...
    }@inputs: flake-utils.lib.eachDefaultSystem
      (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
        inherit (pkgs) lib;
      in
      {
        checks = {
          pre-commit-check = nix-pre-commit-hooks.lib.${system}.run {
            src = ./.;
            hooks = {
              black.enable = true;
              nixpkgs-fmt.enable = true;
              shellcheck.enable = true;
            };
          };
        };

        apps = lib.mapAttrs
          (name: program: { type = "app"; program = toString program; })
          (flake-utils.lib.flattenTree {
            deploy = lib.recurseIntoAttrs (lib.mapAttrs
              (hostname: machine:
                let
                  inherit (krops.packages.${system}) writeCommand;
                  inherit (krops) lib;
                in
                writeCommand "deploy-${hostname}" {
                  target = lib.mkTarget "root@${machine.config.deployment.targetHost}" // {
                    extraOptions = [
                      # force allocation of tty to allow aborting with ^C and to show build progress
                      "-t"
                    ];
                  };
                  source = lib.evalSource (lib.singleton {
                    config.file = {
                      path = toString ./.;
                      useChecksum = true;
                      filters = [
                        {
                          type = "include";
                          pattern = "/machines/${hostname}/";
                        }
                        {
                          type = "exclude";
                          pattern = "/machines/*/";
                        }
                      ];
                    };
                  });
                  command = targetPath: ''
                    nixos-rebuild switch --flake ${targetPath}/config -L --keep-going
                  '';
                }
              )
              self.nixosConfigurations);

            unlock = lib.recurseIntoAttrs (lib.mapAttrs
              (hostname: machine:
                let
                  inherit (machine.config.deployment)
                    targetHost
                    unlockOverV4;
                in
                pkgs.writeShellScript "unlock-${hostname}" ''
                  set -exo pipefail
                  # opening luks fails if gpg-agent is not unlocked yet
                  pass "devices/${hostname}/luks" >/dev/null
                  ssh \
                      ${lib.optionalString unlockOverV4 "-4"} \
                      -p 2222 \
                      "root@$(${pkgs.dnsutils}/bin/dig \
                          +short \
                          @${if unlockOverV4 then "8.8.8.8" else "2001:4860:4860::8888"} \
                          ${targetHost} \
                          ${if unlockOverV4 then "A" else "AAAA"})" \
                      "cat > /crypt-ramfs/passphrase" < <(pass "devices/${hostname}/luks")
                '')
              self.nixosConfigurations);

            showKeyFingerprint = pkgs.writeShellScript "show-key-fingerprint" ''
              gpg --with-fingerprint --with-colons --show-key "keys/''${1}.asc" | awk -F: '$1 == "fpr" { print $10; exit }'
            '';

          });

        devShell = pkgs.mkShell {
          buildInputs = (with pkgs; [
            black
            nixpkgs-fmt
            shellcheck
            sops
            ssh-to-pgp
          ]);

          shellHook = ''
            find ${./keys} -type f -print0 | xargs -0 ${pkgs.gnupg}/bin/gpg --quiet --import
          '' + self.checks.${system}.pre-commit-check.shellHook;
        };
      }) // {
      overlay = import ./pkgs;

      nixosConfigurations = nixpkgs.lib.mapAttrs
        (hostname: { system
                   , extraModules ? [ ]
                   , targetHost ? hostname
                   , unlockOverV4 ? true
                   }: nixpkgs.lib.nixosSystem rec {
          inherit system;

          modules = [
            (./machines + "/${hostname}/configuration.nix")

            {
              _module.args.inputs = inputs;
            }

            # deployment settings
            ({ lib, ... }: {
              options.deployment = {
                targetHost = lib.mkOption {
                  type = lib.types.str;
                  readOnly = true;
                  internal = true;
                };
                unlockOverV4 = lib.mkOption {
                  type = lib.types.bool;
                  readOnly = true;
                  internal = true;
                  description = "Whether to unlock the host over IPv4 (only)";
                };
              };
              config.deployment = {
                inherit
                  targetHost
                  unlockOverV4;
              };
            })
          ] ++ (with inputs; [
            home-manager.nixosModules.home-manager
            sops-nix.nixosModules.sops

            aria2_exporter.nixosModules.aria2_exporter
            bang-evaluator.nixosModules.bang-evaluator
          ]) ++ (builtins.attrValues nixpkgs-overlay.nixosModules)
          ++ extraModules;
        })
        (import ./machines inputs);
    };
}