{ lib,
  expect,
  nix,
  nixos-rebuild,
  path,
  runCommand,
  stdenv,
  writeText,
}:
let
  # Arguably not true, but it holds up for now.
  escapeExpect = lib.strings.escapeNixString;

  expectSetup = ''
    set timeout 180
    proc expect_simple { pattern } {
      puts "Expecting: $pattern"
      expect {
        timeout {
          puts "\nTimeout waiting for: $pattern\n"
          exit 1
        }
        $pattern
      }
    }
  '';

  # In case we want/need to evaluate packages or the assertions or whatever,
  # we want to have a linux system.
  # TODO: make the non-flake test use thise.
  linuxSystem = lib.replaceStrings ["darwin"] ["linux"] stdenv.hostPlatform.system;

in
runCommand "test-nixos-rebuild-repl" {
  nativeBuildInputs = [
    expect
    nix
    nixos-rebuild
  ];
  nixpkgs =
    if builtins.pathExists (path + "/.git")
    then lib.cleanSource path
    else path;
} ''
  export HOME=$(mktemp -d)
  export TEST_ROOT=$PWD/test-tmp

  # Prepare for running Nix in sandbox
  export NIX_BUILD_HOOK=
  export NIX_CONF_DIR=$TEST_ROOT/etc
  export NIX_LOCALSTATE_DIR=$TEST_ROOT/var
  export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
  export NIX_STATE_DIR=$TEST_ROOT/var/nix
  export NIX_STORE_DIR=$TEST_ROOT/store
  export PAGER=cat
  mkdir -p $TEST_ROOT $NIX_CONF_DIR

  echo General setup
  ##################

  export NIX_PATH=nixpkgs=$nixpkgs:nixos-config=$HOME/configuration.nix
  cat >> ~/configuration.nix <<EOF
  {
    boot.loader.grub.enable = false;
    fileSystems."/".device = "x";
    imports = [ ./hardware-configuration.nix ];
  }
  EOF

  echo '{ }' > ~/hardware-configuration.nix


  echo Test traditional NixOS configuration
  #########################################

  expect ${writeText "test-nixos-rebuild-repl-expect" ''
    ${expectSetup}
    spawn nixos-rebuild repl --fast

    expect "nix-repl> "

    send "config.networking.hostName\n"
    expect "\"nixos\""
  ''}


  echo Test flake based NixOS configuration
  #########################################

  # Switch to flake flavored environment
  unset NIX_PATH
  cat > $NIX_CONF_DIR/nix.conf <<EOF
  experimental-features = nix-command flakes
  EOF

  # Make the config pure
  echo '{ nixpkgs.hostPlatform = "${linuxSystem}"; }' > ~/hardware-configuration.nix

  cat >~/flake.nix <<EOF
  {
    inputs.nixpkgs.url = "path:$nixpkgs";
    outputs = { nixpkgs, ... }: {
      nixosConfigurations.testconf = nixpkgs.lib.nixosSystem {
        modules = [
          ./configuration.nix
          # Let's change it up a bit
          { networking.hostName = "itsme"; }
        ];
      };
    };
  }
  EOF

  # cat -n ~/flake.nix

  expect ${writeText "test-nixos-rebuild-repl-expect" ''
    ${expectSetup}
    spawn sh -c "nixos-rebuild repl --fast --flake path:\$HOME#testconf"

    expect_simple "nix-repl>"

    send "config.networking.hostName\n"
    expect_simple "itsme"

    expect_simple "nix-repl>"
    send "lib.version\n"
    expect_simple ${escapeExpect (
      # The version string is a bit different in the flake lib, so we expect a prefix and ignore the rest
      # Furthermore, including the revision (suffix) would cause unnecessary rebuilds.
      # Note that a length of 4 only matches e.g. "24.
      lib.strings.substring 0 4 (lib.strings.escapeNixString lib.version))}

    # Make sure it's the right lib - should be the flake lib, not Nixpkgs lib.
    expect_simple "nix-repl>"
    send "lib?nixosSystem\n"
    expect_simple "true"
    expect_simple "nix-repl>"
    send "lib?nixos\n"
    expect_simple "true"
  ''}
  echo

  #########
  echo Done
  touch $out
''