defmodule Mv.Helpers.SystemActor do
  @moduledoc """
  Provides access to the system actor for systemic operations.

  The system actor is a user with admin permissions that is used
  for operations that must always run regardless of user permissions:
  - Email synchronization
  - Email uniqueness validation
  - Cycle generation (if mandatory)
  - Background jobs
  - Seeds

  ## Usage

      # Get system actor for systemic operations
      system_actor = Mv.Helpers.SystemActor.get_system_actor()
      Ash.read(query, actor: system_actor)

  ## Implementation

  The system actor is cached in an Agent for performance. On first access,
  it attempts to load a user with email "system@mila.local" and admin role.
  If that user doesn't exist, it falls back to the admin user from seeds
  (identified by ADMIN_EMAIL environment variable or "admin@localhost").

  ## Caching

  The system actor is cached in an Agent to avoid repeated database queries.
  The cache is invalidated on application restart. For long-running applications,
  consider implementing cache invalidation on role changes.

  ## Race Conditions

  The system actor creation uses `upsert?: true` with `upsert_identity: :unique_email`
  to prevent race conditions when multiple processes try to create the system user
  simultaneously. This ensures idempotent creation and prevents database constraint errors.

  ## Security

  The system actor should NEVER be used for user-initiated actions. It is
  only for systemic operations that must bypass user permissions.
  """

  use Agent

  require Ash.Query

  @system_user_email "system@mila.local"

  @doc """
  Starts the SystemActor Agent.

  This is called automatically by the application supervisor.
  The agent starts with nil state and loads the system actor lazily on first access.
  """
  def start_link(_opts) do
    # Start with nil - lazy initialization on first get_system_actor call
    # This prevents database access during application startup (important for tests)
    Agent.start_link(fn -> nil end, name: __MODULE__)
  end

  @doc """
  Returns the system actor (user with admin role).

  The system actor is cached in an Agent for performance. On first access,
  it loads the system user from the database or falls back to the admin user.

  ## Returns

  - `%Mv.Accounts.User{}` - User with admin role loaded
  - Raises if system actor cannot be found or loaded

  ## Examples

      iex> system_actor = Mv.Helpers.SystemActor.get_system_actor()
      iex> system_actor.role.permission_set_name
      "admin"

  """
  @spec get_system_actor() :: Mv.Accounts.User.t()
  def get_system_actor do
    case get_system_actor_result() do
      {:ok, actor} -> actor
      {:error, reason} -> raise "Failed to load system actor: #{inspect(reason)}"
    end
  end

  @doc """
  Returns the system actor as a result tuple.

  This variant returns `{:ok, actor}` or `{:error, reason}` instead of raising,
  which is useful for error handling in pipes or when you want to handle errors explicitly.

  ## Returns

  - `{:ok, %Mv.Accounts.User{}}` - Successfully loaded system actor
  - `{:error, term()}` - Error loading system actor

  ## Examples

      case SystemActor.get_system_actor_result() do
        {:ok, actor} -> use_actor(actor)
        {:error, reason} -> handle_error(reason)
      end

  """
  @spec get_system_actor_result() :: {:ok, Mv.Accounts.User.t()} | {:error, term()}
  def get_system_actor_result do
    # In test environment, always load directly to avoid Agent/Sandbox issues
    if Mix.env() == :test do
      try do
        {:ok, load_system_actor()}
      rescue
        e -> {:error, e}
      end
    else
      try do
        result =
          Agent.get_and_update(__MODULE__, fn
            nil ->
              # Cache miss - load system actor
              try do
                actor = load_system_actor()
                {actor, actor}
              rescue
                e -> {{:error, e}, nil}
              end

            cached_actor ->
              # Cache hit - return cached actor
              {cached_actor, cached_actor}
          end)

        case result do
          {:error, reason} -> {:error, reason}
          actor -> {:ok, actor}
        end
      catch
        :exit, {:noproc, _} ->
          # Agent not started - load directly without caching
          try do
            {:ok, load_system_actor()}
          rescue
            e -> {:error, e}
          end
      end
    end
  end

  @doc """
  Invalidates the system actor cache.

  This forces a reload of the system actor on the next call to `get_system_actor/0`.
  Useful when the system user's role might have changed.

  ## Examples

      iex> Mv.Helpers.SystemActor.invalidate_cache()
      :ok

  """
  @spec invalidate_cache() :: :ok
  def invalidate_cache do
    Agent.update(__MODULE__, fn _state -> nil end)
  end

  @doc """
  Returns the email address of the system user.

  This is useful for other modules that need to reference the system user
  without loading the full user record.

  ## Returns

  - `String.t()` - The system user email address ("system@mila.local")

  ## Examples

      iex> Mv.Helpers.SystemActor.system_user_email()
      "system@mila.local"

  """
  @spec system_user_email() :: String.t()
  def system_user_email, do: @system_user_email

  # Loads the system actor from the database
  # First tries to find system@mila.local, then falls back to admin user
  @spec load_system_actor() :: Mv.Accounts.User.t() | no_return()
  defp load_system_actor do
    case find_user_by_email(@system_user_email) do
      {:ok, user} when not is_nil(user) ->
        load_user_with_role(user)

      {:ok, nil} ->
        handle_system_user_not_found("no system user or admin user found")

      {:error, _reason} = error ->
        handle_system_user_error(error)
    end
  end

  # Handles case when system user doesn't exist
  @spec handle_system_user_not_found(String.t()) :: Mv.Accounts.User.t() | no_return()
  defp handle_system_user_not_found(message) do
    case load_admin_user_fallback() do
      {:ok, admin_user} ->
        admin_user

      {:error, _} ->
        handle_fallback_error(message)
    end
  end

  # Handles database error when loading system user
  @spec handle_system_user_error(term()) :: Mv.Accounts.User.t() | no_return()
  defp handle_system_user_error(error) do
    case load_admin_user_fallback() do
      {:ok, admin_user} ->
        admin_user

      {:error, _} ->
        handle_fallback_error("Failed to load system actor: #{inspect(error)}")
    end
  end

  # Handles fallback error - creates test actor or raises
  @spec handle_fallback_error(String.t()) :: Mv.Accounts.User.t() | no_return()
  defp handle_fallback_error(message) do
    if Mix.env() == :test do
      create_test_system_actor()
    else
      raise "Failed to load system actor: #{message}"
    end
  end

  # Creates a temporary admin user for tests when no system/admin user exists
  @spec create_test_system_actor() :: Mv.Accounts.User.t() | no_return()
  defp create_test_system_actor do
    alias Mv.Accounts
    alias Mv.Authorization

    admin_role = ensure_admin_role_exists()
    create_system_user_with_role(admin_role)
  end

  # Ensures admin role exists - finds or creates it
  @spec ensure_admin_role_exists() :: Mv.Authorization.Role.t() | no_return()
  defp ensure_admin_role_exists do
    case find_admin_role() do
      {:ok, role} ->
        role

      {:error, :not_found} ->
        create_admin_role_with_retry()
    end
  end

  # Finds admin role in existing roles
  @spec find_admin_role() :: {:ok, Mv.Authorization.Role.t()} | {:error, :not_found}
  defp find_admin_role do
    alias Mv.Authorization

    case Authorization.list_roles() do
      {:ok, roles} ->
        case Enum.find(roles, &(&1.permission_set_name == "admin")) do
          nil -> {:error, :not_found}
          role -> {:ok, role}
        end

      _ ->
        {:error, :not_found}
    end
  end

  # Creates admin role, handling race conditions
  @spec create_admin_role_with_retry() :: Mv.Authorization.Role.t() | no_return()
  defp create_admin_role_with_retry do
    alias Mv.Authorization

    case create_admin_role() do
      {:ok, role} ->
        role

      {:error, :already_exists} ->
        find_existing_admin_role()

      {:error, error} ->
        raise "Failed to create admin role: #{inspect(error)}"
    end
  end

  # Attempts to create admin role
  @spec create_admin_role() ::
          {:ok, Mv.Authorization.Role.t()} | {:error, :already_exists | term()}
  defp create_admin_role do
    alias Mv.Authorization

    case Authorization.create_role(%{
           name: "Admin",
           description: "Administrator with full access",
           permission_set_name: "admin"
         }) do
      {:ok, role} ->
        {:ok, role}

      {:error, %Ash.Error.Invalid{errors: [%{field: :name, message: "has already been taken"}]}} ->
        {:error, :already_exists}

      {:error, error} ->
        {:error, error}
    end
  end

  # Finds existing admin role after creation attempt failed due to race condition
  @spec find_existing_admin_role() :: Mv.Authorization.Role.t() | no_return()
  defp find_existing_admin_role do
    alias Mv.Authorization

    case Authorization.list_roles() do
      {:ok, roles} ->
        Enum.find(roles, &(&1.permission_set_name == "admin")) ||
          raise "Admin role should exist but was not found"

      _ ->
        raise "Failed to find admin role after creation attempt"
    end
  end

  # Creates system user with admin role assigned
  @spec create_system_user_with_role(Mv.Authorization.Role.t()) ::
          Mv.Accounts.User.t() | no_return()
  defp create_system_user_with_role(admin_role) do
    alias Mv.Accounts

    Accounts.create_user!(%{email: @system_user_email},
      upsert?: true,
      upsert_identity: :unique_email
    )
    |> Ash.Changeset.for_update(:update, %{})
    |> Ash.Changeset.manage_relationship(:role, admin_role, type: :append_and_remove)
    |> Ash.update!()
    |> Ash.load!(:role, domain: Mv.Accounts)
  end

  # Finds a user by email address
  @spec find_user_by_email(String.t()) :: {:ok, Mv.Accounts.User.t() | nil} | {:error, term()}
  defp find_user_by_email(email) do
    Mv.Accounts.User
    |> Ash.Query.filter(email == ^email)
    |> Ash.read_one(domain: Mv.Accounts)
  end

  # Loads a user with their role preloaded (required for authorization)
  @spec load_user_with_role(Mv.Accounts.User.t()) :: Mv.Accounts.User.t() | no_return()
  defp load_user_with_role(user) do
    case Ash.load(user, :role, domain: Mv.Accounts) do
      {:ok, user_with_role} ->
        validate_admin_role(user_with_role)

      {:error, reason} ->
        raise "Failed to load role for system actor: #{inspect(reason)}"
    end
  end

  # Validates that the user has an admin role
  @spec validate_admin_role(Mv.Accounts.User.t()) :: Mv.Accounts.User.t() | no_return()
  defp validate_admin_role(%{role: %{permission_set_name: "admin"}} = user) do
    user
  end

  @spec validate_admin_role(Mv.Accounts.User.t()) :: no_return()
  defp validate_admin_role(%{role: %{permission_set_name: permission_set}}) do
    raise """
    System actor must have admin role, but has permission_set_name: #{permission_set}
    Please assign the "Admin" role to the system user.
    """
  end

  @spec validate_admin_role(Mv.Accounts.User.t()) :: no_return()
  defp validate_admin_role(%{role: nil}) do
    raise """
    System actor must have a role assigned, but role is nil.
    Please assign the "Admin" role to the system user.
    """
  end

  @spec validate_admin_role(term()) :: no_return()
  defp validate_admin_role(_user) do
    raise """
    System actor must have a role with admin permissions.
    Please assign the "Admin" role to the system user.
    """
  end

  # Fallback: Loads admin user from seeds (ADMIN_EMAIL env var or default)
  @spec load_admin_user_fallback() :: {:ok, Mv.Accounts.User.t()} | {:error, term()}
  defp load_admin_user_fallback do
    admin_email = System.get_env("ADMIN_EMAIL") || "admin@localhost"

    case find_user_by_email(admin_email) do
      {:ok, user} when not is_nil(user) ->
        {:ok, load_user_with_role(user)}

      {:ok, nil} ->
        {:error, :admin_user_not_found}

      {:error, _reason} = error ->
        error
    end
  end
end