defmodule X3m.System.Message do
  @moduledoc """
  System Message.

  This module defines a `X3m.System.Message` struct and the main functions
  for working with it.

  ## Fields:

    * `service_name` - the name of the service that should handle this message. Example: `:create_job`.
    * `id` - unique id of the message.
    * `correlation_id` - id of the message that "started" conversation.
    * `causation_id` - id of the message that "caused" this message.
    * `logger_metadata` - In each new process `Logger.metadata` should be set to this value.
    * `invoked_at` - utc time when message was generated.
    * `dry_run` - specifies dry run option. It can be either `false`, `true` or `:verbose`.
    * `request` - request structure converted to Ecto.Changeset (or anything else useful).
    * `raw_request` - request as it is received before converting to Message (i.e. `params` from controller action).
    * `assigns` - shared Data as a map.
    * `response` - the response for invoker.
    * `events` - list of generated events.
    * `aggregate_meta` - metadata for aggregate.
    * `valid?` - when set to `true` it means that raw_request was successfully validated and
      structered request is set to `request` field
    * `reply_to` - Pid of process that is waiting for response.
    * `halted?` - when set to `true` it means that response should be returned to the invoker
      without further processing of Message.
  """
  require Logger
  alias X3m.System.Response

  @enforce_keys ~w(service_name id correlation_id causation_id invoked_at dry_run
                   reply_to halted? raw_request request valid? response events
                   aggregate_meta assigns logger_metadata)a
  defstruct @enforce_keys

  @type t() :: %__MODULE__{
          service_name: atom,
          id: String.t(),
          correlation_id: String.t(),
          causation_id: String.t(),
          logger_metadata: Keyword.t(),
          invoked_at: DateTime.t(),
          dry_run: dry_run(),
          raw_request: map(),
          request: nil | request,
          valid?: boolean,
          assigns: assigns,
          response: nil | Response.t(),
          events: [map],
          aggregate_meta: map,
          reply_to: pid,
          halted?: boolean
        }
  @typep assigns :: %{atom => any}
  @typep request :: map()
  @type error :: {String.t(), Keyword.t()}
  @type errors :: [{atom, error}]
  @type dry_run :: boolean | :verbose

  @doc """
  Creates new message with given `service_name` and provided `opts`:

    * `id` - id of the message. If not provided it tries to find `request_id`
      in `Logger.metadata`. If that one is also missing it generates random one.
    * `correlation_id` - id of "conversation". If not provided it is set to `id`.
    * `causation_id` - id of message that "caused" this message. If not provided it is set to `id`.
    * `reply_to` - sets pid of process that expects response. If not provided it is set to `self()`.
    * `raw_request` - sets raw request as it is received (i.e. `params` from controller action).
    * `logger_metadata` - if not provided `Logger.metadata` is used by default.
  """
  @spec new(atom, Keyword.t()) :: __MODULE__.t()
  def new(service_name, opts \\ []) when is_atom(service_name) do
    id = Keyword.get(opts, :id, _get_request_id())
    correlation_id = Keyword.get(opts, :correlation_id, id)
    causation_id = Keyword.get(opts, :causation_id, correlation_id)
    dry_run = Keyword.get(opts, :dry_run, false)
    reply_to = Keyword.get(opts, :reply_to, self())
    raw_request = Keyword.get(opts, :raw_request)
    logger_metadata = Keyword.get(opts, :logger_metadata, Logger.metadata())

    %__MODULE__{
      service_name: service_name,
      id: id,
      correlation_id: correlation_id,
      causation_id: causation_id,
      invoked_at: DateTime.utc_now(),
      dry_run: dry_run,
      raw_request: raw_request,
      request: nil,
      valid?: true,
      response: nil,
      events: [],
      aggregate_meta: %{},
      reply_to: reply_to,
      halted?: false,
      assigns: %{},
      logger_metadata: logger_metadata
    }
  end

  @doc """
  Creates new message with given `service_name` that is caused by other `msg`.
  """
  @spec new_caused_by(atom, __MODULE__.t(), Keyword.t()) :: __MODULE__.t()
  def new_caused_by(service_name, %__MODULE__{} = msg, opts \\ []) when is_atom(service_name) do
    service_name
    |> new(
      id: gen_msg_id(),
      correlation_id: msg.correlation_id,
      causation_id: msg.id,
      raw_request: opts[:raw_request]
    )
  end

  @spec to_service(t(), atom) :: t()
  def to_service(%__MODULE__{} = sys_msg, service_name),
    do: %__MODULE__{sys_msg | service_name: service_name}

  @doc """
  Assigns a value to a key in the message.
  The "assigns" storage is meant to be used to store values in the message
  so that others in pipeline can use them when needed. The assigns storage
  is a map.

  ## Examples

      iex> sys_msg.assigns[:user_id]
      nil
      iex> sys_msg = assign(sys_msg, :user_id, 123)
      iex> sys_msg.assigns[:user_id]
      123
  """
  @spec assign(__MODULE__.t(), atom, any) :: __MODULE__.t()
  def assign(%__MODULE__{assigns: assigns} = sys_msg, key, val) when is_atom(key),
    do: %{sys_msg | assigns: Map.put(assigns, key, val)}

  @doc """
  Returns `sys_msg` with provided `response` and as `halted? = true`.
  """
  @spec return(__MODULE__.t(), Response.t()) :: __MODULE__.t()
  def return(%__MODULE__{events: events} = sys_msg, response) do
    sys_msg
    |> Map.put(:response, response)
    |> Map.put(:halted?, true)
    |> Map.put(:events, Enum.reverse(events))
  end

  @doc """
  Returns `message` it received with `Response.created(id)` result set.
  """
  @spec created(__MODULE__.t(), any) :: __MODULE__.t()
  def created(%__MODULE__{} = message, id) do
    response = Response.created(id)
    return(message, response)
  end

  @spec ok(__MODULE__.t()) :: __MODULE__.t()
  def ok(message) do
    response = Response.ok()
    return(message, response)
  end

  @spec ok(__MODULE__.t(), any) :: __MODULE__.t()
  def ok(message, any) do
    response = Response.ok(any)
    return(message, response)
  end

  @spec error(__MODULE__.t(), any) :: __MODULE__.t()
  def error(message, any) do
    response = Response.error(any)
    return(message, response)
  end

  def put_request(%{valid?: false} = request, %__MODULE__{} = message) do
    %{message | valid?: false, request: request}
    |> return(Response.validation_error(request))
  end

  def put_request(%{} = request, %__MODULE__{} = message),
    do: %{message | valid?: true, request: request}

  @doc """
  Puts `value` under `key` in `message.raw_request` map.
  """
  def put_in_raw_request(%__MODULE__{} = message, key, value) do
    raw_request =
      (message.raw_request || %{})
      |> Map.put(key, value)

    %{message | raw_request: raw_request}
  end

  @doc """
  Adds `event` in `message.events` list. If `event` is nil
  it behaves as noop.

  After `return/2` (and friends) order of `msg.events` will be the same as
  they've been added.
  """
  @spec add_event(message :: t(), event :: nil | any) :: t()
  def add_event(%__MODULE__{} = message, nil),
    do: message

  def add_event(%__MODULE__{events: events} = message, event),
    do: %{message | events: [event | events]}

  def prepare_aggregate_id(%__MODULE__{} = message, id_field, opts \\ []) do
    id =
      message
      |> Map.from_struct()
      |> get_in([:raw_request, id_field])

    case {id, opts[:generate_if_missing] == true} do
      {nil, true} ->
        id = UUID.uuid4()
        raw_request = Map.put(message.raw_request, id_field, id)
        aggregate_meta = Map.put(message.aggregate_meta, :id, id)

        message
        |> Map.put(:raw_request, raw_request)
        |> Map.put(:aggregate_meta, aggregate_meta)

      {nil, false} ->
        response = Response.missing_id(id_field)
        return(message, response)

      _ ->
        aggregate_meta = Map.put(message.aggregate_meta, :id, id)
        Map.put(message, :aggregate_meta, aggregate_meta)
    end
  end

  @spec _get_request_id :: String.t()
  defp _get_request_id do
    Logger.metadata()
    |> Keyword.get(:request_id, gen_msg_id())
  end

  # taken from https://github.com/elixir-plug/plug/blob/master/lib/plug/request_id.ex
  @spec gen_msg_id :: String.t()
  def gen_msg_id() do
    binary = <<
      System.system_time(:nanosecond)::64,
      :erlang.phash2({node(), self()}, 16_777_216)::24,
      :erlang.unique_integer()::32
    >>

    Base.url_encode64(binary)
  end
end