defmodule Rollbax.Client do
  @moduledoc false

  # This GenServer keeps a pre-built bare-bones version of an exception (a
  # "draft") to be reported to Rollbar, which is then filled with the data
  # related to each specific exception when such exception is being
  # reported. This GenServer is also responsible for actually sending data to
  # the Rollbar API and receiving responses from said API.

  use GenServer

  require Logger

  alias Rollbax.Item

  @api_url "https://api.rollbar.com/api/1/item/"
  @headers [{"content-type", "application/json"}]

  ## GenServer state

  defstruct [:draft, :url, :enabled, hackney_responses: %{}]

  ## Public API

  def start_link(token, environment, enabled, custom, url \\ @api_url) do
    state = new(token, environment, url, enabled, custom)
    GenServer.start_link(__MODULE__, state, [name: __MODULE__])
  end

  def emit(level, timestamp, body, custom, occurrence_data) do
    if pid = Process.whereis(__MODULE__) do
      event = {Atom.to_string(level), timestamp, body, custom, occurrence_data}
      GenServer.cast(pid, {:emit, event})
    else
      Logger.warn("(Rollbax) Trying to report an exception but the :rollbax application has not been started", rollbax: false)
    end
  end

  ## GenServer callbacks

  def init(state) do
    Logger.metadata(rollbax: false)
    :ok = :hackney_pool.start_pool(__MODULE__, [max_connections: 20])
    {:ok, state}
  end

  def terminate(_reason, _state) do
    :ok = :hackney_pool.stop_pool(__MODULE__)
  end

  def handle_cast({:emit, _event}, %{enabled: false} = state) do
    {:noreply, state}
  end

  def handle_cast({:emit, event}, %{enabled: :log} = state) do
    Logger.info [
      "(Rollbax) registered report.\n", event_to_chardata(event),
    ]
    {:noreply, state}
  end

  def handle_cast({:emit, event}, %{enabled: true} = state) do
    case compose_json(state.draft, event) do
      {:ok, payload} ->
        opts = [:async, pool: __MODULE__]
        case :hackney.post(state.url, @headers, payload, opts) do
          {:ok, _ref} -> :ok
          {:error, reason} ->
            Logger.error("(Rollbax) connection error: #{inspect(reason)}")
        end
      {:error, exception} ->
        Logger.error [
          "(Rollbax) failed to encode report below ",
          "for reason: ", Exception.message(exception),
          ?\n, event_to_chardata(event),
        ]
    end
    {:noreply, state}
  end

  def handle_info({:hackney_response, ref, response}, state) do
    new_state = handle_hackney_response(ref, response, state)
    {:noreply, new_state}
  end

  def handle_info(message, state) do
    Logger.info("(Rollbax) unexpected message: #{inspect(message)}")
    {:noreply, state}
  end

  ## Helper functions

  defp new(token, environment, url, enabled, custom) do
    draft = Item.draft(token, environment, custom)
    %__MODULE__{draft: draft, url: url, enabled: enabled}
  end

  defp compose_json(draft, event) do
    item = Item.compose(draft, event)
    # We use `try/1` here instead of `Poison.decode/1`
    # since under some circumstances it
    # still unsafe and will raise an exception.
    try do
      {:ok, Poison.encode!(item, iodata: true)}
    rescue
      exception -> {:error, exception}
    end
  end

  defp event_to_chardata({level, timestamp, body, custom, occurrence_data}) do
    [
      inspect(body),
      "\nLevel: ", level,
      "\nTimestamp: ", Integer.to_string(timestamp),
      "\nCustom data: ", inspect(custom),
      "\nOccurrence data: ", inspect(occurrence_data),
    ]
  end

  defp handle_hackney_response(ref, :done, %{hackney_responses: responses} = state) do
    body = responses |> Map.fetch!(ref) |> IO.iodata_to_binary()

    case Poison.decode(body) do
      {:ok, %{"err" => 1, "message" => message}} when is_binary(message) ->
        Logger.error("(Rollbax) API returned an error: #{inspect message}")
      {:ok, response} ->
        Logger.debug("(Rollbax) API response: #{inspect response}")
      {:error, _} ->
        Logger.error("(Rollbax) API returned malformed JSON: #{inspect body}")
    end

    %{state | hackney_responses: Map.delete(responses, ref)}
  end

  defp handle_hackney_response(ref, {:status, code, description}, %{hackney_responses: responses} = state) do
    if code != 200 do
      Logger.error("(Rollbax) unexpected API status: #{code}/#{description}")
    end

    %{state | hackney_responses: Map.put(responses, ref, [])}
  end

  defp handle_hackney_response(_ref, {:headers, headers}, state) do
    Logger.debug("(Rollbax) API headers: #{inspect(headers)}")
    state
  end

  defp handle_hackney_response(ref, body_chunk, %{hackney_responses: responses} = state)
       when is_binary(body_chunk) do
    %{state | hackney_responses: Map.update!(responses, ref, &[&1 | body_chunk])}
  end

  defp handle_hackney_response(ref, {:error, reason}, %{hackney_responses: responses} = state) do
    Logger.error("(Rollbax) connection error: #{inspect(reason)}")
    %{state | hackney_responses: Map.delete(responses, ref)}
  end
end