defmodule Agnus.DayInfo do
  @moduledoc false

  require Logger
  use GenServer
  use Timex

  use Agnus.Config.Helper

  #
  ## Macros
  #

  defmacrop if_latest_valid(state, do: if_block) do
    quote do
      %{day_info: %{latest: latest} = day_info} = unquote(state)

      if Map.has_key?(day_info, :error) or is_nil(latest) do
        false
      else
        unquote(if_block)
      end
    end
  end

  #
  ## Public API
  #

  def current?(opts \\ []) when is_list(opts) do
    GenServer.call(__MODULE__, {:current?, opts})
  end

  def get_info(:all) do
    state = state()

    if_latest_valid(state) do
      %{day_info: %{latest: latest}} = state
      latest
    end
  end

  def get_info(key) when is_atom(key), do: get_info([key])

  def get_info(keys) when is_list(keys) do
    state = state()

    if_latest_valid(state) do
      %{day_info: %{latest: latest}} = state
      Map.take(latest, keys)
    end
  end

  def keys do
    state = state()

    if_latest_valid(state) do
      %{day_info: %{latest: latest}} = state
      Map.keys(latest)
    end
  end

  def last_refresh() do
    state = state()

    if_latest_valid(state) do
      %{day_info: %{last_fetch: last_refresh}} = state
      last_refresh
    end
  end

  def state, do: :sys.get_state(__MODULE__)

  def trigger_day_info_refresh(opts \\ []) when is_list(opts),
    do: GenServer.cast(__MODULE__, {:action, :day_refresh, opts})

  #
  ## GenServer Start, Init and Terminate Callbacks
  #

  def start_link(args) when is_list(args) do
    GenServer.start_link(
      __MODULE__,
      Map.merge(Enum.into(args, %{}), %{
        autostart: Keyword.get(args, :autostart, true),
        opts: module_opts(default_opts()),
        day_info: %{last_fetch: nil, latest: %{}, previous: %{}},
        starting_up: true
      }),
      name: __MODULE__
    )
  end

  @impl true
  def init(%{autostart: autostart, opts: _opts} = s) do
    log?(s, :init_args) && Logger.info(["init():\n", inspect(s, pretty: true)])

    if autostart do
      Process.flag(:trap_exit, true)

      {:ok, s, {:continue, {:startup}}}
    else
      {:ok, s}
    end
  end

  @impl true
  def terminate(reason, s) do
    log?(s, :init) &&
      Logger.info(["terminating with reason ", inspect(reason, pretty: true)])
  end

  #
  ## GenServer Handle Callbacks
  #

  @impl true
  def handle_call(
        {:current?, _opts},
        _from,
        %{day_info: %{last_fetch: _last_fetch}} = s
      ) do
    {:reply, info_current?(s), s}
  end

  @impl true
  def handle_call(catchall, _from, s), do: {:reply, {:bad_call, catchall}, s}

  @impl true
  def handle_cast({:action, :day_refresh, _opts}, %{} = s) do
    {:noreply, refresh_day_info(s)}
  end

  @impl true
  def handle_cast(catchall, s) do
    Logger.warn(["handle_cast() unhandled:\n", inspect(catchall, pretty: true)])
    {:noreply, s}
  end

  @impl true
  def handle_continue(
        {:startup},
        %{
          opts: _opts,
          day_info: %{last_fetch: nil},
          starting_up: true
        } = s
      ) do
    {:noreply, refresh_day_info(s) |> log_refresh_day_info(), {:continue, {:startup_complete}}}
  end

  @impl true
  def handle_continue(
        {:startup_complete},
        %{opts: _opts, starting_up: true} = s
      ) do
    log?(s, :init) && Logger.info(["startup complete"])
    {:noreply, Map.put(s, :starting_up, false)}
  end

  #
  ## Private Functions
  #

  defp convert_datetimes(%{sunrise: _, sunset: _} = x, opts)
       when is_list(opts) do
    raw_map =
      Map.take(x, [
        :astronomical_twilight_begin,
        :astronomical_twilight_end,
        :civil_twilight_begin,
        :civil_twilight_end,
        :nautical_twilight_begin,
        :nautical_twilight_end,
        :solar_noon,
        :sunrise,
        :sunset
      ])

    Enum.reduce(raw_map, Map.take(x, [:day_length]), fn {k, v}, acc ->
      Map.put(
        acc,
        k,
        Timex.parse!(v, "{ISO:Extended}")
        |> Timex.to_datetime(Keyword.get(opts, :tz, "America/New_York"))
      )
    end)
  end

  defp info_current?(%{} = s) do
    if_latest_valid(s) do
      import Timex, only: [day: 1, now: 1]

      %{opts: opts, day_info: %{last_fetch: last_fetch} = day_info} = s
      tz = Keyword.get(opts, :tz)

      if is_nil(last_fetch) or Map.has_key?(day_info, :error) do
        false
      else
        Timex.day(last_fetch) == Timex.day(now(tz))
      end
    end
  end

  defp refresh_day_info(%{opts: opts, day_info: %{latest: previous}} = s) do
    import HTTPoison, only: [get: 3]
    import Jason, only: [decode: 2]
    import Timex, only: [now: 1, to_date: 1]

    alias HTTPoison.{Response}

    tz = Keyword.get(opts, :tz, "America/New_York")

    today = now(tz) |> to_date()

    api_opts = Keyword.get(opts, :api, [])

    url = Keyword.get(api_opts, :url, nil)
    lat_and_lng = Keyword.take(api_opts, [:lat, :lng])

    params = Keyword.merge(lat_and_lng, formatted: 0, date: today) |> Enum.into(%{})

    with {:current, false} <- {:current, info_current?(s)},
         url when is_binary(url) <- url,
         # append required uri info
         uri <- [url, "/json"],
         # perform the HTTPS fetch and pattern match to get the body
         {:ok, %Response{body: body}} <- get(uri, [], params: params),
         # decode the JSON and pattern match the results
         {:ok, %{results: latest, status: "OK"}} <- decode(body, keys: :atoms),
         # convert the resulting DateTimes to the configured timezone
         latest <- convert_datetimes(latest, opts),
         # build the new day info map noting that the old latest becomes previous
         day_info <- %{previous: previous, latest: latest, last_fetch: now(tz)},
         # remove the error key (if needed)
         day_info <- Map.delete(day_info, :error) do
      # all is well
      # simply update state's day info
      %{s | day_info: day_info}
    else
      # day info is current, avoid unnecessary API calls by doing nothing
      {:current, true} ->
        s

      error ->
        # there was an error, replace previous with what was latest and
        # include the error
        day_info = %{
          previous: previous,
          latest: %{},
          last_fetch: now(tz),
          error: error
        }

        %{s | day_info: day_info}
    end
  end

  #
  ## Constants
  #

  defp default_opts,
    do: [
      log: [init: false, init_args: false, default_opts: true],
      tz: "America/New_York",
      api: [
        url: "https://api.sunrise-sunset.org",
        lat: 40.2108,
        lng: -74.011
      ]
    ]

  #
  ## Logging Helpers
  #
  defp log_refresh_day_info(%{day_info: %{error: error}} = s) do
    Logger.warn([
      "could not fetch day info:\n",
      inspect(error, pretty: true)
    ])

    # always return what was passed so this can be used ina pipeline
    s
  end

  # day_info does not have an error key
  defp log_refresh_day_info(%{day_info: _} = s), do: s
end