defmodule Toolbox.Report do
  @moduledoc """
  Behaviour module for notification reports.

  This behaviour encapsulates notifications which behave like reports = notifications are sent
  periodically.

  ## Example

  ```
  defmodule Scenarios.Template.Reporter do
    @callback Runbox.Scenario.Template.StageBased

    alias Crontab.CronExpression
    alias Runbox.Message
    alias Runbox.Runtime.Stage.Unit
    alias Scenarios.Template.Reporter.DailyReport
    alias Toolbox.Report

    import Crontab.CronExpression

    @impl true
    def init(init_timestamp, %Unit{} = unit) do
      # report schedule defined as crontab expression
      report_schedule = ~e[* * * * *]

      # timezone used to display localized timestamp in notification
      timezone = "Europe/Prague"

      # args to init report state
      init_arg = :ok

      {:ok, outputs, report_state} =
        Report.init(DailyReport, init_timestamp, report_schedule, timezone, init_arg)

      {:ok, outputs, %Unit{unit | state: %{report_state: report_state}}}
    end

    @impl true
    def handle_message(%Message{} = msg, %Unit{} = unit) do
      {:ok, outputs, report_state} = Report.process_message(unit.state.report_state, msg)
      {:reply, outputs, %Unit{unit | state: %{unit.state | report_state: report_state}}}
    end
  end

  defmodule Scenarios.Template.Reporter.DailyReport do
    @callback Toolbox.Report

    alias Runbox.Message
    alias Runbox.Scenario.OutputAction

    @impl true
    def init(:ok) do
      {:ok, [], %{incoming_messages: []}}
    end

    @impl true
    def handle_process_message(%{} = state, %Message{} = msg) do
      {:ok, [], %{incoming_messages: [msg | state.incoming_messages]}}
    end

    @impl true
    def handle_publish_report(%{} = state, publish_timestamp, last_publish_timestamp) do
      report_notification =
        %OutputAction.Notification{
          type: :daily_report,
          data: %{
            "incoming_messages" => state.incoming_messages,
            "publish_time" => publish_timestamp,
            "last_publish_time" => last_timestamp
          }
        }

      {:ok, [report_notification], %{incoming_messages: []}}
    end
  end
  ```
  """

  alias Crontab.CronExpression
  alias Crontab.Scheduler, as: CrontabScheduler
  alias Runbox.Message
  alias Runbox.Runtime.RuntimeInstruction
  alias Timex
  alias Timex.AmbiguousDateTime
  alias Timex.Timezone

  defmodule State do
    @moduledoc false

    @enforce_keys [:report_mod, :report_state, :cron_expr, :timezone]
    defstruct [:report_mod, :report_state, :cron_expr, :timezone]

    @type t :: %__MODULE__{
            report_mod: module,
            report_state: term,
            cron_expr: CronExpression.t(),
            timezone: String.t()
          }
  end

  @type timestamp :: integer
  @typedoc "User state where a module implementing `Toolbox.Report` can store anything it wants."
  @type report_state :: term
  @typedoc "State that the `Toolbox.Report` operates on; should be stored in a unit's state."
  @opaque external_state :: State.t()
  @callback init(term) :: {:ok, [term], report_state}
  @callback handle_process_message(report_state, Message.t()) :: {:ok, [term], report_state}
  @callback handle_publish_report(
              report_state,
              current_report :: timestamp,
              last_report :: timestamp
            ) ::
              {:ok, [term], report_state}

  @doc "Initializes report"
  @spec init(module, timestamp, CronExpression.t(), String.t(), term) ::
          {:ok, [term], external_state()}
  def init(report_mod, init_timestamp, cron_expr, timezone, init_arg) do
    {:ok, outputs, report_state} = report_mod.init(init_arg)

    state = %State{
      report_mod: report_mod,
      report_state: report_state,
      cron_expr: cron_expr,
      timezone: timezone
    }

    # first report publishment is scheduled (via timeout registration)
    next_publish_timeout = create_next_publish_timeout(state, init_timestamp)

    {:ok, [next_publish_timeout | outputs], state}
  end

  @doc "Processes report message"
  # this function head matches only timeout registrations intended to current report module
  # reports implementation are singletons
  @spec process_message(external_state(), Message.t()) :: {:ok, [term], external_state()}
  # credo:disable-for-lines:21 Credo.Check.Design.DuplicatedCode
  def process_message(
        %State{report_mod: report_mod} = state,
        %Message{type: :publish_report_timeout, body: %{report_mod: report_mod}} = msg
      ) do
    # next report publishment should be scheduled (via timeout registration)
    next_publish_timeout = create_next_publish_timeout(state, msg.timestamp)

    prev_publish_timestamp =
      get_prev_publish_timestamp(msg.timestamp, state.cron_expr, state.timezone)

    # report side effects are generated
    {:ok, outputs, report_state} =
      state.report_mod.handle_publish_report(
        state.report_state,
        msg.timestamp,
        prev_publish_timestamp
      )

    state = %State{state | report_state: report_state}
    {:ok, [next_publish_timeout | outputs], state}
  end

  # this function head matches all other timeout registrations which are not intended to current
  # report module
  def process_message(%State{} = state, %Message{type: :publish_report_timeout}) do
    {:ok, [], state}
  end

  # this function head matches all remaining messages and delegates message processing to
  # report module
  def process_message(%State{report_mod: report_mod} = state, %Message{} = msg) do
    {:ok, outputs, report_state} = report_mod.handle_process_message(state.report_state, msg)
    {:ok, outputs, %State{state | report_state: report_state}}
  end

  # public for testing purposes
  @doc false
  @spec get_next_publish_timestamp(timestamp, CronExpression.t(), String.t()) :: timestamp
  def get_next_publish_timestamp(timestamp, cron_expr, timezone) do
    date_time = unix_to_naive_datetime(timestamp, timezone)

    case next_crontab_run_timestamp(cron_expr, date_time, timezone) do
      ^timestamp ->
        # when date_time is the same as calculated next_run_date,
        # crontab returns this date_time:
        # ^date_time = CrontabScheduler.get_next_run_date!(crontab_expr, date_time)
        # this is not what we want here -> we need to update timestamp by adding 1 millisecond
        date_time = unix_to_naive_datetime(timestamp + 1, timezone)
        next_crontab_run_timestamp(cron_expr, date_time, timezone)

      next_report_timestamp ->
        next_report_timestamp
    end
  end

  @doc false
  @spec get_prev_publish_timestamp(timestamp, CronExpression.t(), String.t()) :: timestamp
  def get_prev_publish_timestamp(timestamp, cron_expr, timezone) do
    date_time = unix_to_naive_datetime(timestamp, timezone)

    case prev_crontab_run_timestamp(cron_expr, date_time, timezone) do
      ^timestamp ->
        # when date_time is the same as calculated prev_run_date,
        # crontab returns this date_time:
        # ^date_time = CrontabScheduler.get_next_run_date!(crontab_expr, date_time)
        # this is not what we want here -> we need to update timestamp by adding 1 millisecond
        date_time = unix_to_naive_datetime(timestamp - 1, timezone)
        prev_crontab_run_timestamp(cron_expr, date_time, timezone)

      prev_report_timestamp ->
        prev_report_timestamp
    end
  end

  @spec unix_to_naive_datetime(timestamp, String.t()) :: NaiveDateTime.t()
  defp unix_to_naive_datetime(timestamp, timezone) do
    timestamp
    |> Timex.from_unix(:millisecond)
    |> Timezone.convert(timezone)
    |> resolve_ambiguous_datetime()
    |> DateTime.to_naive()
  end

  @spec naive_datetime_to_unix(NaiveDateTime.t(), String.t()) :: timestamp
  defp naive_datetime_to_unix(naive_datetime, timezone) do
    naive_datetime
    |> Timex.to_datetime(timezone)
    |> resolve_ambiguous_datetime()
    |> DateTime.to_unix(:millisecond)
  end

  # needed because Timezone.convert, Timex.to_datetime and other similar funs can return ambiguous
  # times
  @doc """
  Resolves date time into non-ambiguous DateTime.

  If AmbiguousDateTime is given, it picks one version. If any other non-ambiguous date time is
  given it is returned as is.
  """
  @spec resolve_ambiguous_datetime(AmbiguousDateTime.t() | DateTime.t()) :: DateTime.t()
  def resolve_ambiguous_datetime(%AmbiguousDateTime{} = datetime), do: datetime.after
  def resolve_ambiguous_datetime(%DateTime{} = datetime), do: datetime

  @spec next_crontab_run_timestamp(CronExpression.t(), NaiveDateTime.t(), String.t()) ::
          timestamp
  defp next_crontab_run_timestamp(cron_expr, naive_datetime, timezone) do
    next_run_naive = CrontabScheduler.get_next_run_date!(cron_expr, naive_datetime)
    naive_datetime_to_unix(next_run_naive, timezone)
  end

  @spec prev_crontab_run_timestamp(CronExpression.t(), NaiveDateTime.t(), String.t()) ::
          timestamp
  defp prev_crontab_run_timestamp(cron_expr, naive_datetime, timezone) do
    next_run_naive = CrontabScheduler.get_previous_run_date!(cron_expr, naive_datetime)
    naive_datetime_to_unix(next_run_naive, timezone)
  end

  @spec create_next_publish_timeout(external_state(), timestamp) :: RuntimeInstruction.t()
  defp create_next_publish_timeout(%State{} = state, timestamp) do
    timeout_timestamp = get_next_publish_timestamp(timestamp, state.cron_expr, state.timezone)
    timeout_msg = create_timeout_message(timeout_timestamp, state.report_mod)
    RuntimeInstruction.register_timeout(timeout_msg)
  end

  # public for testing purposes
  @doc false
  @spec create_timeout_message(timestamp, module) :: Message.t()
  def create_timeout_message(timeout_timestamp, report_mod) do
    %Message{
      timestamp: timeout_timestamp,
      type: :publish_report_timeout,
      body: %{report_mod: report_mod}
    }
  end
end