defmodule EventStore.Storage.Appender do
  @moduledoc """
  Append-only storage of events for a stream
  """

  require Logger

  alias EventStore.{EventData,RecordedEvent}
  alias EventStore.Sql.Statements
  alias EventStore.Storage.{Appender,QueryLatestStreamVersion}

  def append(conn, stream_id, expected_version, events) do
    case QueryLatestStreamVersion.execute(conn, stream_id) do
      {:ok, ^expected_version} -> execute_using_multirow_value_insert(conn, stream_id, expected_version, events)
      {:ok, latest_version} -> wrong_expected_version(stream_id, expected_version, latest_version)
      {:error, reason} -> failed_to_append(stream_id, reason)
    end
  end

  defp wrong_expected_version(stream_id, expected_version, latest_version) do
    Logger.warn "failed to append events to stream id #{stream_id}, expected version #{expected_version} but latest version is #{latest_version}"
    {:error, :wrong_expected_version}
  end

  defp failed_to_append(stream_id, reason) do
    Logger.warn "failed to append events to stream id #{stream_id} due to #{reason}"
    {:error, reason}
  end

  defp execute_using_multirow_value_insert(conn, stream_id, expected_version, events) do
    prepared_events = prepare_events(events, stream_id, expected_version)
    statement = build_insert_statement(events)
    parameters = build_insert_parameters(prepared_events)

    conn
    |> Postgrex.query(statement, parameters)
    |> handle_response(stream_id, prepared_events)
  end

  defp build_insert_statement(events) do
    Statements.create_events(length(events))
  end

  defp build_insert_parameters(prepared_events) do
    prepared_events
    |> Enum.reduce([], fn(event, parameters) ->
      parameters ++ [
        event.stream_id,
        event.stream_version,
        event.correlation_id,
        event.event_type,
        event.headers,
        event.payload
      ]
    end)
  end

  defp prepare_events(events, stream_id, expected_version) do
    initial_stream_version = expected_version + 1

    events
    |> Enum.map(&map_to_recorded_event(&1))
    |> Enum.map(&assign_stream_id(&1, stream_id))
    |> Enum.with_index(initial_stream_version)
    |> Enum.map(&assign_stream_version/1)
  end

  defp map_to_recorded_event(%EventData{correlation_id: correlation_id, event_type: event_type, headers: headers, payload: payload}) do
    %RecordedEvent{
      correlation_id: correlation_id,
      event_type: event_type,
      headers: headers,
      payload: payload
    }
  end

  defp assign_stream_id(%RecordedEvent{} = event, stream_id) do
    %RecordedEvent{event | stream_id: stream_id}
  end

  defp assign_stream_version({%RecordedEvent{} = event, stream_version}) do
    %RecordedEvent{event | stream_version: stream_version}
  end

  defp handle_response({:ok, %Postgrex.Result{num_rows: 0}}, stream_id, _events) do
    Logger.info "failed to append any events to stream id #{stream_id}"
    {:ok, []}
  end

  defp handle_response({:ok, %Postgrex.Result{num_rows: num_rows, rows: rows}}, stream_id, events) do
    Logger.info "appended #{num_rows} events to stream id #{stream_id}"

    persisted_events =
      Enum.zip(events, rows)
      |> Enum.map(fn {event, [event_id, created_at]} ->
        %RecordedEvent{event | event_id: event_id, created_at: created_at}
      end)

    {:ok, persisted_events}
  end

  defp handle_response({:error, reason}, stream_id, events) do
    Logger.warn "failed to append events to stream id #{stream_id} due to #{reason}"
    {:error, reason}
  end
end