defmodule KafkaexLagExporter.ConsumerOffsetFetcher do
  @moduledoc "Calculate summarized lag for each consumer group"

  require Logger

  # TODO: change return type
  @spec get(KafkaexLagExporter.KafkaWrapper.endpoint()) :: {any(), any()}
  def get(endpoint) do
    consumer_group_names = KafkaexLagExporter.KafkaUtils.get_consumer_group_names(endpoint)

    consumer_lags =
      KafkaexLagExporter.KafkaUtils.topic_names_for_consumer_groups(
        endpoint,
        [],
        consumer_group_names
      )
      |> Enum.map(fn [consumer_group, topics] ->
        [consumer_group, get_lag_for_consumer(consumer_group, topics)]
      end)

    consumer_lag_sum = get_lag_for_consumer_sum(consumer_lags)

    %{lags: consumer_lags, sum: consumer_lag_sum}
  end

  defp get_lag_for_consumer(consumer_group, topics) do
    topics
    |> Enum.flat_map(fn topic ->
      KafkaexLagExporter.KafkaUtils.lag(topic, consumer_group, :client1)
    end)
  end

  defp get_lag_for_consumer_sum(lags_per_consumer_group) do
    lags_per_consumer_group
    |> Enum.map(fn [topic, lag_per_partition] -> [topic, sum_topic_lag(lag_per_partition, 0)] end)
  end

  defp sum_topic_lag([], acc), do: acc
  defp sum_topic_lag([h | t], acc), do: sum_topic_lag(t, acc + elem(h, 1))
end