defmodule TelemetryMetricsLogflare.BatchCache do
  @moduledoc """
  Caches the batch, dispatches API post request if the batch is larger than configured max batch size or flush is called.
  Doesn't error or drop the message if the API is unresponsive, holds them
  """

  @cache __MODULE__
  use Agent

  alias TelemetryMetricsLogflare.Config
  alias TelemetryMetricsLogflare.ApiClient

  # batch limit prevents runaway memory usage if API is unresponsive
  @batch_limit 1000

  def start_link(_) do
    Agent.start_link(fn -> initial_state() end, name: @cache)
  end

  def put(event, %Config{} = config) do
    if pid = Process.whereis(@cache) do
      new_batch =
        Agent.get_and_update(pid, fn %{count: c, events: events} ->
          events = Enum.take([event | events], @batch_limit)
          count = if c + 1 > @batch_limit, do: @batch_limit, else: c + 1
          batch = %{count: count, events: events}
          {batch, batch}
        end)

      if new_batch.count >= config.max_batch_size do
        send_to_logflare(config)
      end

      new_batch
    else
      nil
    end
  end

  def send_to_logflare(config) do
    with pid when pid != nil <- Process.whereis(@cache),
         %{count: count, events: events} when count > 0 <- Agent.get(pid, & &1) do
      events
      |> Enum.reverse()
      |> post_logs(config)
      |> case do
        {:ok, %Tesla.Env{status: status}} ->
          unless status == 200 do
            IO.warn("Logflare API warning: HTTP response status is #{status}")
          end

          Agent.update(pid, fn %{count: batched_count, events: batched_events} ->
            %{count: batched_count - count, events: batched_events -- events}
          end)

        {:error, reason} ->
          IO.warn("Logflare API error: #{inspect(reason)}")
          :noop
      end
    else
      _ -> :noop
    end
  end

  def clear do
    Agent.update(@cache, fn _ -> initial_state() end)
  end

  defp initial_state() do
    %{count: 0, events: []}
  end

  @spec post_logs(list(map), Config.t()) :: {:ok, Tesla.Env.t()} | {:error, term}
  def post_logs(events, %Config{api_client: _api_client, source_id: source_id} = config)
      when is_list(events) do
    config |> ApiClient.new() |> ApiClient.post_logs(events, source_id)
  end
end