defmodule OpentelemetryEcto do
  @moduledoc """
  Telemetry handler for creating OpenTelemetry Spans from Ecto query events.
  """

  require OpenTelemetry.Tracer

  @doc """
  Attaches the OpentelemetryEcto handler to your repo events. This should be called
  from your application behaviour on startup.

  Example:

      OpentelemetryEcto.setup([:blog, :repo])

  You may also supply the following options in the second argument:

    * `:time_unit` - a time unit used to convert the values of query phase
      timings, defaults to `:microsecond`. See `System.convert_time_unit/3`

    * `:span_prefix` - the first part of the span name, as a `String.t`,
      defaults to the concatenation of the event name with periods, e.g.
      `"blog.repo.query"`. This will always be followed with a colon and the
      source (the table name for SQL adapters).
  """
  def setup(event_prefix, config \\ []) do
    # register the tracer. just re-registers if called for multiple repos
    _ = OpenTelemetry.register_application_tracer(:opentelemetry_ecto)

    event = event_prefix ++ [:query]
    :telemetry.attach({__MODULE__, event}, event, &__MODULE__.handle_event/4, config)
  end

  @doc false
  def handle_event(event, measurements, %{query: query,
                                          source: source,
                                          result: query_result,
                                          repo: repo,
                                          type: type}, config) do
    # Doing all this even if the span isn't sampled so the sampler
    # could technically use the attributes to decide if it should sample or not

    total_time = measurements.total_time
    end_time = :opentelemetry.timestamp()
    start_time = end_time - total_time
    database = repo.config()[:database]

    url = case repo.config()[:url] do
            nil ->
              URI.to_string(%URI{scheme: "ecto", host: repo.config()[:hostname]})  # TODO: add port
            url ->
              url
          end

    span_name =
      case Keyword.fetch(config, :span_prefix) do
        {:ok, prefix} -> prefix
        :error -> Enum.join(event, ".")
      end <> ":#{source}"

    time_unit = Keyword.get(config, :time_unit, :microsecond)

    # TODO: need connection information to complete the required attributes
    # net.peer.name or net.peer.ip and net.peer.port
    base_attributes =
      [
        {"db.type", case type do
                      :ecto_sql_query -> "sql"
                      _ -> Atom.to_string(type)
                    end},
        {"db.statement", query},
        {"source", source},
        {"db.instance", database},
        {"db.url", url},
        {"total_time_#{time_unit}s", System.convert_time_unit(total_time, :native, time_unit)}
        |  case query_result do
             {:ok, _} -> []
             _ -> [{"error", true}]
           end
      ]

    attributes =
      measurements
      |> Enum.into(%{})
      |> Map.take(~w(decode_time query_time queue_time)a)
      |> Enum.reject(&is_nil(elem(&1, 1)))
      |> Enum.map(fn {k, v} ->
      {"#{k}_#{time_unit}s", System.convert_time_unit(v, :native, time_unit)}
    end)

    OpenTelemetry.Tracer.start_span(span_name, %{start_time: start_time,
                                                 attributes: attributes ++ base_attributes})
    OpenTelemetry.Tracer.end_span()
  end
end