defmodule Peep do
  @moduledoc """
  `Telemetry.Metrics` reporter supporting Statsd and Prometheus.

  To use it, start the reporter with `start_link/1`, providing a list of
  `Telemetry.Metrics` metric definitions in a Keyword list matching the schema
  specified in `Peep.Options`:

      import Telemetry.Metrics

      Peep.start_link([
        name: :my_peep,
        metrics: [
          counter("http.request.count"),
          sum("http.request.payload_size"),
          last_value("vm.memory.total"),
          distribution("http.request.latency")
        ]
      ])

  To emit Statsd metrics, `Peep` supports both UDP and Unix Domain Sockets.

  ## Why another `Telemetry.Metrics` library?

  Both `TelemetryMetricsStatsd` and `TelemetryMetrics.Prometheus` are great
  choices for emitting telemetry. However, `Peep` makes several different
  choices that may not be as general-purpose as either of those libraries.

  ### No sampled metrics

  Sampling is a popular approach to reduce the amount of Statsd data flowing out
  of a service, but naive sampling dramatically reduces visibility into the
  shapes of distributions. `Peep` represents distributions using histograms,
  using a small exponential function by default. This sacrifices some
  granularity on individual samples, but one usually doesn't mind too much if a
  sample value of `95` is rounded to `100`, or if `950` is rounded to `1000`.
  These histograms are emitted to statsd using the optional sample rate of
  `1/$count`.

  `Peep` uses `:atomics` stored in `:ets` for performance. New `:atomics` arrays
  are created when a metric with a new set of tags is observed, so there is a
  slight overhead when handling the first telemetry event with a distinct set of
  tags. `Peep` reporter processes are not involved in the handling of any
  `:telemetry` events, so there's no chance of a single process becoming a
  bottleneck.

  ### Distributions are aggregated immediately

  This is a consequence of choosing to represent distributions as
  histograms. There is no step in `Peep`'s processing where samples are
  aggregated in large batches. This leads to a flatter performance profile when
  scaling up.

  ### Statsd packets are stuffed

  This is something that is not (at the time of writing) supported by
  `TelemetryMetricsStatsd`, but the need for stuffing packets became pretty
  clear when handling tens of thousands of telemetry events every second. During
  each reporting period, metrics collected by a `Peep` reporter will be
  collected into a minimum number of packets. Users can set the maximum packet
  size in `Peep.Options`.
  """
  use GenServer
  require Logger
  alias Peep.{EventHandler, Options, Storage, Statsd}

  defmodule State do
    @moduledoc false
    defstruct tid: nil,
              interval: nil,
              handler_ids: nil,
              statsd_opts: nil,
              statsd_state: nil
  end

  def child_spec(options) do
    %{id: peep_name!(options), start: {__MODULE__, :start_link, [options]}}
  end

  def start_link(options) do
    case Options.validate(options) do
      {:ok, options} ->
        GenServer.start_link(__MODULE__, options, name: options.name)

      {:error, _} = err ->
        err
    end
  end

  @doc """
  Fetches all metrics from the worker. Called when preparing Prometheus or
  StatsD data.
  """
  defdelegate get_all_metrics(name_or_pid), to: Peep.Storage

  @impl true
  def init(options) do
    tid = Storage.new(options.name)

    metrics = options.metrics
    handler_ids = EventHandler.attach(metrics, tid, options.global_tags)

    statsd_opts = options.statsd
    statsd_flush_interval = statsd_opts[:flush_interval_ms]

    if statsd_flush_interval != nil do
      set_statsd_timer(statsd_flush_interval)
    end

    statsd_state =
      if options.statsd do
        Statsd.make_state(statsd_opts)
      else
        nil
      end

    state = %State{
      tid: tid,
      handler_ids: handler_ids,
      statsd_opts: statsd_opts,
      statsd_state: statsd_state
    }

    {:ok, state}
  end

  @impl true
  def handle_call(:get_all_metrics, _from, %State{tid: tid} = state) do
    {:reply, Storage.get_all_metrics(tid), state}
  end

  @impl true
  def handle_info(:statsd_flush, %State{statsd_state: nil} = state) do
    {:noreply, state}
  end

  def handle_info(
        :statsd_flush,
        %State{tid: tid, statsd_state: statsd_state, statsd_opts: statsd_opts} = state
      ) do
    new_statsd_state =
      Storage.get_all_metrics(tid)
      |> Statsd.make_and_send_packets(statsd_state)

    set_statsd_timer(statsd_opts[:flush_interval_ms])
    {:noreply, %State{state | statsd_state: new_statsd_state}}
  end

  defp set_statsd_timer(interval) do
    Process.send_after(self(), :statsd_flush, interval)
  end

  defp peep_name!(options) do
    Keyword.get(options, :name) || raise(ArgumentError, "a name must be provided")
  end
end