defmodule Vela.Stats.Collector do
  @moduledoc """
  Tracks cache statistics using lock-free atomic counters.

  The hot path (every get/put) increments counters directly using
  the :counters module — no GenServer message needed. This GenServer
  only wakes periodically to emit a telemetry summary.
  """

  use GenServer

  # Counter indices
  @hits 1
  @misses 2
  @writes 3
  @deletes 4
  @evictions 5
  @errors 6
  @num_counters 6

  @flush_interval 60_000

  # ---- Public API ----

  def start_link(opts) do
    cache_name = Keyword.fetch!(opts, :name)
    GenServer.start_link(__MODULE__, opts, name: collector_name(cache_name))
  end

  def collector_name(cache_name), do: :"vela_stats_#{cache_name}"

  @doc "Increment the hits counter. Called from the hot path — no GenServer involved."
  def incr_hits(cache_name), do: incr(cache_name, @hits)
  def incr_misses(cache_name), do: incr(cache_name, @misses)
  def incr_writes(cache_name), do: incr(cache_name, @writes)
  def incr_deletes(cache_name), do: incr(cache_name, @deletes)
  def incr_evictions(cache_name), do: incr(cache_name, @evictions)
  def incr_errors(cache_name), do: incr(cache_name, @errors)

  @doc "Return current stats as a map."
  def stats(cache_name) do
    ref = get_ref(cache_name)

    %{
      hits: :counters.get(ref, @hits),
      misses: :counters.get(ref, @misses),
      writes: :counters.get(ref, @writes),
      deletes: :counters.get(ref, @deletes),
      evictions: :counters.get(ref, @evictions),
      errors: :counters.get(ref, @errors)
    }
  end

  # ---- GenServer Callbacks ----

  @impl true
  def init(opts) do
    config = Keyword.fetch!(opts, :config)

    ref = :counters.new(@num_counters, [:atomics])

    # Store the ref in persistent_term so callers can access it
    # without going through this GenServer
    :persistent_term.put({:vela_stats_ref, config.name}, ref)

    Process.send_after(self(), :flush, @flush_interval)

    {:ok, %{config: config, ref: ref, window_start: System.monotonic_time()}}
  end

  @impl true
  def handle_info(:flush, state) do
    s = stats(state.config.name)
    duration = System.monotonic_time() - state.window_start

    # Emit a telemetry snapshot — dashboards/exporters can compute rates
    # from successive snapshots. Counters are cumulative and never reset,
    # so stats/1 always returns lifetime totals.
    :telemetry.execute(
      state.config.telemetry_prefix ++ [:stats, :flush],
      Map.put(s, :window_duration, duration),
      %{cache: state.config.name}
    )

    Process.send_after(self(), :flush, @flush_interval)
    {:noreply, %{state | window_start: System.monotonic_time()}}
  end

  # ---- Private ----

  defp incr(cache_name, index) do
    ref = get_ref(cache_name)
    :counters.add(ref, index, 1)
  end

  defp get_ref(cache_name) do
    :persistent_term.get({:vela_stats_ref, cache_name})
  end
end