defmodule Microwaveprop.PromEx.Plugins.Phoenix do
  @moduledoc """
  Local fork of `PromEx.Plugins.Phoenix` that normalizes the `:transport` tag
  value in socket and channel event metrics so non-`String.Chars` transport
  values (e.g., `Phoenix.ChannelTest`'s tuple `{Module, pid}`) are not
  silently dropped as "bad tag value" by `telemetry_metrics_prometheus_core`.

  All other metric groups are delegated to the built-in plugin unchanged.
  """

  use PromEx.Plugin

  alias Phoenix.Socket
  alias PromEx.Plugins.Phoenix, as: BuiltinPhoenix

  @impl true
  def event_metrics(opts) do
    # Delegate everything to the built-in plugin (which also attaches the
    # telemetry proxy handlers), then patch only the transport tag in the
    # two affected groups.
    opts
    |> BuiltinPhoenix.event_metrics()
    |> Enum.map(fn
      %{group_name: :phoenix_channel_event_metrics} = event ->
        %{event | metrics: fix_channel_transport(event.metrics)}

      %{group_name: :phoenix_socket_event_metrics} = event ->
        %{event | metrics: fix_socket_transport(event.metrics)}

      event ->
        event
    end)
  end

  # ── Channel event metrics ──────────────────────────────────────────────

  defp fix_channel_transport(metrics) do
    Enum.map(metrics, fn
      %{event_name: [:phoenix, :channel_joined]} = metric ->
        %{
          metric
          | tag_values: fn %{
                             result: result,
                             socket: %Socket{transport: transport, endpoint: endpoint}
                           } ->
              %{
                transport: normalize_transport(transport),
                result: result,
                endpoint: normalize_module_name(endpoint)
              }
            end
        }

      other ->
        other
    end)
  end

  # ── Socket event metrics ───────────────────────────────────────────────

  defp fix_socket_transport(metrics) do
    Enum.map(metrics, fn
      %{event_name: [:phoenix, :socket_connected]} = metric ->
        %{
          metric
          | tag_values: fn %{result: result, endpoint: endpoint, transport: transport} ->
              %{
                transport: normalize_transport(transport),
                result: result,
                endpoint: normalize_module_name(endpoint)
              }
            end
        }

      other ->
        other
    end)
  end

  # ── Helpers ────────────────────────────────────────────────────────────

  defp normalize_transport(transport) do
    if String.Chars.impl_for(transport) do
      to_string(transport)
    else
      inspect(transport)
    end
  end

  defp normalize_module_name(name) when is_atom(name) do
    name
    |> Atom.to_string()
    |> String.trim_leading("Elixir.")
  end

  defp normalize_module_name(name) do
    String.trim_leading(name, "Elixir.")
  end
end