defmodule MetrixWire do
  @moduledoc """
  Zero-config APM SDK for Elixir.

  Add the dependency and set `METRIXWIRE_KEY` (or `config :metrixwire, api_key: ...`)
  and it is **fully automatic** — every HTTP request becomes a trace, and every DB
  query / outbound HTTP call within it becomes a span. There is no manual span API.

  The SDK ships its own OTP application (`MetrixWire.Application`) which, on boot,
  starts the transport GenServer and attaches every `:telemetry` handler. Phoenix,
  Ecto, Finch and Tesla all emit telemetry, so a single handler set instruments the
  whole app with zero code changes. Bare Plug apps add one line: `plug MetrixWire.Plug`.

  Non-blocking: traces are batched in a GenServer and flushed off the request path
  with a short timeout, and **all** transport errors are swallowed — instrumentation
  must never throw into or block the host application.
  """

  alias MetrixWire.{Config, Context, Trace, Transport}

  @doc """
  Enqueue a finished trace for delivery. Internal — used by instrumentation.
  """
  def enqueue(%Trace{} = trace) do
    Transport.enqueue(trace)
  rescue
    _ -> :ok
  end

  @doc "Whether the SDK is enabled (configured with an API key)."
  def enabled?, do: Config.enabled?()

  @doc "The configured ingest endpoint URL, or nil."
  def endpoint, do: Config.get().endpoint

  @doc "Whether span source-location capture is on."
  def capture_source?, do: Config.get().capture_source

  @doc """
  Manually flush the queue (e.g. before a short-lived process exits).
  """
  def flush, do: Transport.flush()

  @doc """
  Attach an exception to the active trace. Escape hatch mirroring the other SDKs —
  NOT a manual span API; requests/queries are still captured automatically.
  """
  def capture_exception(error, stacktrace \\ nil) do
    case Context.current() do
      %Trace{} = trace -> Context.put(Trace.capture_exception(trace, error, stacktrace))
      _ -> :ok
    end

    :ok
  rescue
    _ -> :ok
  end

  # ── shared utilities used across the SDK ──────────────────────────────────

  @doc "Monotonic milliseconds — safe for measuring durations (never goes back)."
  def monotonic_ms, do: System.monotonic_time(:millisecond)

  @doc ~S(ISO8601 UTC with millisecond precision, e.g. "2026-07-15T10:00:00.000Z".)
  def iso8601(%DateTime{} = dt) do
    dt
    |> DateTime.truncate(:millisecond)
    |> DateTime.to_iso8601()
    |> normalize_iso()
  rescue
    _ -> iso8601(DateTime.utc_now())
  end

  def iso8601(_), do: iso8601(DateTime.utc_now())

  @doc "ISO8601 string for `ms_ago` milliseconds before now (used for span start)."
  def iso8601_ago(ms_ago) when is_integer(ms_ago) do
    DateTime.utc_now()
    |> DateTime.add(-ms_ago, :millisecond)
    |> iso8601()
  end

  def iso8601_ago(_), do: iso8601(DateTime.utc_now())

  # DateTime.to_iso8601/1 yields "...Z" for UTC in recent Elixir, but be defensive
  # about "+00:00" too — the wire format wants a trailing "Z".
  defp normalize_iso(str) do
    str
    |> String.replace_suffix("+00:00", "Z")
    |> ensure_millis()
  end

  # Guarantee millisecond precision (".000") even if truncation dropped it.
  defp ensure_millis(str) do
    cond do
      Regex.match?(~r/\.\d{3}Z$/, str) -> str
      String.ends_with?(str, "Z") -> String.replace_suffix(str, "Z", ".000Z")
      true -> str
    end
  end

  @doc """
  Build the exception meta map from an arbitrary error + optional stacktrace.
  Keeps the top ~8 backtrace frames.
  """
  def exception_meta(error, stacktrace \\ nil) do
    %{
      "type" => exception_type(error),
      "message" => exception_message(error),
      "stack" => format_stack(stacktrace)
    }
    |> Enum.reject(fn {_k, v} -> is_nil(v) end)
    |> Map.new()
  rescue
    _ -> nil
  end

  defp exception_type(error) when is_exception(error), do: inspect(error.__struct__)
  defp exception_type(error) when is_atom(error), do: Atom.to_string(error)
  defp exception_type(_), do: "RuntimeError"

  defp exception_message(error) when is_exception(error) do
    Exception.message(error)
  rescue
    _ -> inspect(error)
  end

  defp exception_message(error) when is_binary(error), do: error
  defp exception_message(error), do: inspect(error)

  defp format_stack(nil), do: nil
  defp format_stack([]), do: nil

  defp format_stack(stacktrace) when is_list(stacktrace) do
    stacktrace
    |> Enum.take(8)
    |> Enum.map_join("\n", &Exception.format_stacktrace_entry/1)
  rescue
    _ -> nil
  end

  defp format_stack(_), do: nil

  @doc "Total memory used by the Erlang VM, in bytes. Best-effort."
  def memory_bytes do
    :erlang.memory(:total)
  rescue
    _ -> nil
  end

  @doc "Memory delta in MB since `start_bytes`. Returns 0 when unavailable."
  def memory_delta_mb(nil), do: 0

  def memory_delta_mb(start_bytes) when is_integer(start_bytes) do
    case memory_bytes() do
      nil -> 0
      now -> max(round((now - start_bytes) / (1024 * 1024)), 0)
    end
  rescue
    _ -> 0
  end
end