metrixwire

defmodule MetrixWire.Span do
  @moduledoc """
  A single unit of work within a trace (`db_query` | `http_call` | `custom`).
  Users never build these — instrumentation creates them. Serialized to the exact
  ingest wire shape.
  """

  defstruct type: "custom",
            description: "",
            started_at: nil,
            duration_ms: 0,
            source_location: nil,
            meta: nil

  @type t :: %__MODULE__{}

  @doc """
  Build a span. `type` is "db_query" | "http_call" | "custom". `started_at` is an
  ISO8601 string. `meta` is an optional map (e.g. `%{"rowCount" => n}`).
  """
  def new(type, description, started_at, duration_ms, opts \\ []) do
    %__MODULE__{
      type: type,
      description: to_string(description),
      started_at: started_at,
      duration_ms: max(round_num(duration_ms), 0),
      source_location: Keyword.get(opts, :source_location),
      meta: normalize_meta(Keyword.get(opts, :meta))
    }
  end

  @doc "Serialize to the ingest wire shape (string keys, camelCase)."
  def to_map(%__MODULE__{} = span) do
    base = %{
      "type" => span.type,
      "description" => span.description,
      "startedAt" => span.started_at,
      "durationMs" => span.duration_ms
    }

    base
    |> maybe_put("sourceLocation", span.source_location)
    |> maybe_put("meta", span.meta)
  end

  defp normalize_meta(meta) when is_map(meta) and map_size(meta) > 0, do: meta
  defp normalize_meta(_), do: nil

  defp maybe_put(map, _key, nil), do: map
  defp maybe_put(map, key, value), do: Map.put(map, key, value)

  defp round_num(v) when is_integer(v), do: v
  defp round_num(v) when is_float(v), do: round(v)
  defp round_num(_), do: 0
end