defmodule AuditTrail.Reader do
  @moduledoc """
  Query audit logs from Loki.

  ## Usage

      AuditTrail.Reader.get_logs(%{
        action:     "item:approved",
        actor_id:   "user-uuid",
        status:     "success",
        resource:   "payment",
        operation:  "update",
        start_date: "2026-06-01",
        end_date:   "2026-06-12",
        search:     "item_id",
        limit:      100
      })

  `resource` and `operation` are optional payload fields (only present on
  logs where the caller opted into resource/CRUD tagging). Filtering on
  them is a JSON field match on the log line, not a stream label, to avoid
  adding cardinality.
  """

  require Logger

  def get_logs(filters \\ %{}) do
    url = AuditTrail.Config.loki_read_url()

    if is_nil(url) do
      {:error, "loki_read_url not configured"}
    else
      warn_if_default_window(filters)
      query = build_query(filters)
      params = build_params(query, filters)

      case Req.get(url, params: params, receive_timeout: 10_000) do
        {:ok, %{status: 200, body: body}} ->
          body
          |> parse_response()
          |> maybe_group(filters)

        {:ok, %{status: code}} ->
          Logger.error("[AuditTrail.Reader] Loki returned #{code}")
          {:error, "Loki returned status #{code}"}

        {:error, reason} ->
          Logger.error("[AuditTrail.Reader] HTTP error: #{inspect(reason)}")
          {:error, reason}
      end
    end
  end

  # `get_logs/1` with no start_date/end_date silently scopes to the last
  # 24h. A query that legitimately returns 0 results looks identical to one
  # that returns 0 because it never looked further back than a day — so
  # make that window explicit in the logs whenever it's implicit for the
  # caller.
  defp warn_if_default_window(filters) do
    if is_nil(filters[:start_date]) and is_nil(filters[:end_date]) do
      Logger.info(
        "[AuditTrail.Reader] get_logs/1 called without start_date/end_date — " <>
          "defaulting to the last 24h. Pass start_date/end_date explicitly for a wider window."
      )
    end
  end

  defp build_query(filters) do
    app = AuditTrail.Config.app_name()
    labels = [~s(app="#{app}")]
    labels = if t = filters[:type], do: [~s(type="#{t}") | labels], else: labels
    labels = if s = filters[:status], do: [~s(status="#{s}") | labels], else: labels
    labels = if id = filters[:actor_id], do: [~s(user="#{id}") | labels], else: labels
    labels = if tn = filters[:tenant], do: [~s(tenant="#{tn}") | labels], else: labels

    selector = "{#{Enum.join(labels, ", ")}}"

    selector = with_line_filter(selector, filters[:search])
    selector = with_json_field_filter(selector, "resource", filters[:resource])
    selector = with_json_field_filter(selector, "resource_id", filters[:resource_id])
    with_json_field_filter(selector, "operation", filters[:operation])
  end

  defp with_line_filter(selector, nil), do: selector
  defp with_line_filter(selector, ""), do: selector
  defp with_line_filter(selector, term), do: ~s(#{selector} |= "#{term}")

  defp with_json_field_filter(selector, _field, nil), do: selector
  defp with_json_field_filter(selector, _field, ""), do: selector

  defp with_json_field_filter(selector, field, value),
    do: ~s(#{selector} | json | #{field}="#{value}")

  defp build_params(query, filters) do
    [
      query: query,
      limit: Map.get(filters, :limit, 100),
      start: to_loki_time(filters[:start_date], :start),
      end: end_bound(filters)
    ]
  end

  # `before:` is the cursor returned by `AuditTrail.get_logs_page/1` — the
  # nanosecond timestamp of the oldest entry in the previous page. Passing
  # it back narrows `end` to just before that entry so the next call picks
  # up strictly older logs instead of repeating the same page.
  defp end_bound(%{before: cursor}) when is_binary(cursor) do
    case Integer.parse(cursor) do
      {ns, ""} -> ns - 1
      _ -> to_loki_time(nil, :end)
    end
  end

  defp end_bound(filters), do: to_loki_time(filters[:end_date], :end)

  defp to_loki_time(nil, :start),
    do:
      DateTime.utc_now()
      |> DateTime.add(-86_400, :second)
      |> DateTime.to_unix(:nanosecond)

  defp to_loki_time(nil, :end),
    do:
      DateTime.utc_now()
      |> DateTime.to_unix(:nanosecond)

  defp to_loki_time(str, type) when is_binary(str) do
    case Date.from_iso8601(str) do
      {:ok, date} -> to_loki_time(date, type)
      _ -> to_loki_time(nil, type)
    end
  end

  defp to_loki_time(%Date{} = d, :start),
    do:
      d
      |> NaiveDateTime.new!(~T[00:00:00])
      |> DateTime.from_naive!("Etc/UTC")
      |> DateTime.to_unix(:nanosecond)

  defp to_loki_time(%Date{} = d, :end),
    do:
      d
      |> NaiveDateTime.new!(~T[23:59:59])
      |> DateTime.from_naive!("Etc/UTC")
      |> DateTime.to_unix(:nanosecond)

  defp parse_response(%{"data" => %{"result" => results}}) do
    results
    |> Enum.flat_map(fn %{"stream" => labels, "values" => values} ->
      Enum.map(values, fn [ts_ns, line] ->
        payload =
          case Jason.decode(line) do
            {:ok, json} -> json
            _ -> %{"raw" => line}
          end

        %{
          timestamp: parse_ts(ts_ns),
          labels: labels,
          payload: payload
        }
      end)
    end)
    |> Enum.sort_by(& &1.timestamp, {:desc, DateTime})
  end

  defp parse_response(_), do: []

  defp maybe_group(logs, %{view: "actors"}) do
    unique =
      logs
      |> Enum.uniq_by(fn log -> get_in(log.payload, ["actor_id"]) end)
      |> Enum.reject(fn log -> is_nil(get_in(log.payload, ["actor_id"])) end)

    {:ok, unique}
  end

  defp maybe_group(logs, _), do: {:ok, logs}

  defp parse_ts(ts_ns) when is_binary(ts_ns) do
    {ns, ""} = Integer.parse(ts_ns)
    DateTime.from_unix!(ns, :nanosecond)
  end
end