prop/lib/microwaveprop/weather/era5_client.ex

defmodule Microwaveprop.Weather.Era5Client do
  @moduledoc """
  Client for the Copernicus Climate Data Store (CDS) API.
  Fetches ERA5 reanalysis data (pressure-level profiles + single-level surface fields)
  for historical contact enrichment where HRRR is unavailable (pre-2014).

  The CDS API is asynchronous: submit a request, poll for completion, download result.
  Results are GRIB2 format, decoded with the existing GRIB2 infrastructure.
  """

  alias Microwaveprop.Weather.Grib2.Extractor
  alias Microwaveprop.Weather.SoundingParams

  require Logger

  @cds_url "https://cds.climate.copernicus.eu/api"
  @poll_interval_ms 5_000
  @max_poll_attempts 120

  @pressure_levels ~w(1000 975 950 925 900 875 850 825 800 775 750 725 700)

  @single_level_vars ~w(
    2m_temperature
    2m_dewpoint_temperature
    surface_pressure
    10m_u_component_of_wind
    10m_v_component_of_wind
    total_column_water_vapour
    boundary_layer_height
  )

  @pressure_level_vars ~w(temperature dewpoint_temperature geopotential)

  @doc """
  Fetch ERA5 profile for a single point and time.
  Returns {:ok, profile_attrs} or {:error, reason}.

  The profile_attrs map has the same shape as HRRR profiles for interoperability.
  """
  def fetch_profile(lat, lon, timestamp) do
    # Round to ERA5 grid (0.25°)
    rlat = Float.round(lat * 4) / 4
    rlon = Float.round(lon * 4) / 4

    # ERA5 hourly — round to nearest hour
    valid_time = DateTime.truncate(timestamp, :second)
    valid_time = %{valid_time | minute: 0, second: 0}

    # Build a small bounding box (0.25° around the point)
    area = [rlat + 0.25, rlon - 0.25, rlat - 0.25, rlon + 0.25]

    with {:ok, single_data} <- fetch_single_levels(valid_time, area),
         {:ok, pressure_data} <- fetch_pressure_levels(valid_time, area) do
      build_profile(rlat, rlon, valid_time, single_data, pressure_data)
    end
  end

  @doc """
  Fetch ERA5 single-level (surface) data for a time and area.
  """
  def fetch_single_levels(valid_time, area) do
    request = %{
      "product_type" => ["reanalysis"],
      "variable" => @single_level_vars,
      "year" => [Calendar.strftime(valid_time, "%Y")],
      "month" => [Calendar.strftime(valid_time, "%m")],
      "day" => [Calendar.strftime(valid_time, "%d")],
      "time" => [Calendar.strftime(valid_time, "%H:00")],
      "area" => area,
      "data_format" => "grib"
    }

    submit_and_download("reanalysis-era5-single-levels", request)
  end

  @doc """
  Fetch ERA5 pressure-level data for a time and area.
  """
  def fetch_pressure_levels(valid_time, area) do
    request = %{
      "product_type" => ["reanalysis"],
      "variable" => @pressure_level_vars,
      "pressure_level" => @pressure_levels,
      "year" => [Calendar.strftime(valid_time, "%Y")],
      "month" => [Calendar.strftime(valid_time, "%m")],
      "day" => [Calendar.strftime(valid_time, "%d")],
      "time" => [Calendar.strftime(valid_time, "%H:00")],
      "area" => area,
      "data_format" => "grib"
    }

    submit_and_download("reanalysis-era5-pressure-levels", request)
  end

  defp submit_and_download(dataset, request) do
    api_key = api_key()

    if is_nil(api_key) do
      {:error, "ERA5_CDS_API_KEY not configured"}
    else
      case submit_request(dataset, request, api_key) do
        {:ok, job_id} -> poll_and_download(job_id, api_key)
        {:error, reason} -> {:error, reason}
      end
    end
  end

  defp submit_request(dataset, request, api_key) do
    url = "#{@cds_url}/retrieve/#{dataset}"

    case Req.post(url,
           json: request,
           headers: [{"PRIVATE-TOKEN", api_key}],
           receive_timeout: 30_000
         ) do
      {:ok, %{status: status, body: body}} when status in [200, 201, 202] ->
        job_id = body["request_id"] || body["jobId"]

        if job_id do
          Logger.info("ERA5: submitted job #{job_id} for #{dataset}")
          {:ok, job_id}
        else
          {:error, "ERA5: no job_id in response: #{inspect(body)}"}
        end

      {:ok, %{status: status, body: body}} ->
        {:error, "ERA5 CDS HTTP #{status}: #{inspect(body)}"}

      {:error, reason} ->
        {:error, "ERA5 CDS request failed: #{inspect(reason)}"}
    end
  end

  defp poll_and_download(job_id, api_key, attempt \\ 0)

  defp poll_and_download(_job_id, _api_key, attempt) when attempt >= @max_poll_attempts do
    {:error, "ERA5: poll timeout after #{@max_poll_attempts} attempts"}
  end

  defp poll_and_download(job_id, api_key, attempt) do
    url = "#{@cds_url}/tasks/#{job_id}"

    case Req.get(url, headers: [{"PRIVATE-TOKEN", api_key}], receive_timeout: 10_000) do
      {:ok, %{status: 200, body: %{"state" => "completed"} = body}} ->
        download_url = body["location"] || get_in(body, ["result", "location"])

        if download_url do
          download_result(download_url, api_key)
        else
          {:error, "ERA5: completed but no download URL in #{inspect(body)}"}
        end

      {:ok, %{status: 200, body: %{"state" => state}}} when state in ["queued", "running"] ->
        Process.sleep(@poll_interval_ms)
        poll_and_download(job_id, api_key, attempt + 1)

      {:ok, %{status: 200, body: %{"state" => "failed"} = body}} ->
        {:error, "ERA5 job failed: #{inspect(body["error"])}"}

      {:ok, %{status: status, body: body}} ->
        {:error, "ERA5 poll HTTP #{status}: #{inspect(body)}"}

      {:error, reason} ->
        {:error, "ERA5 poll failed: #{inspect(reason)}"}
    end
  end

  defp download_result(url, api_key) do
    case Req.get(url, headers: [{"PRIVATE-TOKEN", api_key}], receive_timeout: 120_000, into: :self) do
      {:ok, %{status: 200, body: body}} ->
        {:ok, body}

      {:ok, %{status: status}} ->
        {:error, "ERA5 download HTTP #{status}"}

      {:error, reason} ->
        {:error, "ERA5 download failed: #{inspect(reason)}"}
    end
  end

  defp build_profile(lat, lon, valid_time, single_grib, pressure_grib) do
    # Decode GRIB2 data using existing infrastructure
    with {:ok, single_points} <- extract_point(single_grib, lat, lon),
         {:ok, pressure_points} <- extract_point(pressure_grib, lat, lon) do
      # ERA5 temperatures are in Kelvin, convert to Celsius
      temp_c = kelvin_to_celsius(single_points["TMP:2 m above ground"])
      dewpoint_c = kelvin_to_celsius(single_points["DPT:2 m above ground"])
      # ERA5 surface pressure is in Pa, convert to mb
      pressure_mb = pa_to_mb(single_points["PRES:surface"])
      hpbl_m = single_points["HPBL:surface"]
      # ERA5 TCWV is in kg/m² which equals mm
      pwat_mm = single_points["TCWV:entire atmosphere"]

      # Build pressure-level profile
      profile =
        @pressure_levels
        |> Enum.map(fn level_str ->
          level = String.to_integer(level_str)
          pres_key = "#{level} mb"

          %{
            "pres" => level * 1.0,
            "tmpc" => kelvin_to_celsius(pressure_points["TMP:#{pres_key}"]),
            "dwpc" => kelvin_to_celsius(pressure_points["DPT:#{pres_key}"]),
            "hght" => geopotential_to_height(pressure_points["HGT:#{pres_key}"])
          }
        end)
        |> Enum.reject(fn p -> is_nil(p["tmpc"]) end)

      # Derive refractivity and ducting from profile
      params = SoundingParams.derive(profile)

      attrs = %{
        valid_time: valid_time,
        lat: lat,
        lon: lon,
        profile: profile,
        hpbl_m: hpbl_m,
        pwat_mm: pwat_mm,
        surface_temp_c: temp_c,
        surface_dewpoint_c: dewpoint_c,
        surface_pressure_mb: pressure_mb
      }

      attrs =
        if params do
          Map.merge(attrs, %{
            surface_refractivity: params.surface_refractivity,
            min_refractivity_gradient: params.min_refractivity_gradient,
            ducting_detected: params.ducting_detected || false,
            duct_characteristics: params.duct_characteristics
          })
        else
          attrs
        end

      {:ok, attrs}
    end
  rescue
    e -> {:error, "ERA5 GRIB2 decode failed: #{Exception.message(e)}"}
  end

  defp extract_point(grib_data, lat, lon) do
    case Extractor.extract_points(grib_data, lat, lon) do
      {:ok, fields} when map_size(fields) > 0 -> {:ok, fields}
      {:ok, _} -> {:error, "no data at #{lat},#{lon}"}
      {:error, reason} -> {:error, reason}
    end
  end

  defp kelvin_to_celsius(nil), do: nil
  defp kelvin_to_celsius(k), do: k - 273.15

  defp pa_to_mb(nil), do: nil
  defp pa_to_mb(pa), do: pa / 100.0

  defp geopotential_to_height(nil), do: nil
  defp geopotential_to_height(gp), do: gp / 9.80665

  defp api_key do
    System.get_env("ERA5_CDS_API_KEY")
  end
end