defmodule Mix.Tasks.Hrdps.Probe do
  @shortdoc "Throwaway probe: fetch one HRDPS variable, decode at sample cities"
  @moduledoc """
  One-shot probe for the Canadian HRDPS model. Fetches a single GRIB2
  file from MSC Datamart and uses `wgrib2 -lon` to extract values at
  five Canadian cities, printing a table for visual sanity-checking.

  Designed as a wedge to retire risk before committing to a real
  HrdpsClient: validates URL pattern, GRIB2 decode, and rotated-lat/lon
  reprojection without touching the DB, Oban, or the score grid.

  Throwaway code. Will be deleted once the real client lands.

      mix hrdps.probe                 # latest cycle, f000, 2m temperature
      mix hrdps.probe --hour 6        # f006 from latest cycle
      mix hrdps.probe --cycle 20260429T12Z

  URL pattern (current as of 2026-04-29; ECCC reorganized away from the
  old `/model_hrdps/` root to a date-prefixed structure):

      https://dd.weather.gc.ca/{YYYYMMDD}/WXO-DD/model_hrdps/continental/2.5km/
        {HH}/{FFF}/{YYYYMMDD}T{HH}Z_MSC_HRDPS_{VAR}_{LEVEL}_RLatLon0.0225_PT{FFF}H.grib2
  """
  use Mix.Task

  @datamart_base "https://dd.weather.gc.ca"

  # Variable / level slug for 2m temperature in HRDPS naming. HRRR-side
  # equivalent is `TMP:2 m above ground`; HRDPS uses MSC's per-variable
  # filename convention.
  @variable_slug "TMP_AGL-2m"

  @cities [
    {"YYZ Toronto", 43.68, -79.63},
    {"YVR Vancouver", 49.19, -123.18},
    {"YYC Calgary", 51.11, -114.02},
    {"YOW Ottawa", 45.32, -75.67},
    {"YHZ Halifax", 44.88, -63.51}
  ]

  @impl Mix.Task
  def run(argv) do
    {opts, _, _} =
      OptionParser.parse(argv,
        strict: [cycle: :string, hour: :integer],
        aliases: [c: :cycle, h: :hour]
      )

    {:ok, _} = Application.ensure_all_started(:req)

    forecast_hour = Keyword.get(opts, :hour, 0)
    cycle = opts |> Keyword.get(:cycle) |> resolve_cycle()

    url = build_url(cycle, forecast_hour)
    IO.puts("Fetching: #{url}")

    {:ok, path} = download(url)
    IO.puts("Downloaded #{path} (#{File.stat!(path).size} bytes)")

    print_inventory(path)
    print_city_table(path, cycle, forecast_hour)
  end

  # ----- Cycle resolution -----

  defp resolve_cycle(nil), do: latest_published_cycle()

  defp resolve_cycle(<<year::binary-size(4), month::binary-size(2), day::binary-size(2), "T", hour::binary-size(2), "Z">>) do
    %{date: "#{year}#{month}#{day}", hour: hour}
  end

  defp resolve_cycle(other) do
    Mix.raise("Invalid --cycle #{inspect(other)} — expected YYYYMMDDTHHZ")
  end

  # HRDPS publishes ~3-4h after each cycle (00/06/12/18Z). Walk back from
  # "now - 4h" until we find a cycle whose root directory exists.
  defp latest_published_cycle do
    now = DateTime.utc_now()
    candidates = for h <- 0..3, do: DateTime.add(now, -((4 + h * 6) * 3600), :second)

    candidates
    |> Enum.map(&snap_to_cycle/1)
    |> Enum.uniq()
    |> Enum.find(&cycle_published?/1)
    |> case do
      nil -> Mix.raise("No published HRDPS cycle found in the last 24h")
      cycle -> cycle
    end
  end

  defp snap_to_cycle(%DateTime{} = dt) do
    cycle_hour = div(dt.hour, 6) * 6
    %{date: Calendar.strftime(dt, "%Y%m%d"), hour: String.pad_leading("#{cycle_hour}", 2, "0")}
  end

  defp cycle_published?(%{date: date, hour: hour}) do
    url = "#{@datamart_base}/#{date}/WXO-DD/model_hrdps/continental/2.5km/#{hour}/000/"

    case Req.head(url, retry: false, connect_options: [timeout: 5_000]) do
      {:ok, %{status: 200}} -> true
      _ -> false
    end
  end

  # ----- URL + download -----

  defp build_url(%{date: date, hour: hour}, forecast_hour) do
    fff = String.pad_leading("#{forecast_hour}", 3, "0")
    filename = "#{date}T#{hour}Z_MSC_HRDPS_#{@variable_slug}_RLatLon0.0225_PT#{fff}H.grib2"

    "#{@datamart_base}/#{date}/WXO-DD/model_hrdps/continental/2.5km/#{hour}/#{fff}/#{filename}"
  end

  defp download(url) do
    path = Path.join(System.tmp_dir!(), "hrdps_probe_#{System.unique_integer([:positive])}.grib2")

    case Req.get(url, into: File.stream!(path), retry: false) do
      {:ok, %{status: 200}} -> {:ok, path}
      {:ok, %{status: status}} -> Mix.raise("Datamart returned HTTP #{status} for #{url}")
      {:error, reason} -> Mix.raise("Fetch failed: #{inspect(reason)}")
    end
  end

  # ----- wgrib2 inspection -----

  defp print_inventory(path) do
    {output, 0} = System.cmd("wgrib2", [path], stderr_to_stdout: true)
    IO.puts("\nGRIB2 inventory:")
    IO.puts("  " <> String.trim(output))
  end

  defp print_city_table(path, cycle, forecast_hour) do
    args = [path] ++ Enum.flat_map(@cities, fn {_, lat, lon} -> ["-lon", "#{lon}", "#{lat}"] end)
    {output, 0} = System.cmd("wgrib2", args, stderr_to_stdout: true)

    values = parse_wgrib2_lon_output(output)

    IO.puts("\nHRDPS 2m temperature, cycle #{cycle.date}T#{cycle.hour}Z f#{pad3(forecast_hour)}:\n")
    IO.puts("  city                  lat,lon               temp")
    IO.puts("  --------------------  --------------------  ------")

    @cities
    |> Enum.zip(values)
    |> Enum.each(fn {{name, lat, lon}, kelvin} ->
      celsius = kelvin - 273.15

      IO.puts("  #{String.pad_trailing(name, 20)}  #{format_coord(lat, lon)}  #{format_temp(celsius)}")
    end)
  end

  # wgrib2 -lon emits one line for the GRIB record with all `lon=…lat=…val=…`
  # entries colon-separated. For five cities we get five `val=` matches.
  defp parse_wgrib2_lon_output(output) do
    ~r/val=([\-\d\.]+)/
    |> Regex.scan(output)
    |> Enum.map(fn [_, v] -> String.to_float(v) end)
  end

  defp format_coord(lat, lon), do: "#{:io_lib.format("~6.2f", [lat])},#{:io_lib.format("~7.2f", [lon])}"

  defp format_temp(celsius), do: "#{:io_lib.format("~5.1f", [celsius])}°C"

  defp pad3(n), do: String.pad_leading("#{n}", 3, "0")
end