prop/lib/microwaveprop_web/controllers/weather_tile_controller.ex

defmodule MicrowavepropWeb.WeatherTileController do
  use MicrowavepropWeb, :controller

  alias Microwaveprop.Weather
  alias Microwaveprop.Weather.MapLayers
  alias Microwaveprop.Weather.TileRenderer

  # Fields shipped to the client. The client recolors locally on layer
  # switch from this same payload, so include every field any layer
  # might need.
  @cell_fields ~w(
    temperature dewpoint_depression surface_rh surface_pressure_mb
    surface_refractivity refractivity_gradient bl_height pwat
    temp_850mb dewpoint_850mb temp_700mb dewpoint_700mb
    lapse_rate mid_lapse_rate inversion_strength inversion_base_m
    ducting duct_base_m duct_strength
  )a

  @spec show(Plug.Conn.t(), map()) :: Plug.Conn.t()
  def show(conn, %{"z" => z, "x" => x, "y" => y, "layer" => layer_id, "time" => time_param}) do
    with true <- MapLayers.valid_id?(layer_id),
         {z, ""} <- Integer.parse(z),
         {x, ""} <- Integer.parse(x),
         {y, ""} <- Integer.parse(y),
         {:ok, valid_time, _offset} <- DateTime.from_iso8601(time_param) do
      body = TileRenderer.render_svg(valid_time, layer_id, z, x, y)

      conn
      |> put_resp_content_type("image/svg+xml")
      |> put_resp_header("cache-control", "public, max-age=60")
      |> send_resp(200, body)
    else
      _ ->
        conn
        |> put_resp_content_type("image/svg+xml")
        |> put_resp_header("cache-control", "public, max-age=5")
        |> send_resp(200, TileRenderer.empty_svg())
    end
  end

  @spec cells(Plug.Conn.t(), map()) :: Plug.Conn.t()
  def cells(conn, %{"time" => time_param} = params) do
    with {:ok, valid_time, _offset} <- DateTime.from_iso8601(time_param),
         {:ok, bounds} <- parse_bounds(params),
         {:ok, layers} <- parse_layers(params["layers"]) do
      rows = Weather.weather_grid_at(valid_time, bounds)

      case params["format"] do
        "bin" ->
          conn
          |> put_resp_header("content-type", "application/octet-stream")
          |> put_resp_header("cache-control", "public, max-age=60")
          |> send_resp(200, encode_binary(rows, layers))

        _ ->
          conn
          |> put_resp_header("cache-control", "public, max-age=60")
          |> json(%{
            valid_time: DateTime.to_iso8601(valid_time),
            cells: Enum.map(rows, &cell_payload(&1, layers))
          })
      end
    else
      _ -> bad_request(conn)
    end
  end

  def cells(conn, _params), do: bad_request(conn)

  defp bad_request(conn) do
    conn
    |> put_status(400)
    |> json(%{error: "invalid params"})
  end

  defp parse_bounds(%{"south" => s, "north" => n, "west" => w, "east" => e}) do
    with {:ok, south} <- parse_float(s),
         {:ok, north} <- parse_float(n),
         {:ok, west} <- parse_float(w),
         {:ok, east} <- parse_float(e) do
      {:ok, %{"south" => south, "north" => north, "west" => west, "east" => east}}
    end
  end

  defp parse_bounds(_), do: :error

  defp parse_float(value) when is_binary(value) do
    case Float.parse(value) do
      {f, ""} -> {:ok, f}
      _ -> :error
    end
  end

  defp parse_float(_), do: :error

  defp cell_payload(row, layers) do
    Enum.reduce(layers, %{lat: row.lat, lon: row.lon}, fn field, acc ->
      Map.put(acc, field, Map.get(row, field))
    end)
  end

  defp parse_layers(nil), do: {:ok, @cell_fields}

  defp parse_layers(value) when is_binary(value) do
    fields =
      value
      |> String.split(",", trim: true)
      |> Enum.map(&String.trim/1)
      |> Enum.map(&safe_atom/1)

    cond do
      Enum.any?(fields, &is_nil/1) -> :error
      Enum.empty?(fields) -> {:ok, @cell_fields}
      true -> {:ok, fields}
    end
  end

  defp parse_layers(_), do: :error

  defp safe_atom(name) when is_binary(name) do
    if name in Enum.map(@cell_fields, &Atom.to_string/1) do
      String.to_existing_atom(name)
    end
  end

  # Custom binary cell-pack format. Designed for cheap parsing in JS via
  # DataView + Float32Array (no library, no JSON parse). The header is
  # padded so all f32 arrays land on 4-byte boundaries — DataView reads
  # tolerate misalignment but typed-array views over an ArrayBuffer
  # don't.
  #
  #   "WCEL" (4 bytes)        — magic
  #   u8                      — version (1)
  #   u32 LE                  — cell count
  #   u8                      — layer count
  #   for each layer:
  #     u8                    — name length
  #     u8 × name_length      — name (UTF-8)
  #   u8 × N                  — zero padding to next 4-byte boundary
  #   f32 LE × cell_count     — latitudes
  #   f32 LE × cell_count     — longitudes
  #   for each layer:
  #     f32 LE × cell_count   — values (NaN = null; booleans 0.0/1.0)
  defp encode_binary(rows, layers) do
    cell_count = length(rows)
    layer_names = Enum.map(layers, &Atom.to_string/1)

    header =
      <<"WCEL", 1::8, cell_count::little-32, length(layer_names)::8>> <>
        Enum.map_join(layer_names, fn n -> <<byte_size(n)::8>> <> n end)

    pad_size = padding_to_align(byte_size(header), 4)
    header_padded = header <> :binary.copy(<<0>>, pad_size)

    lats = rows |> Enum.map(&<<float_or_nan(&1.lat)::little-float-32>>) |> IO.iodata_to_binary()
    lons = rows |> Enum.map(&<<float_or_nan(&1.lon)::little-float-32>>) |> IO.iodata_to_binary()

    values =
      layers
      |> Enum.map(fn field ->
        Enum.map(rows, fn row ->
          <<encode_value(Map.get(row, field))::little-float-32>>
        end)
      end)
      |> IO.iodata_to_binary()

    <<header_padded::binary, lats::binary, lons::binary, values::binary>>
  end

  defp padding_to_align(size, align) do
    rem = rem(size, align)
    if rem == 0, do: 0, else: align - rem
  end

  defp encode_value(nil), do: :nan
  defp encode_value(true), do: 1.0
  defp encode_value(false), do: 0.0
  defp encode_value(v) when is_number(v), do: v * 1.0
  defp encode_value(_), do: :nan

  defp float_or_nan(v) when is_number(v), do: v * 1.0
  defp float_or_nan(_), do: :nan
end