defmodule MicrowavepropWeb.GridBounds do
  @moduledoc """
  Bounds clamp + sanity checks for grid-cell HTTP endpoints
  (`/scores/cells`, `/weather/cells`).

  The propagation grid covers HRRR (CONUS) and HRDPS (Canada to
  60°N): roughly lat 25-60, lon -141 to -52 — about 35° × 90° =
  3150 sq degrees. Any caller-provided viewport is clamped to a
  generous superset of that bbox, and viewports whose area exceeds
  the supported region are rejected as `:viewport_too_large` so a
  malicious caller cannot trigger global / multi-million-cell
  binary responses.

  `nil` bounds are passed through (caller intends the full grid).
  """

  # Generous superset of HRRR + HRDPS — a few extra degrees on each
  # edge so legitimate clients zoomed slightly past the data extent
  # still get a 200 with the actual covered cells.
  @lat_min 20.0
  @lat_max 65.0
  @lon_min -150.0
  @lon_max -50.0

  # Comfortably above the HRDPS+HRRR union (~3150 sq deg), but tight
  # enough that the fully-clamped extent (~4500 sq deg) still trips
  # the cap — a "give me everything" request is exactly the DoS
  # shape we want to reject.
  @max_viewport_area_sq_deg 4000.0

  @type bounds :: %{optional(String.t()) => float()} | nil

  @doc """
  Clamp `bounds` to the supported grid extent. Returns:

    * `{:ok, nil}` when bounds are nil (caller wants the full grid)
    * `{:ok, clamped_bounds}` when the clamped viewport area is OK
    * `{:error, :viewport_too_large}` when the clamped area still
      exceeds `@max_viewport_area_sq_deg` (catches degenerate
      requests like global bounds that survive clamping)
    * `{:error, :invalid_bounds}` when north < south or east < west
  """
  @spec clamp(bounds()) ::
          {:ok, bounds()} | {:error, :viewport_too_large | :invalid_bounds}
  def clamp(nil), do: {:ok, nil}

  def clamp(%{"south" => s, "north" => n, "west" => w, "east" => e})
      when is_number(s) and is_number(n) and is_number(w) and is_number(e) do
    south = clamp_value(s, @lat_min, @lat_max)
    north = clamp_value(n, @lat_min, @lat_max)
    west = clamp_value(w, @lon_min, @lon_max)
    east = clamp_value(e, @lon_min, @lon_max)

    cond do
      north < south or east < west ->
        {:error, :invalid_bounds}

      (north - south) * (east - west) > @max_viewport_area_sq_deg ->
        {:error, :viewport_too_large}

      true ->
        {:ok, %{"south" => south, "north" => north, "west" => west, "east" => east}}
    end
  end

  def clamp(_), do: {:error, :invalid_bounds}

  @doc "Maximum viewport area in square degrees."
  @spec max_viewport_area_sq_deg() :: float()
  def max_viewport_area_sq_deg, do: @max_viewport_area_sq_deg

  defp clamp_value(v, lo, hi), do: v |> max(lo) |> min(hi)
end