defmodule CobblemonUi.SpriteCache do
  @moduledoc """
  Downloads and caches Pokémon sprite images locally.

  Sprites are stored under `$CACHE_DIR/sprites/<species>.png`.
  Use `ensure_sprite/1` to lazily download a sprite on first access,
  or `sprite_path/1` to get the expected local filesystem path.
  """

  use GenServer
  require Logger

  @source_url "https://img.rankedboost.com/wp-content/plugins/k-Pokemon/assets/sprites-official"

  # ---------------------------------------------------------------------------
  # Client API
  # ---------------------------------------------------------------------------

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  @doc """
  Ensures the sprite for the given species is downloaded locally.
  Returns `:ok` if the file exists (or was just fetched), or `{:error, reason}`.
  """
  @spec ensure_sprite(String.t()) :: :ok | {:error, term()}
  def ensure_sprite(species) when is_binary(species) do
    key = String.downcase(species)
    path = sprite_path(key)

    if File.exists?(path) do
      :ok
    else
      GenServer.call(__MODULE__, {:download, key}, 30_000)
    end
  end

  @doc """
  Returns the local URL path for a species sprite (for use in templates).
  Always returns the path even if the file hasn't been downloaded yet.
  """
  @spec sprite_url(String.t()) :: String.t()
  def sprite_url(species) do
    "/sprites/#{String.downcase(species)}.png"
  end

  @doc "Returns the filesystem path where a sprite would be stored."
  @spec sprite_path(String.t()) :: String.t()
  def sprite_path(species) do
    Path.join(sprites_dir(), "#{String.downcase(species)}.png")
  end

  @doc "Returns the base sprites directory."
  @spec sprites_dir() :: String.t()
  def sprites_dir do
    dir = System.get_env("CACHE_DIR", ".")
    Path.join(dir, "sprites")
  end

  # ---------------------------------------------------------------------------
  # Server callbacks
  # ---------------------------------------------------------------------------

  @impl true
  def init(_opts) do
    File.mkdir_p!(sprites_dir())
    {:ok, MapSet.new()}
  end

  @impl true
  def handle_call({:download, species}, _from, in_progress) do
    path = sprite_path(species)

    if File.exists?(path) do
      {:reply, :ok, in_progress}
    else
      result = download_sprite(species, path)
      {:reply, result, in_progress}
    end
  end

  # ---------------------------------------------------------------------------
  # Private helpers
  # ---------------------------------------------------------------------------

  defp download_sprite(species, dest) do
    url = "#{@source_url}/#{species}.png"

    case Req.get(url, decode_body: false) do
      {:ok, %Req.Response{status: 200, body: body}} when is_binary(body) ->
        File.mkdir_p!(Path.dirname(dest))
        File.write!(dest, body)
        Logger.debug("[SpriteCache] Downloaded sprite for #{species}")
        :ok

      {:ok, %Req.Response{status: status}} ->
        Logger.warning("[SpriteCache] Failed to download #{species}: HTTP #{status}")
        {:error, {:http_error, status}}

      {:error, reason} ->
        Logger.warning("[SpriteCache] Failed to download #{species}: #{inspect(reason)}")
        {:error, reason}
    end
  end
end