defmodule PhoenixImage do
  @moduledoc """
  On-the-fly image resizing for Phoenix with disk caching and responsive components.

  ## Setup

  1. Configure your OTP app:

      ```elixir
      # config/config.exs
      config :phoenix_image, otp_app: :my_app
      ```

  2. Add to your supervision tree:

      ```elixir
      # lib/my_app/application.ex
      children = [
        PhoenixImage,
        MyAppWeb.Endpoint
      ]
      ```

  3. Forward requests in your router:

      ```elixir
      # lib/my_app_web/router.ex
      forward "/resize", PhoenixImage.Plug
      ```

  4. Import the component:

      ```elixir
      # lib/my_app_web.ex, in html_helpers/0
      import PhoenixImage.Component
      ```

  ## Configuration

  All options are set via application config:

      config :phoenix_image,
        otp_app: :my_app,            # Required
        prefix: "/resize",            # Default: "/resize"
        secret: "random-secret",     # Signing secret (falls back to endpoint's secret_key_base)
        quality: 90,                  # Default: 90
        source_dir: "priv/static",    # Default: "priv/static"
        cache_dir: "priv/static/cache/resize"  # Default: "priv/static/cache/resize"
  """

  @default_quality 90
  @default_prefix "/resize"
  @default_source_dir "priv/static"
  @default_cache_dir "priv/static/cache/resize"

  @doc """
  Returns a child spec for the `PhoenixImage.Dimensions` GenServer.

  Add `PhoenixImage` to your application's supervision tree to start
  the ETS-backed dimension cache.
  """
  def child_spec(_opts) do
    %{
      id: PhoenixImage.Dimensions,
      start: {PhoenixImage.Dimensions, :start_link, [[]]},
      type: :worker
    }
  end

  @doc """
  Returns the configured OTP app name.

  Raises if `:otp_app` is not configured.
  """
  def otp_app do
    Application.get_env(:phoenix_image, :otp_app) ||
      raise "phoenix_image :otp_app not configured"
  end

  @doc """
  Returns the configured URL prefix. Default: `"/resize"`.

  Must match the `forward` path in your router.
  """
  def prefix, do: Application.get_env(:phoenix_image, :prefix, @default_prefix)

  @doc """
  Returns the signing secret used for URL signatures.

  Falls back to the Phoenix endpoint's `secret_key_base` if not configured.
  """
  def secret do
    case Application.get_env(:phoenix_image, :secret) do
      nil ->
        app = otp_app()
        endpoint = Application.get_env(app, :endpoint)

        if endpoint do
          endpoint.config(:secret_key_base)
        else
          raise "phoenix_image :secret not configured and no endpoint found"
        end

      secret ->
        secret
    end
  end

  @doc """
  Signs a URL path with HMAC-SHA256, returning a 12-character URL-safe base64 signature.
  """
  def sign_url(path) do
    :crypto.mac(:hmac, :sha256, secret(), path)
    |> Base.url_encode64(padding: false)
    |> binary_part(0, 12)
  end

  @doc """
  Verifies a URL path against a signature using constant-time comparison.

  Returns `:ok` if the signature is valid, `:error` otherwise.
  """
  def verify_url(path, signature) do
    expected = sign_url(path)
    if Plug.Crypto.secure_compare(expected, signature), do: :ok, else: :error
  end

  @doc """
  Returns the JPEG output quality (1-100). Default: `90`.
  """
  def quality, do: Application.get_env(:phoenix_image, :quality, @default_quality)

  @doc """
  Returns the source directory for original images, relative to the OTP app's priv.

  Default: `"priv/static"`.
  """
  def source_dir, do: Application.get_env(:phoenix_image, :source_dir, @default_source_dir)

  @doc """
  Returns the cache directory for resized images, relative to the OTP app's priv.

  Default: `"priv/static/cache/resize"`.
  """
  def cache_dir, do: Application.get_env(:phoenix_image, :cache_dir, @default_cache_dir)
end