philomena/lib/camo/image.ex

defmodule Camo.Image do
  def image_url(input) do
    uri = URI.parse(input)

    cond do
      is_nil(uri.host) ->
        ""

      is_nil(camo_key()) ->
        input

      uri.host in [cdn_host(), camo_host()] ->
        URI.to_string(%{uri | scheme: "https", port: 443})

      true ->
        camo_digest = :crypto.mac(:hmac, :sha, camo_key(), input) |> Base.url_encode64(padding: false)

        camo_uri = %URI{
          host: camo_host(),
          path: "/" <> camo_digest <> "/" <> Base.url_encode64(input, padding: false),
          scheme: "https"
        }

        URI.to_string(camo_uri)
    end
  end

  defp cdn_host do
    Application.get_env(:philomena, :cdn_host)
  end

  defp camo_key do
    Application.get_env(:philomena, :camo_key)
  end

  defp camo_host do
    Application.get_env(:philomena, :camo_host)
  end
end