Google url signer for Elixir

url_signer.ex

defmodule GoogleMaps.UrlSigner do
  @module_doc """
  Generate a signed url for google maps API.
  https://developers.google.com/maps/documentation/streetview/get-api-key#generating_valid_signatures
  """
  def call(private_key, url) do
    parsed_url = URI.parse(url)
    path_and_query = "#{parsed_url.path}?#{parsed_url.query}"

    # Create a signature using the private key and the URL(path and query).
    # And encode the signature into base64 for url use form.
    signature =
      Base.url_decode64!(private_key)
      |> create_signature(path_and_query)
      |> Base.url_encode64

    "#{parsed_url.scheme}://#{parsed_url.host}#{path_and_query}&signature=#{signature}"
  end

  defp create_signature(key, data) do
    :crypto.hmac(:sha, key, data)
  end
end