mercury_client

defmodule Mercury.Multipart do
  @moduledoc false

  # Hand-rolled multipart/form-data encoder (single file field) used for the
  # attachment-upload endpoints. Kept dependency-free rather than reaching
  # for an extra multipart-building library.

  @spec encode(field_name :: String.t(), filename :: String.t(), content :: binary()) ::
          {binary(), String.t()}
  def encode(field_name, filename, content) when is_binary(content) do
    boundary = "mercury-elixir-" <> random_boundary()

    body =
      IO.iodata_to_binary([
        "--",
        boundary,
        "\r\n",
        "Content-Disposition: form-data; name=\"",
        field_name,
        "\"; filename=\"",
        escape(filename),
        "\"\r\n",
        "Content-Type: application/octet-stream\r\n",
        "\r\n",
        content,
        "\r\n",
        "--",
        boundary,
        "--\r\n"
      ])

    {body, "multipart/form-data; boundary=" <> boundary}
  end

  defp random_boundary do
    16
    |> :crypto.strong_rand_bytes()
    |> Base.encode16(case: :lower)
  end

  defp escape(name), do: String.replace(name, "\"", "\\\"")
end