@spec compute_check_digit(String.t()) :: non_neg_integer()

Computes the modulo-10 recursive check digit for a numeric string.

@spec generate(String.t() | nil, String.t()) ::
  {:ok, String.t()} | {:error, String.t()}

Generates a QR payment reference from an optional customer identification number and a reference number.

Parameters

• customer_id — optional BESR-ID from bank (numeric, max 11 digits), can be nil or ""
• reference_number — the reference number (numeric, required)

Returns

{:ok, reference} with the 27-digit reference, or {:error, reason}.

Example

iex> SwissQrBill.Reference.QrReferenceGenerator.generate("210000", "313947143000901")
{:ok, "210000000003139471430009017"}