steuer-id.lisp

;; c.f. <https://de.wikipedia.org/wiki/Steuerliche_Identifikationsnummer#Aufbau_der_Identifikationsnummer>

(defun check-steuer-id (id)
  (let ((product 0))
    (loop
      with digits = (mapcar (lambda (digit) (parse-integer (string digit))) (coerce (format NIL "~D" id) 'list))
      for i from 1 to 10
      for digit = (elt digits (1- i))
      for sum = (mod (+ digit product) 10)
      when (zerop sum)
        do (setf sum 10)
      do (setf product (mod (* sum 2) 11))
      finally
         (let ((check-digit (- 11 product)))
           (when (eql check-digit 10)
             (setf check-digit 0))
           (return (eql (elt digits 10) check-digit))))))