Reservoir sampling over a list in Common Lisp

sample-list.lisp

(defun sample-list (items &optional (size 1))
  (let ((sample (make-array size :fill-pointer 0)))
    (loop for item in items
          for enum from 1
          if (<= enum size)
            do (vector-push item sample)
          else
            do (let ((rand (random enum)))
                 (when (< rand size)
                   (setf (aref sample rand) item)))
          finally (return sample))))