(defun binary-gap (n)
  (let* ((bits (write-to-string n :base 2))
         (trimmed (string-trim "0" bits))
         (groups (split-on-char trimmed #\1))
         (gap 0))
    (dolist (zero groups)
      (setf gap (max gap (length zero))))
    gap))

(defun split-on-char (s ch)
  (loop with start = 0
        for pos = (position ch s :start start)
        collect (subseq s start pos)
        while pos
        do (setf start (1+ pos))))