(defun entropy (string)
  (let ((table (make-hash-table :test 'equal))
        (entropy 0))
    (mapc (lambda (c) (setf (gethash c table) (+ (gethash c table 0) 1)))
          (coerce string 'list))
    (maphash (lambda (k v)
               (decf entropy (* (/ v (length input-string))
                                (log (/ v (length input-string)) 2))))
             table)
    entropy))