(defun read-same-characters (&optional (stream *standard-input*)
                                       (eof-error-p t)
                                       eof-value)
  (loop for len upfrom 1
        for c = (read-char stream eof-error-p nil)
        while c
        collect c into chars
        while (eql c (peek-char nil stream nil nil))
        finally (return (if c
                            (cons c len)
                            eof-value))))
 
(defun hash-table-to-alist (hash-table)
  (loop for key being the hash-keys in hash-table using (hash-value val)
        collect (cons key val)))
 
(let* ((stats
         (with-open-stream (s (open "/dev/stdin" :direction :input))
           (loop with stats = (make-hash-table)
                 for (char . len) = (read-same-characters s nil nil)
                 while char
                 for char-maxlen = (gethash char stats 0)
                 when (> len char-maxlen)
                   do (setf (gethash char stats) len)
                 finally (return (hash-table-to-alist stats)))))
       (maxlen (apply #'max (mapcar (lambda (x) (cdr x)) stats)))
       (longest-repeated-characters
         (remove-if (complement (lambda (x) (eql (cdr x) maxlen)))
                    stats)))
  (format t "~S~%" longest-repeated-characters))
 

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