(defun lzw-compress (sequence elements)
	(let ((dictionary (make-hash-table :test #'equalp))
				(index -1)
				(seq nil))
		(map nil
				 (lambda (e)
					 (setf (gethash (list e) dictionary) (incf index)))
				 elements)
		(nconc (loop
							for e across sequence
							do (setf seq (list* e seq))
							unless (gethash seq dictionary)
							collect (gethash (cdr seq) dictionary)
							and
							do (setf (gethash seq dictionary) (incf index)
											 seq (list (car seq))))
					 (list (gethash seq dictionary)))))
 
(defun lzw-decompress (list elements)
	(let ((dictionary (make-array (length elements)
																:adjustable t
																:fill-pointer t
																:initial-contents (map 'list #'list elements))))
		(loop
			 for index in list
			 for ee = e
			 and e = (if (< index (length dictionary))
									 (aref dictionary index)
									 e)
			 for s = (append (last e) ee)
			 unless (find s dictionary :test #'equalp)
			 do (vector-push-extend s dictionary)
			 append (reverse (setf e (aref dictionary index))))))
 
 
(defun test (data &optional elements)
	(let* ((data-elements (or elements (remove-duplicates data)))
				 (compressed (lzw-compress data data-elements))
				 (decompressed (lzw-decompress compressed data-elements)))
		(format t "~%characters:   ~s~%uncompressed: ~a~%compressed:   ~a~%decompressed: ~{~a~}~%" 
						data-elements data compressed decompressed)
		nil))
 
(test "010101010" "0123")
(test "01230123012" "0123")
(test "00001110000" "0123")
(test "akamakigami aomakigami kimakigami")
 

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