8-Puzzle: Moving a Tile

(defun state-new-pos (state dir)

(let ((dimen (truncate (sqrt (length (state-tiles state)))))

(blank-pos (state-blank-pos state)))

(multiple-value-bind (row col) (truncate blank-pos dimen)

(case dir

((:UP)

(if (= row 0)

NIL

(+ (* dimen (- row 1)) col)))

((:DOWN)

(if (= row (- dimen 1))

NIL

(+ (* dimen (+ row 1)) col)))

((:LEFT)

(if (= col 0)

NIL

(+ (* dimen row) (- col 1))))

((:RIGHT)

(if (= col (- dimen 1))

NIL

(+ (* dimen row) (+ col 1))))))))

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