(defstruct (world (:print-function
						 (lambda (s d o)
							(declare (ignore d))
							(format s "#<world: ~D>" (world-numdots world)))))
	size
	current
	numdots
	next
	(xmin 1000000)
	(xmax -1)
	(ymin 1000000)
	(ymax -1))

(defun setnext (world i j)
	(let* ((current (world-current world))
			(next (world-next world))
			(neighbors (count-neighbors current i j)))
	 (if (zerop (bit current i j))
	(cond ((not (= neighbors 3))
			 (setf (bit next i j) 0))	
			(t (setf (bit next i j) 1)
		 (incf (world-numdots world)))) 
			(cond ((or (= neighbors 2) 
			(= neighbors 3))
			 (setf (bit next i j) 1))	
			(t (setf (bit next i j) 0)
						(decf (world-numdots world)))))
	 (unless (zerop (bit next i j))
		(when (< i (world-xmin world)) (setf (world-xmin world) i))
		(when (> i (world-xmax world)) (setf (world-xmax world) i))
		(when (< j (world-ymin world)) (setf (world-ymin world) j))
		(when (> j (world-ymax world)) (setf (world-ymax world) j)))))

(defun count-neighbors (array i j)
	(+ (bit array (1- i) (1- j))
		(bit array i		(1- j))
		(bit array (1+ i) (1- j))
		(bit array (1- i) j)
		(bit array (1+ i) j)
		(bit array (1- i) (1+ j))
		(bit array i		(1+ j))
		(bit array (1+ i) (1+ j))))

(defun next-cycle (world)
	(let* ((lim (world-size world))
			(current (world-current world))
			(next (world-next world))
			(xlb (max 1 (1- (world-xmin world))))
			(xub (min (- lim 2) (1+ (world-xmax world))))
			(ylb (max 1 (1- (world-ymin world))))
			(yub (min (- lim 2) (1+ (world-ymax world)))))
	 (dotimes (i (1+ (- xub xlb)))
		(dotimes (j (1+ (- yub ylb)))
			(setnext world (+ i xlb) (+ j ylb))))
	 (dotimes (y lim)
		(dotimes (x lim)
			(setf (bit current x y) (bit next x y))))))

(defun print-world (world generations)
	(let ((lim (world-size world))
			(current (world-current world)))
	 (dotimes (y lim)
		(dotimes (x lim)
			(if (zerop (bit current y x))
				(princ " ")
				(princ "*")))
		(terpri))
	 (format t "~&~d Generations, ~d Organisms." 
				generations (world-numdots world))))

(defun propagate (world cycles)
	(print-world world cycles)
	(do ()
		((zerop (world-numdots world))
		 (format t "~2&POPULATION 0 ... ~d generations" cycles))
	 (next-cycle world)
	 (incf cycles)
	 (print-world world cycles)))

(defun life (source)
	(let* ((size (length (car source)))
			(life (make-world
					 :size size
					 :current (make-array (list size size) :element-type "bit
																		:initial-contents source)
					 :next (make-array (list size size) :element-type "bit
																	:initial-element 0)
					 :numdots 0)))
	 (dotimes (i size)
		(dotimes (j size)
			(unless (zerop (bit (world-current life) i j))
			 (incf (world-numdots life))
			 (when (< i (world-xmin life)) (setf (world-xmin life) i))
			 (when (> i (world-xmax life)) (setf (world-xmax life) i))
			 (when (< j (world-ymin life)) (setf (world-ymin life) j))
			 (when (> j (world-ymax life)) (setf (world-ymax life) j)))))
	 (propagate life 0)))


                  
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