(defglobal
	?*N* = 8
)

(deffunction free_table
	(?n)
	(loop-for-count (?nr 1 ?n) do
		(loop-for-count (?letter 1 ?n) do
			(assert(free ?nr ?letter))
		)
	)
	(return)
)

;(deffacts alphabet
;	(letters a b c d e g h i j k l m n o p q r s t u v w x y z)
;)
	
(defrule init-table
	(initial-fact)
=>
	(free_table ?*N*)
)

(defrule place_first_queen
	(not (queen ? ?))
	(free ? ?)
=>
	(assert(queen 1 1))
	(assert(level 2))
)

; level - next' column index

(defrule next_queen
	(declare (salience 10))
	?level_fact <- (level ?column)
	?free_fact <- (free ?line ?column)
	(not (non_free ?line ?column))
	(test (<= ?column ?*N*))
=>
	(assert (queen ?line ?column))
	(retract ?free_fact)
	(retract ?level_fact)
	(assert (level (+ 1 ?column)))
)

(defrule update_spaces
	(declare (salience 20))
	(queen ?l ?c)
	?free_fact <- (free ?fl ?fc)
	(or
		(test (= ?fl ?l))
		(test (= ?fc ?c))
		(test (= (- ?fc ?c) (- ?fl ?l)))
		(test (= (+ ?fc ?fl) (+ ?l ?c)))
		(non_free ?fl ?fc)
	)
=>
	(retract ?free_fact)
)



(defrule back
	(declare (salience 0))
	?level_fact <- (level ?level)
	?queen_fact <- (queen ?line ?column)
	(test (= ?level (+ 1 ?column)))
	(not (free ? ?level))
=>
	(free_table ?*N*)
	(retract ?queen_fact)
	(retract ?level_fact)
	(assert (non_free ?line ?column))
	(assert (level ?column))
)

(defrule exit
	(declare (salience 1000))
	(level ?level)
	(test (> ?level ?*N*))
=>
	(halt)
)

(defrule control_non_free_fields
	(declare (salience 10))
	(level ?level)
	?non_free_fact <- (non_free ?line ?column)
	(test (>= ?column ?level))	
=>
	(retract ?non_free_fact)
)