1 665c255d 2023-08-04 jrmu (defun make-position (row col)
 2 665c255d 2023-08-04 jrmu   (cons row col))
 3 665c255d 2023-08-04 jrmu (defun position-row (pos)
 4 665c255d 2023-08-04 jrmu   (car pos))
 5 665c255d 2023-08-04 jrmu (defun position-col (pos)
 6 665c255d 2023-08-04 jrmu   (cdr pos))
 7 665c255d 2023-08-04 jrmu (defun positions-equal (a b)
 8 665c255d 2023-08-04 jrmu   (equal a b))
 9 665c255d 2023-08-04 jrmu (defvar empty-board '())
10 665c255d 2023-08-04 jrmu (defun adjoin-position (row col positions)
11 665c255d 2023-08-04 jrmu   (append positions (list (make-position row col))))
12 665c255d 2023-08-04 jrmu (defun attacks? (a b)
13 665c255d 2023-08-04 jrmu   (let ((a-row (position-row a))
14 665c255d 2023-08-04 jrmu 	(a-col (position-col a))
15 665c255d 2023-08-04 jrmu 	(b-row (position-row b))
16 665c255d 2023-08-04 jrmu 	(b-col (position-col b)))
17 665c255d 2023-08-04 jrmu     (cond
18 665c255d 2023-08-04 jrmu       ((= a-row b-row) t)
19 665c255d 2023-08-04 jrmu       ((= a-col b-col) t)
20 665c255d 2023-08-04 jrmu       ((= (abs (- a-col b-col))
21 665c255d 2023-08-04 jrmu 	  (abs (- a-row b-row))) t)
22 665c255d 2023-08-04 jrmu       (t nil))))
23 665c255d 2023-08-04 jrmu (defun safe? (k positions)
24 665c255d 2023-08-04 jrmu   (let ((kth-pos (nth (1- k) positions)))
25 665c255d 2023-08-04 jrmu     (if (null (find-if
26 665c255d 2023-08-04 jrmu 	       (lambda (pos)
27 665c255d 2023-08-04 jrmu 		 (and (not (positions-equal kth-pos pos))
28 665c255d 2023-08-04 jrmu 		      (attacks? kth-pos pos)))
29 665c255d 2023-08-04 jrmu 	       positions))
30 665c255d 2023-08-04 jrmu 	t
31 665c255d 2023-08-04 jrmu 	nil)))