gistfile1.scm

(require (only-in "../lib.ss" enumerate-interval flatmap nil))

; checks not horiz
(define (safe? k queens)
  (define (problem-horiz? k-queen-row queens num-at)
    (cond ((eq? queens nil) #f) ; we've tried them all
          ((= k num-at) #f) ; this is comparing against itself
          ((= k-queen-row (car queens)) #t) ; the row of current queen is the same as kth queen
          (else (problem-horiz? k-queen-row (cdr queens) (+ 1 num-at)))))
  (define (problem-diag? k-row k-col queens num-at)
    ;(display k-row) (display " ") (display k-col) (display queens) (display num-at) (display "\n")
    (let ((queen-top-diag-at-k (+ (car queens) (- k-col num-at)))
          (queen-bot-diag-at-k (- (car queens) (- k-col num-at))))
   ;  (display queen-top-diag-at-k) (display " ") (display queen-bot-diag-at-k) (display "\n")
          (cond ((eq? queens nil) #f) ; we've tried them all 
                ((= k num-at) #f)     ; this is comparing against itself, thus tried all previous
                ((or (= k-row queen-top-diag-at-k)
                     (= k-row queen-bot-diag-at-k)) #t)
                (else (problem-diag? k-row k-col (cdr queens) (+ 1 num-at))))))
  (and (not (problem-horiz? (list-ref queens (- k 1)) queens 1))
       (not (problem-diag? (list-ref queens (- k 1)) k queens 1))))

(define (adjoin-position row col rest-of-queens)
  (append rest-of-queens (list row))) ; cheats a little bit, given that the col is implicit to the ordering

(define (queens board-size)
  (define empty-board '())
  (define (queen-cols k)  
    (if (= k 0)
        (list empty-board)
        (filter
         (lambda (positions) (safe? k positions))
         (flatmap ; all second lists within the main list, flatten
          (lambda (rest-of-queens) ; for each possibility
            (map (lambda (new-row) ; for each new row
                   (adjoin-position new-row k rest-of-queens)) ;add this row to each possibility 
                 (enumerate-interval 1 board-size)))
          (queen-cols (- k 1)))))) ; this is all possibilities
  (queen-cols board-size))

(queens 8 )