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(define (shl bits)
(define len (vector-length bits))
(define res (make-vector len))
(define (copy i)
(if (= i (- len 1))
#t
(and
(vector-set! res i
(vector-ref bits (+ i 1)))
(copy (+ i 1)))))
(copy 0)
(set! copy '())
(vector-set! res (- len 1) #f)
res)
(define (shr bits)
(define len (vector-length bits))
(define res (make-vector len))
(define (copy i)
(if (= i (- len 1))
#t
(and
(vector-set! res (+ i 1)
(vector-ref bits i))
(copy (+ i 1)))))
(copy 0)
(set! copy '())
(vector-set! res 0 #f)
res)
(define (empty-bits len) (make-vector len #f))
(define vs vector-set!)
(define vr vector-ref)
(define res 0)
(define (queen n)
(define (search l m r step)
(define (col-iter c)
(if (= c n)
#f
(and
(if (and (eq? (vr l c) #f)
(eq? (vr r c) #f)
(eq? (vr m c) #f))
(and
(vs l c #t)
(vs m c #t)
(vs r c #t)
((lambda () (search l m r (+ step 1)) #t))
(vs l c #f)
(vs m c #f)
(vs r c #f))
)
(col-iter (+ c 1))
)))
(set! l (shl l))
(set! r (shr r))
(if (= step n)
(set! res (+ res 1))
(col-iter 0)))
; (set! col-iter '()))
(search (empty-bits n)
(empty-bits n)
(empty-bits n)
0)
res)
(display (queen 8))
(define shl '())
(define shr '())
(define empty-bits '())
(define res '())
(define queen '())
(set-gc-resolve-threshold! 0) ; force cycle resolve
(display "\n")
(display (gc-status))
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