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8b5eb68238
They can be close()d explicitly with close-file-descriptor, and will close() on gc, but only explicitly closing the last port on them will close the fileno. Notably needed for network sockets where we open separate input and output ports on the same socket.
36 lines
982 B
Scheme
36 lines
982 B
Scheme
;;; NQUEENS -- Compute number of solutions to 8-queens problem.
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;; 2006/08 -- renamed `try' to `try-it' to avoid Bigloo collision (mflatt)
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(define trace? #f)
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(define (nqueens n)
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(define (one-to n)
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(let loop ((i n) (l '()))
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(if (= i 0) l (loop (- i 1) (cons i l)))))
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(define (try-it x y z)
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(if (null? x)
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(if (null? y)
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(begin (if trace? (begin (write z) (newline))) 1)
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0)
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(+ (if (ok? (car x) 1 z)
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(try-it (append (cdr x) y) '() (cons (car x) z))
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0)
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(try-it (cdr x) (cons (car x) y) z))))
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(define (ok? row dist placed)
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(if (null? placed)
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#t
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(and (not (= (car placed) (+ row dist)))
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(not (= (car placed) (- row dist)))
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(ok? row (+ dist 1) (cdr placed)))))
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(try-it (one-to n) '() '()))
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(let ((input (with-input-from-file "input.txt" read)))
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(time
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(let loop ((n 500) (v 0))
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(if (zero? n)
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v
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(loop (- n 1) (nqueens (if input 8 0)))))))
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