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  1. ; find the nearest prime
  2.  
  3. (define (primes n)
  4.   (let ((sieve (make-vector n #t)) (ps (list)))
  5.     (do ((p 2 (+ p 1))) ((= n p))
  6.       (when (vector-ref sieve p)
  7.         (set! ps (cons p ps))
  8.         (do ((i (* p p) (+ i p))) ((<= n i))
  9.           (vector-set! sieve i #f))))
  10.     (list->vector (reverse ps))))
  11.  
  12. (define ps (primes 542))
  13.  
  14. (define (bsearch x xs)
  15.   (let loop ((lo 0) (hi (- (vector-length xs) 1)))
  16.     (let ((mid (+ lo (quotient (- hi lo) 2))))
  17.       (cond ((< hi lo)
  18.               (if (< (- (vector-ref xs hi) x)
  19.                      (- x (vector-ref xs lo)))
  20.                   (vector-ref xs lo)
  21.                   (vector-ref xs hi)))
  22.             ((< x (vector-ref xs mid)) (loop lo (- mid 1)))
  23.             ((< (vector-ref xs mid) x) (loop (+ mid 1) hi))
  24.             (else (vector-ref xs mid))))))
  25.  
  26. (define (find-prime x) (bsearch x ps))
  27.  
  28. (display (find-prime 123.7)) (newline)
  29. (display (find-prime 4)) (newline)
Success #stdin #stdout 0.02s 42848KB
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https://ideone.com/xPH7v6
language:
Scheme (guile 2.2.4)
created:
6 years ago
visibility:
public

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