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fixing bug in prime-above

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This commit is contained in:
Alex Shinn 2021-04-14 16:47:29 +09:00
parent 3fc9c22245
commit cfbd64f085
2 changed files with 34 additions and 8 deletions
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12
lib/chibi/math/prime-test.sld
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@ -32,7 +32,19 @@
(test 1009 (nth-prime 168))
(test 1013 (nth-prime 169))
(test 2 (prime-above 1))
(test 3 (prime-above 2))
(test 5 (prime-above 3))
(test 5 (prime-above 4))
(test 7 (prime-above 5))
(test 907 (prime-above 888))
(test 911 (prime-above 907))
(test-not (prime-below 2))
(test 2 (prime-below 3))
(test 3 (prime-below 4))
(test 3 (prime-below 5))
(test 5 (prime-below 6))
(test 5 (prime-below 7))
(test 797 (prime-below 808))
(test 1 (totient 2))

26
lib/chibi/math/prime.scm
View file

@ -146,20 +146,34 @@
;;> Returns the first prime less than or equal to \var{n}, or #f if
;;> there are no such primes.
(define (prime-below n)
(and (>= n 3)
(let lp ((n (if (even? n) (- n 1) n)))
(if (prime? n) n (lp (- n 2))))))
(cond
((> n 3)
(let lp ((n (if (even? n) (- n 1) (- n 2))))
(if (prime? n) n (lp (- n 2)))))
((= n 3)
2)
(else
#f)))
;;> Returns the first prime greater than or equal to \var{n}. If the
;;> optional \var{limit} is given and not false, returns \scheme{#f}
;;> if no such primes exist below \var{limit}.
(define (prime-above n . o)
(let ((limit (and (pair? o) (car o))))
(let lp ((n (if (even? n) (+ n 1) n)))
(cond
((and limit (>= n limit)) #f)
((< n 2)
2)
(limit
(let lp ((n (if (even? n) (+ n 1) (+ n 2))))
(cond
((>= n limit) #f)
((prime? n) n)
(else (lp (+ n 2)))))))
(else (lp (+ n 2))))))
(else
(let lp ((n (if (even? n) (+ n 1) (+ n 2))))
(cond
((prime? n) n)
(else (lp (+ n 2)))))))))
;;> Returns the factorization of \var{n} as a monotonically
;;> increasing list of primes.